biochem test 4 Flashcards
(146 cards)
1
Q
electron transport chain
A
- occurs in mitochondria
- a proton gradient is established across the mitochondrial inner membrane
2
Q
chemiosmosis
A
- the movement of protons down the concentration gradient
- grom high [H+] to low [H+]
- ATP is produced
3
Q
proton flow
A
- proton gradient is initiated by outward pumping of H+ from the mitochondrial matrix by three large protein complexes
- the inward flow of H+ through the membrane-bound ATP synthase protein accomplishes ATP synthesis
- complex I and IV pump protons (complex III does not actively pump protons)
4
Q
How is the harvested energy stored and used?
A
the energy released from slow combustion of glucose is stored as potential chemical energy in the form of a proton gradient across the inner membrane
5
Q
electron transport system (chain)
A
- describes combined redox reactions that occur sequentially in a set of protein complexes embedded in the inner mitochondrial membrane
- NADH is oxidized to form NAD+
- O2 is reduced to H2)
6
Q
proton circuit
A
- can be uncoupled so ATP synthesis no longer occurs
- uncoupling causes proton “leakage” and production of heat
- protons leak back into matrix without generating ATP
7
Q
Complex I protein
A
NADH-ubiquinone oxidoreductase
8
Q
Complex II protein
A
succinate dehydrogenase
9
Q
Complex III protein
A
Ubiquinone-cytochrome c oxidoreductase
10
Q
Complex IV protein
A
cytochrome c oxidase
11
Q
Complex V protein
A
ATP synthase complex
12
Q
mitochondrial electron transport system-1
A
- NADH oxidation occurs in complex I
- takes place on the matrix side of the inner mitochondrial membrane
- two electrons initiate multiple redox reactions (ends with oxygen being reduced to water_
- two electrons enter the electron transport system through FADH2 oxidation
- the flow of electrons is facilitated by the sequential arrangement of electron carriers
13
Q
redox loops
A
- separation of protons and electrons on opposite sides of the membrane
- Q cycle (complex III)
14
Q
proton pumps
A
- dependent on protein complex conformational changes
- complexes I and IV
15
Q
complex I
A
- Protein: NADH-ubiquinone oxidoreductase
- NADH is oxidized while coenzyme Q is reduced
- largest complex
- covalently bound to flavin mononucleotide
- FMN accepts 2 electrons from NADH
16
Q
FMN reactions
A
- may be reduced one electron at a time to form semiquinone and reduced flavin mononucleotide
(can accept electrons0
17
Q
Coenzyme Q Function
A
- acts as a mobile electron carrier and transports electrons from complex I to complex III and from complex II to III
- entry point for 2 electrons from the citrate cycle, fatty acid oxidation, and glycerol-3-phosphate dehydrogenase
- converts 2 electrson transport system (complexes I and II) to 1 electron to cytochrome c (complex III)
18
Q
coenzyme Q reduction
A
- can carry two electrons
- semiquinone intermediate
19
Q
Complex II
A
- Protein: succinate dehydrogenase
- catalyzes oxidation-reducation of succinate to fumerate
- coupled redox reaction using FAD
- reduces coenzyme Q to QH2
20
Q
complex III
A
- Protein: ubiquinone-cytochrom c oxidoreductase
- reduces cytochrome c, while translocating 4H+
- docking site for QH2 and cytochrome c
- contains binding sites for ubiquinone
- transfers electrons through an iron-sulfur cluster center
21
Q
Q cycle
A
- converts 2 electron transport process into two 1 electron transfers
- electrons come from coenzyme Q
- occurs in complex III
- cytochrome c is reduced in the process
- cytochrome c transports 1 electron from complex III to complex IV
22
Q
complex IV
A
- protein: cytochrome c oxidase
- accepts 4 electrons total, one at a time
- cytochrome c is oxidized, while oxygen is reduced to water
- 2 H+ are translocated across the membrane
- 4 electrons reduce 1 molecule of O2 to 2 molecules of H2O
- each cytochrome c delivers one electron
- 4 protons consumed come from mitochondrial matrix
23
Q
risk of using oxygen as terminal electron acceptor
A
- oxygen is a very strong oxidizing agent and if electrons are allowed to “leak” out of the ETC to react prematurely with oxygen, it can form reactive oxygen species including superoxide and peroxide
- we are protected from exposure to small amounts of ROS by the 2 scavenger enzymes
24
Q
ATP synthase
A
- enzyme that couples proton movement down a concentration gradient to mechanical work
25
proton-motive force
the combined chemical and charge gradient inherent in the proton gradient
26
F0
"stick" subunit in membrane, proton-conducting
- acts as proton channel crossing the inner mitochondrial membrane
27
F1
"ball" subunit in matrix, ATPase activity
- encodes catalytic activity
28
rotor
| subunits
γ, δ, ε subunits
29
headpiece
catalytic piece α3β3 unit
30
strator
stabilizing arm (immobile)
31
open
product release and substrate binding
32
loose
ADP + Pi trapping
33
tight
catalysis
34
γ subunit controls...
