biochem - carbohydrate metabolism Flashcards
(80 cards)
1
Q
functions of glycolysis (3)
A
- generate ATP without need for O2
- provide substrates for further oxidation and ATP generation
- provide intermediates for biosynthesis and regulation (glucose-6-P can be converted to many important molecules)
2
Q
location of glycolysis
A
- present in all cells
- glycolysis occur in cytoplasm
*cells that do not have mitochondria (eg RBC) rely solely on glycolysis for ATP
3
Q
how is hexokinase activity regulated
A
- product inhibition -> inhibited by high G-6-P concentration
4
Q
how is the liver able to undergo glycolysis even with high glucose concentration
A
- presence of specialized isoenzyme -> GLUCOKINASE -> continued activity in high glucose conditions (can continuously produce glycogen with glucose)
5
Q
how much ATP is obtained from one glucose molecule
A
- 4 generated but 2 consumed -> overall net gain of 2 ATP molecules
6
Q
what are the requirements for glycolysis
A
- 2 NAD+, 2Pi, 2ATP per molecule of glucose
7
Q
when can there be a shortage of NAD+ or Pi
A
NAD+
- if NAD+ is not regenerated from NADH
Pi
- if Pi is trapped in sugar phosphate form that is not metabolised (eg aldolase deficiency) -> cannot break down F-1,6-P2
8
Q
how is NAD+ regenerated under oxygen/ no oxygen
A
- oxidative phosphorylation in mitochondria (aerobic)
- lactic acid fermentation (anaerobic)
9
Q
how is glycolysis regulated in localized tissues
A
- allosteric control -> affect enzymatic activity
10
Q
how is glycolysis regulated systemically
A
- hormonal control -> can have effect on both enzymatic activity and local tissue allosteric control
11
Q
which enzyme is involved in local allosteric regulation
A
- hexokinase (HK)
- PFK-1
- PK
12
Q
described allosteric changes in HIGH energy need
A
- PFK-1 upregulated by F-2,6-P2 (formed from fructose-6-P when F6P accumulates, catalysed by PFK-2/FBP-2 complex) -> cause breakdown of F6P even faster
- PFK-1 upregulated by high levels of AMP (product of ATP degradation, signals ATP depletion)
13
Q
describe allosteric changes in LOW energy need
A
- PFK-1 downregulated by ATP (builds up when muscle is relaxed)
- PFK-1 downregulated by citrate
- HK downregulated by G6P
- PK downregulated by ATP
14
Q
describe action of the PFK2/FBP2 enzyme
A
- PFK2 portion of enzyme catalyses formation of F-2,6-P from F6P -> upregulate PFK-1
- FBP2 portion of enzyme catalyses breakdown of F-2,6-P -> downregulate PFK-1
15
Q
describe hormonal control of PFK-2 in liver
A
- hormones (insulin/ glucagon/ epinephrine) regulate PFK-2/FBP-2 complex
- controls level of F-2,6-P2 -> controls activity of PFK-1
16
Q
describe the effects of glucagon/ epinephrine on PFK2/FBP2
A
- phosphorylates PFK2 -> only FBP2 active -> breakdown F-2,6-P -> downregulate glycolysis
17
Q
describe effect of insulin on PFK2/FBP2
A
- opposite of glucagon & epinephrine
18
Q
describe hormonal control of PK in liver
A
- glucagon phosphorylate (inactivates) PK
- insulin activates PK
19
Q
when is glucagon/ epinephrine released by liver
A
- low glucose levels -> conserve glucose by decreasing glycolysis
20
Q
when is insulin released
A
- high glucose levels -> increase breakdown of glucose to form other byproducts (eg glycogen) or energy
21
Q
how does glycolysis byproducts produce 2,3-BPG & its effects
A
- 1,3-BPG can be converted to 2,3-BPG via MUTASE
- 2,3-BPG binds to HbO2 -> decreases affinity of Hb for O2 -> releases O2
22
Q
genetic diseases in glycolysis of glucose
A
- GENETIC -> pyruvate kinase (PK) deficiency
23
Q
effects of PK deficiency
A
RBC
- glycolysis is IMPT for RBC energy -> PK deficiency cause RBC lysis (lack of energy)
- block in pathway cause more 2,3-BPG product formation in RBC
LIVER
- increase compensatory synthesis of PK in liver cells
24
Q
diseases of fructose/ galactose glycolysis
A
FRUCTOSURIA
- fructokinase deficiency -> fructose accumulation excreted in urine (benign condition)
FRUCTOSE INTOLERANCE
- aldolase B deficiency -> accumulation of fructose-1-P + depletion of phosphate (required for glycolysis) -> poor feeding, unable to thrive
GALACTOSEMIA
- galactose-1-P uridyltransferase deficiency -> galactose-1-p build up (toxic)
