quiz 2 Flashcards
(160 cards)
1
Q
essential fatty acids
A
linoleate (w-6) and linolenate (w-3)
mammals lack the enzymes to introduce double bonds at carbon atoms beyond C9 in the hydrocarbon chain
these are precursors for other needed fatty acids
2
Q
fatty acid synthesis
A
occurs in cell cytoplasm
when high rates of intramitochondrial generation of acetyl co-A and citrate, citrate is transported out of mitochondria
enzymes (malic enzyme, ACL, ACC, FAS) then make saturated FA (palmitic acid (C16))
requires substantial investment of ATP and NADPH so pathway operates at maximum rates when glucose is readily available
happen predominantly in liver and lactating mammary gland
3
Q
fatty acid oxidation
A
mostly in liver and in muscle (but in all tissues except for the brain and RBCs)
requires presence of functioning mitochondria and readily available oxygen
called beta oxidation because the oxidation begins at the beta carbon in the hydrophobic chain
occurs within the mitochondria
4
Q
ketogenesis
A
only in the liver
partial oxidation of FA creating water soluble fuels (ketone bodies) from water-insoluble compound
requires mitochondria
5
Q
ATP citrate lyase
A
citrate + ATP + CoA + H2O = acetyl CoA + ADP + Pi + oxaloacetate
ATP investment to get the pathway started
step 1 in FA synthesis
6
Q
acetyl-coA carboxylase (ACC)
A
converts acetyl CoA to malonyl CoA by adding carboxyl group
contains biotin (vitamin B7)
rate-limiting, highly regulated step
also requires ATP
has two isozymes - first ACC-alpha - in the liver and mammary gland - in cytosol
second = ACC-beta - in muscle and liver - attached to outside of mitochondria and creates inhibitory processes
process has two steps:
first: e-biotin + ATP + HCO3 = E-N-carboxybiotin + ATP + Pi
second: E-N-carboxybiotin + acetyl-CoA = malonyl CoA + E-biotin
7
Q
Fatty acid synthase (FAS)
A
dimeric enzyme with multiple catalytic centers
uses the vitamin pantothenic acid as part of ACP domain - this acid immobilizes reaction intermediates
adds two cycles of carbon addition to malonyl coA
creates double bonds in the process that require reduction by NADPH
only expressed in lipogenic tissues
has 8 catalytic domains and exists as a dimer
ACP domain uses vitamin B5 (coenzyme A)
malonyl attaches to serine on B5
the acetyl group attaches to the other dimer
the enzyme transfers 2 carbons from the malonyl to the acetyl group, making butyryl and then loads another malonyl group and transfers another two carbons - continues until 16 carbons long resulting in palmitic acid
8
Q
malic enzyme
A
only expressed in FA synthesizing tissue
its activity links OAA formation by ACL to NADPH synthesis
source of NADPH needed to reduce FA made by FAS
converts malate to pyruvate so the pyruvate can go to making more NADPH - results in 8 moles of NADPH (get the other 6 needed for FA synthesis from the pentose phosphate pathway)
9
Q
isocitrate dehydrogenase (IDH)
A
when high activity in mitochondria because high glucose levels the high activity will inhibit IDH - this results in a backup in the cycle and extra citrate which is then exported from the mitochondria to be converted into FA
10
Q
malate dehydrogenase
A
makes oxaloacetate into malate with NADH (part of FA synthesis)
11
Q
transport of FA
A
from liver to adipose tissue in VLDL
stored as TG in adipose tissue
in ingested it’s transported as chylomicrons
lipoprotein lipase allows for its transportation
12
Q
fructose versus glucose
A
when glucose metabolized, high ATP levels inhibit phosphofructokinase I which limits the downstream products which limits the synthesis of FA
fructose metabolism is upstream of fructokinase so its inhibition doesn’t inhibit fructose metabolism so get much larger cytoplasmic pool of acetyl coA from fructose than from glucose
fructose also induces transcription of genes for FA synthesis in liver to greater extent than glucose does - get more ACC, FAS
fructose also binds with greater affinity to sweet receptors
13
Q
lipoprotein lipase (LPL)
A
enzyme that breaks down triglycerides to FA to allow the FA to enter the adipose store - they are reassembled back into TG once inside the adipose cell
14
Q
enzymes needed to get FA out of adipose tissue
A
TG can't be transported out of the adipose cell so it has to broken down into FA chains first ATG-L - takes off the first FA chain HSL - takes off the second FA chain MGL - takes off the third ATG-L and HSL are highly regulated
15
Q
perlipin
A
enzyme involved in getting TG stores out of adipose cells
positions the ATG-L and HSL enzymes
needed to create the vacuoles for glycolysis
one of the ways the transport/breakdown of TG is regulated
16
Q
albumin
A
FA are detergents and so can’t circulate freely or they would cause cell damage
they’re bound to albumin when circulating so that they don’t lyse cell membranes
these are free FA (even though they’re bound to something)
17
Q
CD36
A
cell channel that takes up FA after they dissociate from albumin
18
Q
low density lipoprotein (LDL)
A
have much less triacylglycerol than VLDL
high concentration of cholesterol and CE
primary function = provide cholesterol to peripheral tissues
bind to cell-surface membrane LDL receptors (apo-B-100/apo-e receptors) that recognize apo B-100 (and also apo-e)