ATP release
35
conformational changes in ATP synthase
- protons flow through F0 component
- F1 subunit undergoes a conformational change from L, T, and O
- rotor rotates 120 degrees
- ADP and Pi bind to form ATP
- ATP is released only in the open position
- the dissipation of the proton gradient is what releases ATP
36
how does proton flow drive rotation
- a subunit is believes to have 2 hydrophilic "half channels"
- a half channels do not span the membrane
- each half channel lines up directly with a c subunit
- c subunits contain ASP residue near the middle
- protonation of ASP allows c-ring to rotate, releasing H+ when it gets to the proton-poor environment of a's other half-channel
- each proton enters the half-channel from the intermembrane space, follows a complete rotation of the rins, and exits through the other half channel into the matrix
- 10 c subunits are in the ring, so 10 protons drive 1 revolution
- c ring is tightly bound to and turns the γ and ε subunit, resulting in the generation of 3 ATP/revolution
37
transport systems in mitochondria
- biomolecules requires for the electron transport system and oxidative phosphorylation must be shuttles back and forth across the membrane
- 2 shuttles: malate-asparate shuttle (liver), glycerol-3-phosphate (muscle)
- transfre 2 electrons from cytosolic NADH to carrier molecules
38
Glycerol-3-phosphate shuttle
- important in muscle
- delivers electrons from NADH to mitochondrial matrix using FAD
- electrons from NADH go into electron transport system through conenzyme Q
- consists of 2 isozymes of glycerol-3-phosphate dehydrogenase
- the inner mitochondrial membrane is impermeadble to NADH from glycolysis
- instead, electrons from NADH travel to the mitochondria to be used in the ETC and oxidative phosphorylation
39
malate-aspartate shuttle
- primary shuttle in liver and heart cells
- all reactions are reversible
- supply of NAD+ is maintained
- 2 membrane carriers
- 4 enzymes
- electrons are carried into matrix on carrier molecules
40
ATP/ADP transport
- import of ADP and Pi into mitochondrial matrix is accomplished by two translocase proteins located in the inner mitochondrial matrix
- ATP/ADP translocase exports one ATP for every ADP imported
- Phosphate translocase translocates one Pi and one H+ into the matrix
- antiporter membrane trasport protein
41
phosphate translocase
- similar to a channel
- can be symporter or antiporter
- ssymporter= H2PO4- and H+ in
- antiporter= H2PO4 in, OH- out
- electrically neutral translocation
42
regulation of oxidative phosphorylation
- ADP and ATP control aerobic respiration
- the ratior of NADH/NAD+ in the mitochondrial matrix controls multiple steps in the citrate cycle
43
regulation of the ETC
- governed by the need for ATP (= regulation of the ETC is governed by high ADP)
- electrons do not flow down the ETC unless ATP is needed (=ADP present)
- Adding ADP to isolated mitochondria increases oxygen consumption
44
chemical uncoupling
- oxygen consumption is affected
- cyanide inhibits complex 4-> oxygen levels off (= not converting oxygen to water-> not using oxygen)-> no ATP synthesis
- DNP uncouples-> proton diffusion (no proton gradient)-> nothing to uncouple because complex 4 is not functioning
- oligomycin inhibits ATP syntahse-> stop synthesis of STP, not consuming oxygen-> proton gradient not channeling
- DNP= uncoupler-> uncoupled transport of protons and ATP synthesis-> protons flow back into matrix-> ETC can keep goinf because complex 4 is still functionins
45
uncoupling facilitates...