- presentations: cataracts (galactose converted to galacticol and deposited in lens); liver enlargement, brain damage
25
functions of TCA cycle (3)
- generate energy
- provide intermediates for biosynthesis
- provide feedback regulator (citrate) to other pathways
26
location of TCA
- inside mitchondria
- pyruvate -> converted to acetyl CoA -> sent into mitochondria
27
Regulation of PDH complex (2)
- allosteric regulation
- phosphorylation of PDH (by kinase & phosphatase)
28
how is PDH regulated under high energy need (eg exercising)
allosteric regulation
- increase CoASH and NAD+ -> allosteric activation of PDH
- muscle activity increase Ca2+ -> activate phosphatase -> dephosphorylate inactive, phosphorylated PDH to active PDH
- presence of ADP and pyruvate inhibits kinase -> prevents phosphorylation of PDH
29
how is PDH regulated under low energy need (resting)
- NADH increase (not consumed by TCA) -> inhibit PDH
- acetyl-CoA accumulates along with NADH (not used up by TCA cycle) -> activate kinase -> phosphorylate PDH
30
function of anaplerotic reactions
- replenish oxaloacetate when it is depleted for biosynthesis
31
can ethanol replenish TCA cycle intermediates and increase metabolism?
- NO. ethanol is converted to acetyl-CoA -> 2C compound, cannot replenish TCA cycle intermediates
32
how is TCA regulated at high energy demand (eg exercise)
Isocitrate dehydrogenase
- high ADP concentration -> activate isocitrate DH
- Ca2+ produced by muscles -> activate isocitrate
a-ketoglutarate dehydrogenase
- Ca2+ activate a-KG
33
how is TCA regulated at low energy demand
isocitrate dehydrogenase
- end product inhibition: NADH
malate dehydrogenase
- NADH levels rise -> end product inhibition
citrate synthase
- citrate levels rise -> end product inhibition
34
genetic diseases of TCA cycle
- pyruvate dehydrogenase deficiency
- TCA enzymes deficiency
35
PDH deficiency pathogenesis
- pyruvate not metabolised to acetyl-coA -> converted to lactate -> ACIDOSIS (cause neurodegeneration)
- poor muscle tone,
developmental delay, seizures
*treat: anti-epileptics, diet (10% carbs)
36
TCA enzymes deficiency (very rare) pathogenesis
- fumarase/ succinate DH/ a-KG DH
- similar presentations and pathogenesis as PDH deficiency (backflow of pyruvate)
37
acquired disorders of TCA cycle
- thiamine deficiency
- arsenic/ mercury poisoning
38
thiamine deficiency pathogenesis
- thiamine required for pyruvate DH and a-KG DH function
- thiamine deficiency -> decrease ATP production
presentations
- poor muscle control; neurodegeneracy; HF
39
arsenic/ mercury poisoning pathogenesis
- inhibit lipoic acid (required by PDH and a-KG DH complexes) -> decrease ATP production; neurodegeneracy; coma/death
40
location of oxidative phosphorylation
- mitchondrial membrane
41
types of shuttles that transport NADH into mitochondria
- glycerol-3-phosphate shuttle -> transports into mitochondria as FADH2, less biochemical steps but less energy efficient
- malate-aspartate shuttle -> transports into mitochondria as NADH, more biochemical steps but more efficient (more commonly found in cardiac muscles/ tissues that need higher energy pdn efficiency)
42
which complex in ETC cannot pump protons out of mitchondrial membrane
complex II
43
overall ATP yield from oxidative phosphorylation
8NADH x 2.5 = 20
4FADH2 x 1.5 = 6
44
side products of oxidative phosphorylation
- partial reduction of O2 -> forms reactive oxygen species (toxic)
- heat generated when H+ passes through UCP protein instead (UCP present in brown fat) of ATP synthase -> short circuit route
45
how is oxidative phosphorylation regulated at high ATP needs
- high ATP consumption -> increase [ADP] in mitochondria -> increase conversion at ATP synthase -> increase need for O2 at ETC
46
how is oxidative phosphorylation regulated at low ATP needs
- ATP not utilized -> decrease need for ATP at ATP synthase
47
genetic diseases of OXPHOS
- OXPHOS mitochondrial disease; depends on maternal inheritance and replicative segregation
48
OXPHOS mitochondrial disease presentations
- disruption in gene decrease ETC complex & ATP production
- MELAS SYNDROME - Myopathy, Encephalomyopathy,
Lactic Acidosis and Stroke)
49
which complexes are coded by mitochondrial DNA?