steps of uptake and degradation:
1: LDL receptors negatively charged glycoproteins - clustered in pits on cell membranes - intracellular side of pit coated with clathrins
2: LDL binds and LDL-receptor complex internalized via endocytosis (binding encouraged by T3 hormone)
3: vesicle loses clathrin coat and fuses with other similar vesicles - makes endosomes
4: pH of endosome falls - allows for separation of LDL from receptor
5: receptors migrate to one side of endosome and LDLs stay in lumen (structure called CURL at this point)
6: receptor recycled - lipoprotein degraded in lysosomes, releasing free cholesterol, AA, FA and phospholipids
19
Q
pancreatic lipase
A
enzyme responsible for the hydrolysis of ingested TG in the small intestine
20
Q
adipocyte triglyceride lipase (APL)
A
in adipocyte
removes first fatty acid chain from triglyceride
21
Q
hormone-sensitive lipase (HSL)
A
in adipocyte
removes second FA chain from what was originally TG (but is no diglyceride because ATGL must act before HSL)
22
Q
monoglyceride lipase (MGL)
A
in adipocyte
takes last FA chain off of what was once TG (is now monoglyceride - can only act after ATGL and HSL have already removed the first two FA chains)
23
Q
non-esterified fatty acids (NEFA)
A
also known as free fatty acids
FA circulating in the plasma bound to albumin
can be converted to ketone bodies in the liver
24
Q
glycerol
A
what the FA are bound to to make TG
when TG are broken down, the TG is also released into the plasma and can be used by the liver and kidney as a gluconeogenic precursor
25
fatty acyl-CoA synthases
step one of FA oxidation
when FA are taken up into cells, these enzymes catalyze the formation of the fatty acyl thioester conjugate with coenzyme A
requires ATP
accelerated by pyrophosphatase enzyme
occurs on the outer mitochondrial membrane
26
pyrophosphatase
enzyme that breaks down PPi created by the acyl CoA synthases in the first step of FA oxidation
breaking down the PPi makes the reaction essentially reservable and speeds up the reaction because it's removing one of the products, pushing the reaction equilibrium towards the products
27
carnitine acyltransferase I (CAT-1) aka carnitine parlmitoyl transferase (CPT-1)
step two of FA oxidation
the fatty acyl-CoA can't enter the mitochondria
this enzyme conjugates the FA with carnitine (derived from lysine) via a transesterification reaction (removes the CoA and replaces it with carnitine)
this is inhibited by malonyl CoA - when there's lots of FA synthesis there's lots of malonyl CoA and so FA synthesis is inhibited - allosteric inhibition - this allows for the rate of FA synthesis to be tied to levels of glucose present (lots of malonyl-CoA when glycolytic rates high)
reversible
only occurs in the presence of O2
occurs on the outer membrane of mitochondria
28
CAT-II
step 4 of FA oxidation
when the carnitine-FA moiety is transported into the cell, it is then converted back into FA-acyl CoA by this enzyme
this is the rate limiting step
29
carnitine acylcarnitine translocase (CACT)
step 3 of FA oxidation
transports the carnitine FA across the inner mitochondrial membrane and transfers carnitine out of mitochondria to be used in earlier steps
30
beta-oxidation pathway
step 5 in FA oxidation
occurs once FA are in the mitochondrial membrane
each step releases a 2-carbon fragment in the form of acetyl-CoA
each palmitoyl CoA (16 carbons) undergoes 7 oxidation cycles yielding 8 acetyl CoAs
acetyl CoA goes into Kreb cycle if there's enough oxaloacetate present (need some continuous oxidation of glucose for this)
the initial dehydrogenase reaction results in FADH2 and the second one results in NADH - these are used by electron transport chain for ATP synthesis
31
energy yields from beta oxidation of FA
palmitoyl CoA + 7 FAD + 7 NAD+ + H2O + CoA => 8 Acetyl-CoA + 7 FADH2 + 7 NADH + 7 H+
metabolism of 8 mol acetyl CoA in Krebs cycle = + 80 mol ATP
oxidation of 7 mol FADH2 = + 10.5 mol ATP
oxidation of 7 mol NADH = + 17.5 mol ATP
ATP utilization in fatty acyl-CoA ligase = - 2 mol ATP
= 106 mol ATP per mol palmitat
take away point: about 3x as much ATP produced from one mol FA as from one mol glucose
32
ketogenesis
conversion of FA to ketones
ketones are water soluble and are used by muscle and brain (can get through BBB whereas FA can't)
occurs only in the liver!
33
regulation of ketogenesis
when oxaloacetate levels are low because glucose isn't available there won't be much conversion of acetyl CoA to citrate
acetyl CoA will be disposed of via an alternate pathway that only occurs in the liver that genearte acetoacetate and B-hydroxybutyrate (the ketone bodies) and some acetone (eliminated through lungs, hence fruity breath of those in ketosis)
34
ketone bodies
generated in liver by ketogenesis
acetoacetate and beta-hydroxybutarate
both are organic acids and so when in high concentrations result in acidosis
35
ketone oxidation
largely in brain and muscle
need mitochondria, oxygen, oxaloacetate and succinyl-CoA
because need succinyl-CoA must have some glucose as well
energy yield for ketones exceeds that of glucose
36
why can't we make glucose from FA?