thermogenesis (since ATP production is reduced, heat is generated)
46
brown and white adipose tissue
- differ in size of lipid droplets inside the cell and the number of mitochondria
- brown adipose tissue has high levels of UCP1
- brown: smaller fat droplets leave more space for mitochondria
- white: large fat droplet fills the cytosolic space in each cell
47
- NADH is produced in...
- its primary purpose is...
- therefore it is mainly used in______ processes
- glycolysis and CAC
- energy generation
- catabolic (glucose broken down to ATP)
48
- NADPH is produced in...
- its primary purpose is...
- therfore, it is mainly used in______ processes
- photosynthesis/PPP
- biosynthesis
- anabolic (more simple molecules-> more complex)
49
pentose phosphate pathway
crucial source of NADPH for use in reductive biosynthesis and for protection from oxidative stress
- occurs in cytoplasm in most organisms
50
two main functions for NADPH
1. reduction potential in biosynthetic reactions (fatty acid biosynthesis, cholecterol biosynthesis, nucleotide biosynthesis)
2. protection of the cell from oxidative damage (reduction of glutathione)
51
oxidative phase
- NADPH is generated when glucose-6-phosphate is oxidaized to ribulose 5-phosphate
- responsible for generation of NADPH
- payoff upfront
52
non-oxidative phase
- catalyzes interconversion of 3,4,5,6, and 7 carbon sugars in nonoxidative reactions
- controlled primarily by the availability of substrates
- shuffles carbons around into different sugars depending on cellular needs
- can feed precursors into the synthesis of RNA, DNA, ATP, NAD(P)H, FADH, FMN, Coenzyme A
53
NADP+
rate-limiting substrate
54
what is required for PPP
glucose-6-phosphate and NADP+
55
steps of oxidative phase
1. glucose-6-phosphate is oxidized to 6-Phosphoglucono-δ-lactone, producing 1 NADPH
2. lactonase promotes hydrolysis: water hydrolyzes the lactone, opening the ring and yielding 6-phosphogluconate
3. 6-phosphogluconate then undergoes oxidative decarboxylation to yield Ribulose-5-phosphate (6 C to 5C) and 1 NADPH
56
non-oxidative phase
- ribulose-5-phosphate can be isomerized into ribose-5-phosphate for nucleotide synthesis or alternatively epimerized into Xylulose-5-phosphate
- multi carbon units are shuffled arounf to produce biosynthetic precursos or glycolytic intermediates
- rate os non-oxidative phase is controlled primarily by substrate availability
57
transaldolase
transfers 3 carbond instead of 2 carbonds
- makes even-numberes carbon sugars from odd-numbered sugars
58
transketolase
transfers 2 carbons
59
second transketolase-catalyzed step
occurs to make fructose-6-phosphate and glyceraldehyde-3-phosphate from C4 and C5 sugars
60
transketolase #1
transketolase transfers a 2 carbon unit from a 5 carbon ketose to a 5 carbon aldose yielding a 3 carbon aldose and a 7 carbon ketose
61
transaldolase
| reaction
takes the 3 carbon and 7 carbon products from transketolase 1 and performs a 3 carbon transfer to make 6 carbon and 4 carbon products
62
transketolase #2
transfers a 2 carbon unit from a 5 carbon ketose to a 4 carbon aldose yielding a 3 carbon aldose and a 6 carbon ketose
63
why is it called a shunt
stop off to pick up some NADPH then head back to glycolysis
64
mode 1
need more ribose than NADPH
65
mode 2
need for ribose and NADPH equal
66
mode 3
need more NADPH than ribose
67
mode 4
need NADPH (amino acids) and ATP
68
NADPH protects against oxidative damage by...
reducing glutatione
- an important cellular antioxidant, glutathione is a tripeptide
- one glutathione can be oxidized with another to form a disulfide bond
69
superoxide
can be produced by oxidative phosphorylation
- by regenerating reduced glutatione, NADPH can help protect against oxidative damage by superoxide, peroxide and the hydroxyl radical
- reducing qeuivalents in the form of NADPH are produced by glucose-6-phosphate dehydrogenase
70
deficiency of glucose-6-phosphate dehydrogenase
- G-6-P dehydrogenase catalyzes the first step in oxidative branch of PPP
- drug indiced hemolytic anemia
- results in hemoglobin aggregation in red blood cells (Heinz bodies)
- confers evolutionary advatage in some circumstances (protects against falciparum malaria--> parasite needs NADPH for growth; infection by parasite causes oxidative stress in the infected cell, PPP compromized both cell and parasite die from oxidative damage)
- defect is inherited on the X chromosome
- Death of NADPH in all cells
- results in 10 fold reduction in enzymatic activity in RBC
- inthe absence of oxidative stress this deficiency is benign-> demonstrates that atypical reaction to drugs may have a genetic basis
71
symptoms of G6PD deficiency
- pale skin, especially around the mouth
- jaundice, or yellowing of the skin, eyes, and mouth
- dark-colored urine
- fever
- weakness and extreme tiredness
- dizziness
- confucion
- trouble with physical activity
- abdominal and back pain
- shortness of breath or fast breathing
72
how to store excess energy sources
- cells would love to store as much glucose as possible for energy generation when needed
- a common mechanism for cells to store excess metabolites is to polymerize them into large macromolecular structures
- glucose is polymerized into large, branched macromolecules that can be seen in cells as large glycogen molecules
73
how do cells liberate glucose from glycogen
1) release of glucose-1-phosphate
2) remodeling to permit further degradation
3) conversion of glucose-1-phosphate to glucose-6-phosphate
74
non-reducing end
end with free OH group on carbon 4
75
glycogen breakdown
- uses phosphate, not water
- phosphorolysis of glycogen is catalyzed by glycogen phosphorylase (catalyzes removal of glycosyl residues from non-reducing end of glycogen)
- retains configuration at anomeric carbon
76
phosphorolysis is beneficial because...
- it yields glucose-1-phosphate (no energy cost to feed it into glycolysis, phosphorylated glucose stays in the cell)
- to ensure glucose-1-phosphate is the product, glycogen phosphorylase must prevent hydrolysis of glycogen
**uses PLP**
77
glycogen phosphorylase
can break alpha 1,4-glycosidic bonds but stops when it reaches a terminal 4 residues away from branch point
78
transferase
- shifts a block of 3 glycosyl residues from one outer branch to another
79
alpha 1,6-glucosidase
"debranching enzyme"
80
final step
isomerize to glucose-6-phosphate
- phosphoglucomutase exchanges phosphates between a serine residue within its own structure and glucose-1-phosphate
81
phosphorylase a
- liver
- binding of 2 glucose shifts states
- R state favored (active)
- sufficient glucose shifts to T state
- glucose is to be generated unless enzyme is signalling otherwise
- not sensitive to AMP because liver does not undergo dramatic changes in energy
82
phosphorylase b
- muscle
- T state favored
- activated by high AMP (binds to nucleotide binding site and stabilizes the R state confirmation)
- ATP and G6P act as negative allosteric effectors
- active primarily during muscle contraction
83
glycogen phosphorylase
allosteric enzyme that can be further modulated by phosphorylation