the PDH reaction is essentially irreversible so pyruvate can't be used to form acetyl-CoA
there's never any net synthesis of oxaloacetate during FA oxidation - mammals can't form oxaloacetate de novo from acetyl-CoA and so that component of making glucose would be missing
37
glucose availability
liver and kidney are principal organs of gluconeogenesis - use AA, glycerol and lactate as precursors
limited amount of glycogen and even less that can support blood glucose because the glycogen stored in the skeletal muscle can't be released because skeletal muscle lacks glucose-6-phosphatase
when glycogen is depleated gluconeogenesis is used to make the glucose needed for the brain and RBCs and make the oxaloacetate needed for FA/ketone oxidation
gluconeogenesis requires lots of ATP - energy derived from oxidation of FA in the liver
38
metabolic cycle
ensures the ready supply of immediate energy and the constant replenishment of depleted energy
has two phases:
1: anabolic = period that begins with the ingestion of food and continues until the ingested nutrients are assimilated, utilized or stored as reserve energy
2: catabolic = between termination of anabolic phase and next meal - reserve stores are utilized for energy
levels of insulin glucagon and epinephrine control phases
39
short-term regulation of metabolism
seconds-minutes
accomplished by changes in catalytic activity of performed enzymes/proteins with no change in the enzyme content of the cell
mechanisms: allosteric regulation and covalent modification of enzymes
40
long-term regulation of metabolism
changes in enzyme/protein content of cell but also may include changes in specific activity
mechanisms: changes in rate of gene transcription, mRNA turnover, mRNA translation and protein degradation
41
glucose (as regulator of metabolism)
stimulates its own storage by enhancing net glycogen and fatty acid synthesis
42
fatty acids (as regulators of metabolism)
diminish rates of FA synthesis and increase FA oxidation
43
regulation of metabolism by cellular energy status
ATP and AMP can regulate metabolism
5'-AMP activates AMPK (AMP-activated protein kinase) which inactivates enzymes in several synthetic pathways that use ATP and activates other pathways that increase ATP generation
this is regulated by the energy levels because when ATP becomes depleted adenylate kinase converts 2 ADP to ATP + AMP and the AMP activates AMPK allosterically
ATP will inhibit AMPK if there's high levels of energy production
44
glucagon and epinephrine
increased levels do the following:
1: activate net hepatic glycogen breakdown in the liver
2: in liver, activate gluconeogenesis
3: in adipose tissue activates lypolysis
4: in skeletal muscle activates FA utilization - Beta-oxidation of FA so they can be liberated from the adipose tissue
5: in liver activates ketone synthesis
45
insulin receptor activation
receptor has two outer alpha subunits and two inner beta subunits
insulin binds to the alpha subunit and changes the shape of the receptor
the beta subunits are a tyrosine kinase - activation results in autophosphorylation
insulin receptor subunit (IRS) and shc can now dock on the receptor
this pulls in other proteins
ultimately activates AKT
46
insulin actions
```
inhibits all of the things glucagon and epinephrine by activating AKT which phosphroylates PDE, resulting in the breakdown of cAMP to 5'AMP - this decreases PKA activity
favors FA and glycogen storage
1: stimulates glucose transport
2: inhibits gluconeogenesis
3: stimulates FA synthesis
(see other cards for more detail)
```
47
insulin stimulation of glucose transport
in skeletal muscle, heart tissue, and adipose tissue
via AKT action
AKT targests vessicle containing GLUT4
vessel fuses with cell membrane and allows glucose to enter cell
when we exercise AMP also activates AMPK which also traffics GLUT 4
as a result, if we exercise right after taking glucose we could get hyperglycemic cause have both mechanisms at once so diabetics have to anticipate exercise
in starvation: glucose falls, insulin falls but no change in the amount of glucose transport to brain and RBCs because they have glucose receptors that are always on the cell membranes
48
insulin inhibition of gluconeogenesis
ATK phosphorylates FOX01
FOX01 normally stimulates the transcription of PEPCK
when phosporylated FOX01 can't go into the nucleus to increase the transcription of PEPCK
PEPCK is needed for gluconeogenesis
49
insulin stimulation of FA synthesis pathway
insulin turns on SREBP
| SREBP is a transcription factor for ACC and FAS which are involved in FA synthesis
50
autophagy
when fasting no insulin so skeletal muscle mass decreases
AKT activity results in decreased protein synthesis and increased autophagy = normal way of removing damaged cells but in extreme situation will canabilize healthy cells
51
glucagon and epi activation of gluconeogenesis
when levels are high, get more glycerol and AA acids - precursors for gluconeogenesis
PKA turns on set of pathways in gluconeogenesis that encourage gluconeogenesis over glycolysis
52
low insulin regulation of FA use
when PKA activated because insulin levels are low HSL and ATGL are turned on
droplets of lipid in adipose tissue are surrounded by perilipin
perilipin can now more readily bind to the HSL and ATGL and helps them position on the lipid droplet
53
malonyl-CoA regulation of FA cycle
in skeletal muscle
if enough malonyl-CoA CAT-1 is inhibited and FA can't get into the mitochondria
AMPK or PKA phosphroylates ACC
this decreases malonyl CoA levels and now the caratine system is not inhibited so the FA cycle can begin
in liver: phosphorylated ACC (alpha and beta) are inactivated
glucagon and epi both inactivate ACC
insulin activates ACC
54
effects of lack of insulin
get high hepatic gluconeogenesis => decreased skeletal muscle disposal, high blood glucose, once blood glucose is over 180 the kidneys won't retain all of it and there will be glucose in the urine and water follows it resulting in dehydration and weight loss
high lipolytic rate in adipose tissue, liver makes ketones from these but these are acids so blood pH will decrease, acetoacetate is broken down to acetone and makes the breath smell fruity