84
liver vs muscle cells
- standard states are opposite in twwo cell types
- leads to different resting activities and different direction of regulation
- muscle standard state= T
- liver standard state= R
- Phosphorylase b tends toward T state-> T state can be active but it needs to bind to AMP, which happens when AMP is high. High AMP is a sign that muscle cells have low ATP and needs to catabolize glucose. So in the T state, muscle glycogen phosphorylase is responsive to the energy needs of the cell)
85
Phosphorylation converts phosphorylase b into phosphorylase a
- in the liver, most glycogen phosphorylase is in the phosphorylated a forom (active)
- phosphorylase a is insenisitive to energy charge (ATP and AMP have little effect)
- respond to the presence of elevated glucose by switching to the T state
- glucose is negative allosteric inhibitor
86
how is phosphorylase b phosphorylated
epinephrine (muscle) or glucagon (liver) initiates cyclic AMP signal-transduction cascade
87
phosphorylase kinase
converts phosphorylase b into a form by attaching phosphoryl group
- activation initiated when Ca binds to δ subunit
88
glycogen breakdown
- allosteric regulation of phosphorylase by AMP, ATP, glucose-6-P, glucose
- phosphorylation of glycogen phosphorylase by effectors of cAMP cascade
- hormones and signal transduction pathways that control this
89
glycogen synthesis
- works on outer shell of granule
- glycogen is synthesized when have lots of energy
- follows seperate pathway from degradation even though it involves nearly the exact reverse reactions
- readily reversible (uses UTP)
90
glycogen synthase
- elongates glycogen polymers
- can add glycosyl residues only to polysaccharide chain already containing at least 4 residues
- catalyzes only 1,4 linkages
- new glycosyl units are added to nonreducing terminal residues of glycogen
91
branching and rate os synthesis/degradation
- branching increases the rate of both glycogen synthesis and degradation because more reactions can happen at the same time
92
glycosyltransferase in synthesis
can carry the transferred oligosaccharide to its new position before the second nucleophilic attack
93
initiation of glycogen synthesis
- gycogen synthase can onlly work on existing polymers
- requires priming with small oligosaccharides
- accomplished by a dimeric glycosyltransferase called glycogenin
- when the dimer comes together, there is cross-glycosylation to build up to an 8-sugar primer on the opposing monomer
- glycogen synthase then takes over
94
glycogen is an efficient glucose storage molecule
- cost of 1 ATP to regenerate UTP for the activation of glucose
- rest does not require energy input
- about 90% of glucose residues are phosphorylysed out of glycogen leading to no energy cost to g\feed into glycolysis
95
glycogen synthase allosteric and phosphoregulation
- two phosphorylation states, each with capability of allosteric regulation
- non-phosphorylated form is more active (synthase a)
- G6P is an allosteric activator-> switches from T to R state
- good activator because too much glucose=don't need it-> store as glycogen
96
how does PP1 work (degradation)
inactivates phosphorylase a and phosphorylase kinase by dephosphorylating them
97
how does PP1 stimulate glycogen synthesis
- when BG is high, insulin stimulates synthesis of glycogen by inactivating glycogen synthase kinase
98
what is the whole body stimulus that triggers insulin release
glucose in blood
99
what type of signaling cascade does insulin initiate
phosphorylation signaling cascade
100
fatty acids have multiple roles in the body
1. fuel molecules for energy storage (breakdown to yield energy, synthesis to store energy)
2. building blocks for phospholipids and glycolipids
3. anchors for lipoproteins (one way to target proteins to a membrane location)
4. derivatives serve as intracellular messengers and ligands
101
fatty acids
concentrated stores of energy
- breakdown os fatty acids tields about twice the energy tha breakdown of glucose yields
- fatty acids are ideal storage molecules that the body is more than happy to synthesize when we consume excess calories
- fatty acid breakdown is done to generate ATP and reducing equivalents
102
general steps of fatty acid degradation
- fatty acids are first activated by esterification to a tiol-containing group
- the molecule is oxidized to form a double bond between carbond 2 and 3
- the double bond is then hydrated to yield an alcohol at the beta carbon
- a second oxidation occurs to form a ketone from the hydroxyl group
- the bond between carbons 2 and 3 is cleaved yielding activated acetate and a fatty acid two carbons shorter
103
digestion of dietary fat
- lipds are mainly ingested as triacylglycerols (must be degraded to fatty acids for absorption across intestinal epithelium)-> not water soluble
- TAGs are incorporated into mixed micelles with bile salts so they can be accessed by degradative enzymes
- pancreatic TAG lipases catalyze hydrolysis of 2 fatty acid-glycerol ester linkages, but not the one at C-2 of the glycerol
1. bile slats emulsify dietary fats in the small intestine, forming mixed micelles
2. intestinal lipases degrade triacylglycerols
3. fatty acids and other breakdown products are taken up by the intestinal mucosa and converted into TAGs
104
digestion
- pancreatic lipase works at the lipid-water interface
- it requires pancreatic co-lipase in a 1:1 ratio
- the lipase active site binds TAG better when the enzyme is in contact with a micelle containing phosphatidylcholine and bile salts
- bile salts are made in liver, but stored in gall bladder
105
fatty acid transport
- TAG cannot be transported directly across the mucosal cell membrane, so they are digested by lipases first
- in intestinal mucosal cells, the TAGs are resynthesized and packaged into particles--> VLDLs abd chylomicrons
- chylomicrons are released into lymphatic system and eventually reach the bloodstream to be transported throughout the body
106
chylomicron
- a lipoprotein for blood transport of triglycerides, cholesterol, and fat-soluble vitamins that originate from the diet
- largest and least dense of the lipoproteins
- core of TAG and cholesterol esters
- surface of amphipathic molecules like cholesterol, phospholipids, and apoliproteins
- once they reach their destination, TAG are once again degraded to fatty acids and monoacylglycerides
- then they are teken up into the destination cells and resynthesized
107
fatty acids are fuel for muscle cells
- chylomicrons serve as the vehicle for transport of fatty acids to adipose tissue and skeletal muscle. In capillaries near these tissues, the apolipoprotein CII on the chylomicron activates lipoprotein lipase
- the enzyme catalyzes hydrolysis of TAG to fatty acids and monoacylglycerol
- fatty acids enter cells for use as fuel (muscle) or stored (adipose tissue)
108
storage in adipose tissue
- adipose tissue is the storage place for triacylglycerols
- fatty acids freed by lipoprotein lipase from triacylglycerol in chylomicrons enter cells
- triacylglycerols are resynthesized
- in adipose cell cytoplasm, triacylglycerols coalesce into fat droplets that can grow to occupy nearly the entire cell
- surrounded in phospholipid monolayer containing proteins for triacylglycerol mobilization
109
triacylglycerides are stored...