high lipolytic rate so loss of TG and less creation of new TG - get depletion in adipose mass
breakdown of protein so AA release, increases lipolytic rate, increase glycerol release, also lose volume of adipose tissue and muscle
55
PKA activation pathway
```
g-protein coupled receptor activated
activates G protein subunit
subunit activates adenylate cyclase
adenylate cyclase makes cAMP from ATP
cAMP activates PKA
```
cAMP phosphodiesterase (PDE) breaks down cAMP (PDE is activated by insulin)
56
sources for glycerol phosphate
glycerol phosphate is the initial acceptor of FA during TAG synthesis
in liver and adipose can be produced from glucose
using reactions of glycolytic pathway to make DHAP
DHAP is reduced by glycerol phosphate dehydrogenase to glycerol phosphate
in liver glycerol kinase can convert free glycerol to glycerol phosphate
57
synthesis of TAG
from glycerol phosphate and fatty acyl CoA
| 4 reactions that sequentially add 2 FA from FA coA, removes phosphate, adds third FA
58
Fatty Acyl CoA
activated form of fatty acid
| biosynthesized by fatty acyl CoA synthetases using FA, coenzyme A and ATP
59
fate of TAG in different tissues
in adipose: TAG stored in cytosol of cells in nearly anhydrous form
in liver: little TAG stored - most exported in VLDL into blood
in intestine mucosal cells: TAG major lipid cargo for chylomycrons
60
very low density lipoproteins (VLDLs)
have apolipoprotein B-100, cholesteryl esters, cholesterol, phospholipid, and TAG
how TAG is exported from liver to rest of body and transported through blood
made in liver
mostly triacylglycerol
job to carry this from liver to peripheral tissues where its degraded by lipoprotein lipase in same manner as chylomicrons
get apo C-II and apo E from HDL
like chylomicrons, decrease in size as circulate as triacylglycerol is degraded and C and E apoproteins are returned to HDL - retain apo B-100
eventually triacylglycerols transferred from VLDL to HDL in exchange for CE by cholesteryl ester transfer protein (CETP) - becomes LDL
61
glycerophospholipids
phospholipids that contain a glycerol
formed from phosphatidic acid and an alcohol
serine + PA = phosphatidylserine
ethanolamine + PA = phosphatidylethanolamine (cephalin)
choline + PA = phosphatidylcholine (lecithin)
inositol + PA = phosphatidylinositol
glycerol + PA = phosphatidylglycerol
62
cardiolipin
synthesized in mitochondria from phosphatidylglycerol
consists of two molecules of phosphatidic acid esterified through their phosphate groups to an additional molecule of glycerol
only antigenic glycerophospholipid (with syphillis)
important component of inner mitochondrial membrane and bacterial membranes
63
sphingophospholipids
have backbone of sphingosine rather than glycerol
synthesis:
1: long-chain FA attached to amino group of sphingosine through amide linkage = ceramide (can also be precursor for glycolipids)
2: alcohol group of carbon 1 of sphingosine esterified to phosphorylcholine = sphingomyelin
64
phospholipid synthesis
way 1: donation of phosphatidic acid from CDP-diacylglycerol to an alcohol
way 2: donation of the phosphomonoester of the alcohol from CDP-alcohol to 1,2-diacyglycerol
(CDP is the nucleotide cytidine diphosphate)
both ways make an activated intermediate and release CMP
both require activation of the diacylglycerol or alcohol to be added by a linkage with CDP
happens in the smooth ER and are then transported to the golgi where they're sorted and transported to cell membranes or secreted via exocytosis
65
synthesis of phosphatidic acid (PA)
precursor for other phosphoglycerides
1: start with glycerol phosphate and FA-acetyl CoA
2: acetyltransferase puts FA on glycerol phosphate and removes CoA - makes lysophosphatidic acid
3: acetyltransferase does this again - makes phosphatidic acid
66
synthesis of phosphatidylethanolamine (PE) and phosphatidylcholine (PC)
choline and ethanolamine obtained from diet or from turnover of body's phospholipids
PC can be made from PS and PE in the liver with enzyme phosphatidylethanolamineserine transferase - liver needs to be able to do this because it makes a lot of bile
steps in liver:
1: PS decarboxylated to phosphatdicylethanolamine (PE) by PS decarboxylase
2: three methylation steps to make PC - s-adenosylmethionine (SAM) is the methyl group donor
steps everywhere else and in liver when there's enough choline:
1: choline or ethanolamine phosphorylated by kinases
2: converted to activated form (CDP-choline or CDP-ethanolamine)
3: choline-phosphate or ethanolamine-phosphate is transferred from the nucleotide (CDP) to a molecule of diacylglycerol - leaves CMP
67
choline
must reutilize because although we can make it, we don't make enough of it so we have to acquire some from our diets
68
dipalmitoyl-phosphatidylcholine (DPPC) aka dipalmitoylecithin
major component of lung surfactant
made by type II pneumocytes
made same way as PC but positions 1 and 2 on the glycerol are occupied by palmitate
69
respiratory distress syndrome (RDS)
in preterm infants - because not enough surfactant production
determine lung maturity of fetus by finding ratio of DPPC to sphinomyelin in amniotic fluid (ratio of 2 or above is evidence of maturity)
can accelerate lung maturity by giving mother glucocorticoids shortly before delivery
70
synthesis of of PC from PS
in the liver
71
synthesis of phosphatidylserine
via base exchange reaction - ethanolamine in PE is exchanged for free serine
72
synthesis of phosphatidylinositol (PI)
from free inositol and CDP-diacylglycerol
| releases CMP
73
phosphatidylinositol (PI)
unusual because often has stearic acid on carbon 1 and arachidonic acid on carbon 2 of the glycerol
reservoir of arachidonic acid in membranes - therefore substrate for prostaglandin synthesis when required
also has role in:
1: signal transmission across membranes
2: membrane protein anchoring
(see other cards for details)
74
role of PI in signal transmission
when membrane-bound PI is phosphorylated get polyphophoinositids (such as PIP2)