in lipid storage droplets
110
mobilization of fatty acids from storage
- occurs at the result of hormone signaling
- stimulated by glucagon and epinephrine
- cAMP stimulates PKA to phosphorylate Perilipin and hormone-sensitive lipase
- pose transport problem because of their poor solubility in water
- free fatty acids carried through blood by albumin proteins
- glycerol formed in this way travels to the liver where it is converted to glyceraldehyde-3-phosphate which can then participate in wither glycolysis or gluconeogenesis both of which occur in the liver
111
perilipin
restructures the fat droplet making FAs more accessible and releases a coactivator for the first lipase
112
HS lipase
performs the second (DAG) lipase activity
113
Monoacylglycerol (MAG) lipase
performs the final hydrolysis to yield 3 fatty acids and glycerol from a triacylglycerol
114
chanarin-dorfman syndrome
- the coactivator required by ATGL is missing or defective
- fats accumulate throughout the body because they cannot be released by ATGL
- other symptoms: dry skin, enlarged liver, muscle wekness, mild cognitive disability
115
FA activation and transport to the mitochondria
- once arrive at destination tissue, fatty acids are activated by thioesterification before being transported into the mitochondria for oxidation
- 2 step reaction where fatty acid is first converted into a mixed anhydride with the alpha phosphate of ATP , then FA is transferred to the thiol group of CoA
- reaction is driven toward products because phosphate is rapidly hydrolyzed by a phosphatase
- Long-chain FA are activated on the outer mitochondrial membrane but shuttles to the matrix in the form of acylcarnitines
- the fatty acid is transeterified from CoA to the hydroxyl group of carnitine
- once inside the matrix, fatty acyl-CoA is re-formed to undergo beta oxidation
116
beta oxidation- first oxidation
- first step: removal of 2 hydrogen atoms, creating a double bond between the alpha and beta carbond
- because FADHs is so strongly bound to acyl-CoA dehydrogenase, these electrons take an interesting path to enter the electron transport chain
117
beta oxidation- hydration
- next reaction is hydration across the newly formed alpha-beta double bond by enoyl-CoA hydratase
- the hydration results in the placement of a hydroxyl group on the beta-carbon
- the alcohol is now set up to be oxuduzed to a ketone in the next step
118
beta oxidation- second oxidation
- the newly formed hydroxyl group is oxidized to a ketone by hydroxyacyl CoA dehydrogenase
- this time NAD+ is the electron acceptorl new NADH can feed into the ETC at complex I
119
beta oxidation- thiolysis
- final step is cleavage os a carbon-carbon bond
- accomplished through thiolysis by thiolase
- yields a shorter fatty acid and acetyl CoA that can enter the krebs sycle
120
beta oxidation produces ____ ATP
106beta oxidation of unsaturated and odd numbered fatty acids
121
beta oxidation of unsaturated and odd numbered fatty acids
fatty acids with odd number of carbons are oxidized the same way as even except that propionyl CoA and acetyl CoA are final products
122
Acetyl CoA makes_____ when nutrients are scarce
- ketone bodies
- Acetyl CoA can only enter the citric acid cycle is oxaloacetate is present
- after 3 days of starvation, carbs are scarce, and acetyl CoA accumulates because it cannot enter TCA
- brain also starts to run out of glucose
- under these conditions, acetyl CoA is converted to ketone bodies which the heart, brain, and muscle can use as fuel under starvation conditions
123
fatty acid degradation vs synthesis
(where does it take place, intermediates, activities, units, electrons)
degradation
- takes place in mitochondrial matrix
- intermediates linked to CoA
- activities spread across multiple enzymes
- fatty acid broken down into 2 carbon units: acetyl CoA
- electrons stripped from fatty acid donated to NAD+ and FAD
Sythesis
- takes place in cytoplasm
- intermediates linked to Acyl Carrier Protein
- Many activities associated together in the same polypeptide chain-- Fatty Acid synthase
- 2 carbon acetyl units added from carrier molecule malonyl-CoA
- electrons donated to growing fatty acid from NADPH (anabolic pathway)
124
for both degradation and synthesis:
- further modifications carried out by additional enzymes
- 2 carbon subtractions and additions act at thioester end of fatty acid
125
malonate is...
a symmetric molecule with a carboxylic acid group on each end
- fatty acid synthesis starts with carboxylation os acetyl CoA to malonyl CoA
- thus, it is different from the 3-carbon product of the degradation of odd-numbered fatty acids, which was Propionyl CoA
- only a 2 carbon unit is eventually added to the growing fatty acid chain
126
Acetyl-CoA carboxylase
the committed step for fatty acid synthesis
- irreversible
- the bicarbonate molecule plays the rols of water in "hydrolyzing" ATP, ultimately leaving one of its oxygen atoms in the inorganic phosphate molecule so that only CO2 is transferred to biotin
- two active sites (biotin is on an arm that swings first to the site for adding of bicarbonate, then it swings to the other active site for transfer to acetyl-CoA)
127
Fatty acid synthesis starts by...