when PIP2 is degraded by phospholipase C makes IP3 and DAG which are involved in mediating the mobilization of intercellular Ca and activation of protein kinase C
75
role of PI in membrane protein anchoring
some proteins can be covalently attached via a carbohydrate bridge to membrane-bound PI
allows these proteins rapid lateral mobility on surface of plasma membrane
phospholiase C can then cleave the protein from its anchor and release diacylglycerol
76
synthesis of sphingomyelin
1: palmitoyl CoA condenses with serine to make sphingosine
2: sphingosine is acylated at the amino group to make ceramide
3: phosphorylcholine from PC is transferred to ceramide = sphingomyelin and diacylglycerol
77
sphingomyelinase
lysosomal protein that degrades sphingomyelin
| hydrolytically removes the phosphorylcholine, leaving ceramide
78
degradation of phosphoglycerides
phospholipases hydrolyze the phosphodiester bonds
| done by phospholipase A1, A2, C (also a D that's only found in plants)
79
lysophosphoglyceride
when FA from carbon 1 or 2 is removed from a phsophoglyceride
substrate for lysophospholipases
80
phospholipase A1
in many mammalian tissues
| removes first FA group from phosphoglycerides in the process of degrading them
81
phospholipase A2
in many mammilan tissues and in pancreatic juice and in snake and bee venom
severs second FA group from phosphoglyceride in process of degrading them
when acts on PI releases arachidonic acid (precursor for prostaglandins)
lots of the proenzyme in pancreatic secretions - activated by trypsin and requires bile salts for activity
inhibited by glucocorticoids
82
phospholipase C
removes third FA group from phosphoglyceride during degradation
in liver lysosomes and is the alpha-toxin of some bacteria
when membrane bound is activated by PIP2 system and plays role in producing second messengers
83
fatty acyl CoA transferase
enzyme that replaces alternative FA on phosphoglycerols after they've been removed by phospholipases A1 or A2 - way of remodeling phospholipids
mechanism used to make surfactant and make sure that carbon 2 of PI is bound to arachidonic acid
84
degradation of sphinogyelin
1: sphingomyelinase hydrolytically removes phosphorylcholine = ceramide
2: ceramidase cleaves ceramide into sphinogsine and a free FA
these are intercellular messengers (ceramide involved in response to stress and sphingosine inhibits protein kinase C)
85
Niemann-Pick disease (Types A and B)
autosomal recessive disease caused by inability to degrade sphingomyelin
deficient in sphingomyelinase (type of phospholipase C)
in type A (sever cases) have enlarged liver and spleen because of lipid that can't be degraded
infants have rapid and progressive neurodegeneration and die in early childhood
type B doesn't affect neural tissue but affects lungs, spleen, liver and bone marrow and so life expectancy is still only into early adulthood
86
cholesterol esters (CEs)
most plasma cholesterol is esterified (has a FA attached at C3)
makes structure even more hydrophobic than free cholesterol
87
synthesis of cholesterol
in virtually all tissues but mostly in liver, intestine, adrenal cortex and reproductive tissues and brain
all carbon atoms provided by acetate
NADPH provides reducing equivalents
driven by hydrolysis of high-energy thioester bond of acetyl CoA and the terminal phosphate bond of ATP
in both cytoplasm and with enzymes on ER membrane
88
steps in synthesis of cholesterol
1: two acetyl CoA molecules condense to form acetoacetyl COA with thiolase as enzyme - releases one CoA
2: HMG-CoA synthase adds another molecule of acetyl CoA = HMG CoA
3: HMG CoA reductase (on ER membrane) reduces HMG CoA to mevalonic acid (rate-limiting step!) - in cytosol - uses 2 NADPH, releases CoA so irreversible
4: 2 steps to convert mevalonic acid to 5-pyrophosphomevalonic acid by adding two phosphate groups - uses 2 ATP - (they don't tell us the enzymes for most of the future steps)
5: this is decarboxylated to make a 5-carbon isopentenyl pyrophosphate (IPP)
6: IPP is isomerized to 3,3-dimethylallyl pyrophosphate (DPP)
7: IPP and DPP condense to form a 10-carbon geranyl pyrophosphate (GPP) - pyrophosphate released
8: IPP and GPP condense to make 15-carbon farnesyl pyrophosphate (FPP) - pyrophosphate released
9: squaline synthase combines two FPP molecules and reduces it to make squalene (30 carbons) - releases pyrophosphate
10: squaline monoxygenase uses NAPDH and O2 to lanosterol - hydroxylation triggers cyclization
11: multistep process where 3 carbons are removed double bonds removed - pathway not completely solved yet but results in 27 carbon cholesterol molecule
11:
89
smith-lemli-opitz syndrome (SLOS)
autosomal recessive disorder of cholsterol biosynthesis caused by partial deficiency in 7-dehydrocholesterol-7 reductase
90
HMG CoA reductase regulation
1: expression of the gene is controlled by SREBP2 that binds to DNA at SRE - SREBP2 is associated with ER membrane until cholesterol levels are low and proteolytic cleavage liberates it and it can move to the nucleus where it upregulates the production of HMG CoA reductase and other enzymes
2: cholesterol content affects the stability of HMG CoA reductase (more cholesterol destabilizes so more degradation)
3: phosphorylation inactivates - protein kinase is activated by AMP so cholesterol synthesis is decreased when ATP availability is decreased since protein kinase phosphorylates
4: increase insulin results in upregulation, glucagon reduces
5: statin drugs are structural analogs of HMG CoA and are therefore competitive inhibitors of HMG CoA reductase
91
degradation of cholesterol
ring structure can't be metabolized to CO2 and O2 in humans
intact sterol nucleus eliminated from body by conversion to bile acids and salts which are excreted
also used to make oxysterols and steroid hormones
92
7alpha-hydroxycholesterol
first intermediate in bile acid synthesis pathway that utilizes cholesterol as a substrate
93
24S-hydroxy cholesterol
most abundant oxysterol produced in brain
enzyme cholesterol 24-hydroxylase is microsomal enzyme in various regions of brain
b/c of BBB lipids can't get into brain so most of cholesterol needed must be made there
allows for cholesterol excretion from the brain
94
27-hydroxycholsterol
most abundant circulating oxysterol in blood of humans and mice
synthesized by cholesterol 27-hydroxylase - in many tissues, presumably in mitochondria
may be responsible for sterol-mediated degradation of HMG-CoA reductase
95
25-hydroxycholesterol
intermediate in bile acid biosynthesis pathway
| made by cholesterol 27-hydroxylase
96
synthesis of hormones from cholesterol
cytochrome 450P in inner mitochondrial membrane catalyzes conversion of cholesterol to pregnenolone via two successive intermediates:
1: 20alpha-hydrocycholesterol
2: 20alpha,22R-dihydroxycholesterol
uses NAPDH, O2 and cholesterol as substrates
pregnenolone is precursor for all other steroid hormones
97
how cells sense cholesterol level
CDL receptor levels are regulated by intercellular levels of free cholesterol
when free cholesterol levels fall, HMG CoA reductase is unregulated so can make more cholesterol
98
SREBP
sterol response element binding proteins
bind to DNA sequences involved in cholesterol metabolism (SREs)
also appear during adipocyte differentiation
there's three, but only two genes:
SREBP-1: one gene, two variants (different promoter), involved primarily in regulation of FA synthesis
SREBP-2: another gene, involved primarily in regulation of cholesterol metabolism
99
activation of SREBP-2
proteases in golgi where SREBP-2 is located clip it at 2 sites
the 6HLH-zip subunit can now go to the nucleus and bind to SRE
100
SCAP
SREBP cholesterol activating protein
when cholesterol levels are inadequate insigs (a cholesterol sensor) cleaves SCAP
SCAP can then transport SREBP to the golgi from the ER so that it can be activated
101
SREBP1
clipping promoted by insulin (so activated by insulin)
mediator of the effects of insulin but not of those of glucose
regulates genes for ATP cytrase lyase, acetyl Coa Carboxylase and FA synthase (FAS)
Acetyl CoA carboxylase is the rate limiting step in FA synthesis because regulates the amount of citrate being exported by the mitochondrion that can enter the FA synthesis process
when there's lots of glucose, lots of pyruvate is also made and the mitochondria has too much of the combustion products (created by the above enzymes) and so some of them (mainly citrate) are exported into the cytosol where they are converted into FA
102
SREBP-1c
if SREBP-1c is inhibited, there's impaired induction of FAS by glucose and insulin
103
glucose sensing and regulation of FA production
FAS has an SRE region and Eboxes
X-glucose-5-phosphate (product of glucose breakdown) dephosphroylates ChREBP
when dephos, ChREBP can get into the nucleus and bind to the Ebox, up regulating the production of FAS so that FA synthesis can occur
this happens in liver, pancreatic B cells and some cancer cells
104
miRNA-33
involved in regulation of cholesterol production
part of the SREBP-1 gene
needed for the stabilization of the mRNA for SREBP-1
105
ACAT
prevents cholesterol buildup in the metabolism - not controlled by SREBP but rather under allosteric control by cholesterol (its substrate)
106
low density lipoprotein (LDL)
major cholesterol carrier in the blood
recognizes and binds to the LDL receptor and then moves within the coated pits of the membrane where its concentrated
coated pits invaginate and form coated vessels
coated vessels fuse with lysosomes
internalized apo-B100 is degraded to AA and cholsteryl esters are hydrolyzed to free cholesterol and FA
cholesterol goes to plasma membrane or is reesterified in ER by ACAT
in nonhepatic cells these cholesterol esters form droplets for storage
in hepatic cells these esters become part of the neutral lipid core of VLDL
107
Niemann-Pick Type C disease
autosomal recessive
progressive neurological disease, hepatic enlargement
development normal in early childhood and then slowly dement
life expectancy to teen
happens because of mutation in NPC 1 and NPC2 genes - these code for proteins involved in the transport of cholesterol out of the endosomes and lysosomes
results in unesterified cholesterol, sphingomyelin, phospholipids and glycolipids, especially GM2 ganglioside, accumulating in the organs including spleen and liver
unlike in types A and B, accumulation of sphingomyelin believed to be secondary
108
fillipin
fluorescent dye that can be used to stain for cholesterol accumulation in cells (lab usage)
109
NPC1 gene
encodes a membrane bound protein with 13-16 transmembrane segments located in endosomes
mutated in niemann-pick disease type c
binds cholesterol with high affinity
110
NPC2 gene
soluble protein within endo/lysosomes
binds cholesterol with high affinity
mutated in niemann-pick type c disease
111
cholesterol sensing enzymes
in ER: HMGR, SCAP, ACAT-1
| in late endosomes: NPC-1
112
major cholesterol sources
1: derived from LDL
2: synthesized de novo in the ER
3: involved in cholesterol/cholesteryl ester (CE) cycle
113
Niemann-pick disease, types A and B
molecular lesion is at the lysosomal sphinogomyelinase resulting in the accumulation of sphingomyelin in the lysosome
114
gangliosides
carbohydrate-rich sphingolipids that contain acidic sugars
concentrated at the outer leaflets of plasma membranes
participate in signal transduction processes involving the cell surface
to make: oligosaccharide is linked to ceramide by a glucose residue (like formation of sphingomyelin) - uses UDP-sugar (glucose or galactose) - involves ordered, stepwise addition of sugars to ceramide - occurs in ER and golgi
115
degradation of glycosphingolipids
internalized by endocytosis
| degraded by lysosomal enzymes - follows "last on, first off" rule
116
sphingolipidoses
diseases caused by deficiencies in the storage and degradation process of sphingolipids
often seen in nerve tissue - causes neurologic deterioration
tay-sachs disease
117
tay-sachs disease
gangliosides in high concentration in gray matter
normally degraded in lysosomes
clinical presentation: weakness and retarded psychomotor development, demented and blind by age two, dead by age three
neurons become swollen with lipid-filled lysosomes
high concentration of ganglioside GM2 because its teerminal N-acetylgalactosamine residue is removed very slowly or not at all