adding initial substrates to the acyl carrier protein (ACP)
- catalyzed by acetyl transacylase and malonyl transacylase (specific)
- ACP carris a covalently attached phosphopantethine group
- this permanent modification makes ACP like CoA without the nucleotide
128
reaction pathway of Fatty acid synthesis
- the steps of fatty acid synthesis are the opposit reactions to fatty acid degradation
- in bacteria, each reaction is catalyzed by a seperate enzyme (still in a complex)
- in higher organisms all the activities are contained in the fatty acid synthase
129
condensation reaction
- once acetate and malonate are loaded onto ACP, they are condensed in the fatty acid synthase complex
- the acceptor (acetyl group) is handed over to a cysteine in a different part of the enzyme
- the donor (malonyl group) remains on ACP in the complex
- the 2 carbon unit is then transferred to the acceptor, driven by decarboxylation of the malonyl group
- 2 carbonsare added at the thioester end of the chain
130
acceptor group ends up...
back on ACP
- donor=Malonyl
- product is an activated beta-keto acid attached to ACP
- the chain grows at the THIOESTER end
- growing chain bounces on and off the beta-ketoacyl synthase subunit during elongation
- when handed back to the synthase subunit, empty ACP is reloaded from a new malonyl-CoA
131
reduction
- need to reduce ketone to make a hydrocarbon chain
- remaining attached to ACP, the growing fatty acid chain migrates to the ketoreductase domain
- the beta-ketone is reduced to an alcohol
- NADPH is oxidized
132
last two steps to complete fatty acid synthesis
- still remaining attached to ACP, the intermediates migrate to two more active sites to complete the reduction from the ketone all the way down to the methylene group
- NADPH is once again the reducing agent that reduces the dehydrated intermediate
133
order of active sites
1) KS (beta-ketoacyl synthase)
2) KR (Ketoreductase)
3) DH (Dehydrogenase)
4) ER (Enoyl Reductase)
134
one catalytic cycle by fatty acid synthesis adds...
a 2 carbon unit to a growing acyl chain
135
termination
the cycle is repeated until the desired C16 fatty acid is synthesized
- 7 turns required to synthesize and fully oxidize palmitate
- once palmitate is synthesized, the palmitoyl-ACP becomes the right size to be a substrate for thioesterase which hydrolyzes it from ACP
136
thioesterase
acts as a molecular ruler determining when synthesis is finished
137
what is the cost of each turn of the synthesis cycle
1 ATP and 2 NADPH
138
how many turns to male palmitate
7
139
final tally for cost
8 acetyl CoA, 7 ATP, and 14 NADPH
140
Acetyl CoA is regenerated...
in the cytoplasm from sytoplasmic CoA and an acetyl group that has been carried via citrate
**citrate is a positive regulator of lipogenesis**
141
elongation past 16 carbons
- in eukaryotes, longer fatty acids are synthesized on the cytoplasmic face of the ER
- Manoyl-CoA is the 2 carbon donor, driven by decarboxylation
142
generation os unsaturated fatty acids
- a set of membrane-bound electron transporters allow for the unsaturation of fatty acids
- fatty acids get oxidized
143
eicosanoids
- derived from polyunsaturated FAs
- need essential fatty acids
144
control of acetyl CoA carboxylase
- inactivated by phosphorylation when C(H2O)s and ATP are low
- citrate can alleviate this when making filaments of inactive enzyme (have lots of energy/high ATP and acetyl CoA)
- the citrate mechanism is antagonized by palitoyl-CoA, a sign that there is an excess of fatty acids in the cell
145
insulin
- stimulates acetyl CoA carboxylase (stimulates fatty acid synthesis)
- activates Akt to phosphorylate AMPK
- activates protein phosphatase (PP2A) to dephosphorylate acetyl CoA carboxylase
- insulin signals an abundance of energy
146
glucagon, epinephrine
- inhibit ACC (inhiibits FA synthesis)
- high concentration os AMP activates AMPK
- glucagon signals that energy is needed