enzyme b-N-acetylhexosaminidase is missing or deficient
autosomal recessive
118
reverse cholesterol transport
metabolic pathway by which excess cholesterol in peripheral tissues is transported to the liver for elimination from the body
cholesterol gathered by apoA-1
119
apolipoprotein A-1
made by liver
lipid-poor
circulates to peripheral cells and picks up cholesterol and phospholipids
matures into spherical particles that form bulk of HDL
120
LCAT (aka PCAT)
converts discoidal HDL to the spherical shaped HDL3
catalyzes the following:
lecithin + cholesterol => lysolecithin and cholesteryl esters
then adds more CE to HDL3 to make HDL2
121
HDL3
made by LCAT from HDL
| has more cholesteryl esters and is spherical instead of discoidal
122
SR-B1
HDL receptor that recognizes HDL2 and HDL3
in plasma membranse of liver cells and adrenal cortex cells
allows for catabolism and reutilization of CE
one of two ways that HDL delivers CE
123
cholesterol ester transfer protein
one of two ways that HDL delivers CE
catalyzes tranfer of CE from HDL2 and HDL3 to VLDL or LDL or IDL in exchange for PL or TG
The VLDL, LDL, or IDL is then recognized by the LDLR in the liver
124
tangier disease
immigrants from UK living on Tangier Island in chesapeake
large yellow-orange tonsils, neuropathies, splenomegaly, hepatomegaly, ocular abnormalities, hypocholesterolemia and CVD
due to accumulation of CE in reticuloendothelial cells of many tissues including tonsils, thymus, lymph nodes, bone marrow, spleen, liver, gall bladder, and intestinal mucosa
also get lipid deposits in neuronal schwann cells, SMCs and fibroblasts
due to mutations in ABCA1 gene which results in severe HDL deficiency
125
ABCA1
pump that transports cellular PL and cholesterol to apoA1
member of ABC transporter family
use ATP to transport substrates between different cellular compartments
mutation results in failure to lipidate apoplipoproteins, resulting in the rapid catabolism of the lipid-poor apoA-1 and no formation of HDL - results cholesterol accumulation in peripheral tissue and in tangier's disease
126
functions of bile acids
1: their synthesis is a disposal mechanism to counterbalance teh cholesterol synthesis pathway
2: stabilize oil-water interface so oils do not aggregate into large oil particles
3: detergent like actions essential in intestine for uptake of hydrophobic nutrients like fat-soluble vitamins
4: intermediates and end-products of synthesis regulate expression of genes that synthesize cholesterol, FA and bile acids
127
bile
watery mixture of organic and inorganic compounds
PC (lecithin) and bile salts (conjugated bile acids) most important organic components - inorganic = ions and carbonate solutions
128
bile acids
have 24 carbons with 2-3 hydroxyl groups and a side chain that terminates in a carboxyl group (with pka of about 6)
amphipathic so have a polar and nonpolar face and can act as emulsifying agents in intestine
129
synthesis of bile acids
in liver
hydroxyl groups are inserted at specific positions on the steroid structure
double bond of the cholesterol B ring is reduced
hydrocarbon chain is shortened by 3C
carboxyl group is introduced at end of the chain
results most commonly in cholic acid and chenodeoxycholic acid (primary bile acids)
rate-limiting step = introduction of hydroxyl group at carbon 7 of the steroid nucleus by cholesterol-7-alpha-hydroxylase
130
cholesterol-7-alpha-hydroxylase
rate-limiting step in bile acid synthesis
introduces hydroxyl group at carbon 7 of the steroid nucleus
ER associated cytochrome P450 enzyme
only in liver
down-regulated by cholic acid
unregulated by cholesterol
131
synthesis of bile salts
for bile acids to leave liver must first be conjugated with glycine or taurine by an amide bond
3:1 ratio of glycine use to taurine use
adds carboxyl group with lower pKa so that fully ionized (charged) at physiologic pH
makes them more effective detergents than bile acids (which are neutral at physiologic pH cause have a pKa of 6)
only significant mechanism for cholesterol excretion
132
action of intestinal flora on bile salts
can remove glycine and taurine from bile salts to regenerate bile acids
can convert some primary bile acids into secondary ones by removing the hydroxyl group
133
enterohepatic circulation
95 percent of secreted bile salts reabsorbed, mostly in ileum
transported from intestinal mucosal cells to the portal blood
take up by liver parenchymal cells
liver converts the acids back to salts the resecretes them into bile
continuous process of secretion, reuptake and resecretion = enterohepatic circulation
134
cholestyramin
positively-charged insoluble resin that acts as a bile acid sequestrant
binds bile acids in the gut to prevent reabsorption and promote secretion
used in treatment of high cholesterol
dietary fiber also does this
135
cholelithiasis
bile salt deficiency
makes gallstones because more cholesterol enters the bile than can be solubilized by the bile salts and lecithin present
results in precipitation of the cholesterol, which crystalizes in gallbladder
136
lingual lipase
```
made from glands at back of tongue
primarily targets TG molecules
first step in digestion of FA
in stomach
acid-stable
important in patients without pancreatic lipase (such as cystic fibrosis patients)
```
137
gastic lipase
contributes to degradation of FA in stomach
secreted by gastic mucosa
acid-stable
important in patients without pancreatic lipase (such as cystic fibrosis patients)
138
emulsification of dietary lipid
in duodenum
increases surface area of hydrophobic lipid droplets so that the digestive enzymes can act effectively
accomplished by used of detergent properties of biles salts and mechanical mixing due to peristalsis
139
pancreatic lipase
degrades triacylglycerol
removes FA at carbons 1 and 3, creating mixture of 2-monoacylglycerol and free FA
anchored to lipidaqueous interface by enzyme colipase
140
orlistat
anti-obesity drug
| inhibits gastric and pancreatic lipases, decreasing fat absorption, resulting in loss of weight
141
cholesterol ester hydrolase (cholesterol esterase)
hydrolyzes CE
made by pancreas
makes cholesterol plus free FA
activity increased in presence of bile salts
142
phosopholipase A2
proenzyme in pancreatic juice
activated by trypsin
requires bile salts for optimum activity
removes one FA from carbon 2 of a phosopholipid, leaving lysophospholipid
then phospholipid further broken down by lysophospholipase
143
lysophospholipase
enzyme responsible for second step of phospholipid degradation in the intestine
removes fatty acid at carbon 1 from lysophosphatidylcholine resulting in glycerylphosphoryl base that is excreted, further degraded or absorbed
144
mixed micelles
formed by bile salts, free FA, free cholesterol and 2-monoacylglycerol in jejunum
soluble in aqueous environment because form with hydrophobic parts on the inside
hydrophilic surface facilitates transport of hydrophobic parts through brush boarder membrane where they're absorbed
145
resynthesis of triacylglycerol and cholesteryl esters
absorbed lipids go to ER
FA: converted to activated form by fatty acyl CoA synthetase
2-monoacylglycerol: fatty acyl coA derivates are used to convert the 2-monoacylglycerols to triacyglycerols by enzyme triacylglycerol synthase (has two enzymes - monoacylglycerolacyltransferase and diacylglycerolacyltransferase)
lysophospholipids: reacylated by acyltransferases to make phospholipids
cholesterol: esterified by acyl CoA:cholesterolacyltranferase to a FA
146
lipid malabsorption
results in increased lipid and fat-soluble vitamins in the feces
caused by disturbances in digestion or absorption
can result in cystic fibrosis and shortened bowel
147
chylomicrons
used to secrete lipids from enterocytes
TGs and CEs hydrophobic and so aggregate
packaged as droplets surrounded by thin layer of phospholipids, unesterified cholesterol, and apolipoprotein B-48 - layer stabilizes particle and increases its solubility
released into lacteals to lymphatic ducts to left vein to enter blood
largest in size and lowest in density of lipoproteins - highest percentage of lipid and smallest percentage of protein
148
plasma lipoproteins
spherical macromolecular complexes of lipids and specific proteins
include chylomicrons, VLDL, LDL, HDL
have neutral lipid core (of triacylglycerol, CE) surrounded by shell of amphipathic apolipoproteins, phospholipid and nonesterified cholesterol
149
apoplipoproteins
provide recognition sites for cell-surface receptors
activators or coenzymes for enzymes involved in lipoprotein metabolism
some required as structural components of particles
divided by structure and function into 5 classes, A-E and then subclasses
150
apoplipoprotein B
2 forms: apo B-48 and apo B-100
starts in RER
glycosylated in ER and golgi
48 has 48 percent of apo B gene - in chylomicrons - made in intestinal cells - post-transcriptional editing of a cytosine to a uracil makes stop codon in mRNA
100 has entire gene - made in liver and found in VLDL and LDL
151
microsomal triacylglycerol transfer protein (MTP/MTTP)
loads apo B-48 with lipid during transition from the ER to the Golgi where the particles are packed into secretory vesicles
152
nascent chylomicron
particle released by intestinal mucosal cells is functially incompete
once in plasma, recieves apo E and c apolipoproteins (including apo C-II which is necessary for activation of lipoprotein lipase that degrades triacyglycerol in the chylomicron)
all of these apolipoproteins come from HDL
153
lipoprotein lipase
extracellular enzyme anchored by heparin sulfate to the capillary walls of most tissues (but mostly adipose, cardiac and skeletal -none in adult liver!)
activated by apo C-II on circulating lipoprotein particles
hydrolyzes the triacylglycerol in these particles to make FA and glycerol
FA stored by adipose or used for energy by muscle
glycerol used by liver
synthesis and transfer to the membrane is stimulated by insulin
154
chylomicron remnants
as chylomicron circulates, >90% of triacylglycerol degraded by lipoprotein lipase and c apoproteins returned to HDL
now called remnant
liver cells have chylomicron remnant receptors for apo E and take up remnants
endocytosed and fused with lysosomes - contents hydrolytically degraded into AA, free cholesterol and FA
155
fatty liver
when there is an imbalance between triacylglycerol synthesis and secretion of VLDL
results from conditions such as obesity, diabetes mellitus, and chronic ethanol ingestion
156
abetalipoproteinemia
rare hypolipoporoteinmia caused by defect in triacylglycerol transfer protein (MTP) resulting in inability to load apo B with lipid and so no chylomicrons or VLDL are formed and triacylglycerols accumulate in the liver and intestine
157
cholesteryl ester transfer protein (CETP)
exchanges remaining triacylglycerols in VLDL for CE in HDL
| results in conversion of VLDL to LDL (intermediate sized protein IDL is present during conversion)
158
apo E
three different alleles - e2, e3 and e4
most common is e3
apo e2 binds poorly to chylomicron receptors and so patients homozygous for e2 are deficient in clearance of chylomicron remnants and IDLs = familial type III hyperlipoproteinemia - lower HDL levels and hypercholesterolemia and atherosclerosis too
e4 confers increased susceptibility to late-onset alzheimer's
159
regulation of endocytosed cholesterol on cellular cholesterol homeostasis
1: HMG CoA reductase is inhibited by high cholesterol so de novo cholesterol synthesis decreases
2: synthesis of new LDL receptor protein reduced by decreasing expression of LDL receptor gene - limits further entry of LDL cholesterol into cells
3: if cholesterol not required immediately for some structural or synthetic purpose, it's esterified by ACAT - ACAT activity enhanced by intracellular cholesterol
160
statins
inhibit HMG CoA reductase
depletes endogenously synthesized cholesterol in liver and other cell bodies
results in fall in cellular cholesterol level - sends signal to activate SREBP pathway
results in production of more mature form of SREBP which activates genes in cholesterol biosynthetic pathway including HMG CoA reductase and HMG CoA synthetase and gene that encodes LDL receptor - results in reduced blood LDL level