- complex I, III, IV
- ATP synthase
*female with defect in complex II will LIKELY NOT pass on mutation to children (nuclear mutation, lower chance of passing on than mitochondrial mut)
50
acquired diseases of mitochondrial DNA
- mitochondrial poison
51
types of mitochondrial poisons
complex I
- rat poison
complex III
- fish poison
complex IV
- cyanide, CO
proton leak across mt membrane (through UDP)
- herbicides/ pesticides
52
functions of HMP shunt (2)
- generate NADPH
- generate ribulose-5-phosphate for nucleotide synthesis
53
where does HMP shunt function
- cytoplasm
54
which cells are HMP shunt majorly found in
- adipocytes (NADPH for fatty acid synthesis)
- liver (NADPH for fatty acid syn & drug metabolism)
- adrenal cortex (NADPH for steroid synthesis)
- RBC (NADPH for glutathione reduction, helps to neutralise ROS)
- WBC (NADPH for superoxide generation)
55
what can be used as an assay to gauge thiamine levels
- transketolase activity in RBCs (transketolase requires thiamine pyrophosphate prosthetic group to function)
56
how is HMP shunt regulated
- rate of G6PD enzyme is regulated -> based off NADPH/ NADP+ ratio
- supply of NADPH low -> G6PD more active -> increase HMP shunt
57
functions of NADPH (5)
- reducing power for biosynthesis (eg cholesterol synthesis)
- detoxification in liver
- generating ROS in WBCs
- generating NO for vasodilation
- glutathione reduction
58
congenital disease of HMP shunt
- G6PD deficiency
59
G6PD deficiency pathogenesis
- X-linked recessive, range of clinical symptoms due to different mutations in G6PD gene
- G6PD deficiency -> decreased production of NADPH in RBCs needed to maintain glutathione
- glutathione reduces ROS -> ROS builds up (oxidative stress) causing oxidation of proteins -> reduce membrane plasticity -> hemolysis
presentations
- jaundice (excessive hemolysis), kernicterus, anemia
60
contraindications in G6PD deficient patients
- antimalarials
- sulfur based antibiotics (eg cotrimoxazole)
*increases ROS in body -> cannot be removed by lack of reduced glutathione
61
what is a benefit of having G6PD deficiency
- resistance to malaria infections (NADPH is utilized by plasmodium in RBC to survive)
62
function of gluconeogenesis
- maintaining blood glucose levels during long fasts (glycogen used for short fasts)
*order of sources for maintaining glucose: diet carbs -> glycogen (4-24hr) -> gluconeogenesis (>24hr)
63
location of gluconeogenic pathway
- cytosolic (except pyruvate carboxylase in mitochondria; glucose-6-phosphatase in endoplasmic reticulum)
organs
- liver and kidney
64
substrates for gluconeogenesis
LACTATE; ALANINE; other amino acids
- entry via pyruvate
GLUTAMINE, other amino acids
- entry via oxaloacetate
GLYCEROL (from lipolysis)
- entry via glycerol-3-P
65
how is gluconeogenesis regulated at high energy needs
allosteric regulation
- ATP, citrate increase activity of F-1,6-BP
- acetyl CoA increase activity of pyruvate carboxylase
66
how is gluconeogenesis regulated at low energy needs
allosteric regulation
- AMP inhibits F-1,6-BP
- ADP inhibits both PEPCK and PC
67
how is gluconeogenesis regulated at high glucose levels
hormonal regulation
- increase insulin secretion -> increase F-2,6-P -> inhibits F-1,6-BP
68
how is gluconeogenesis regulated at low glucose levels
hormonal regulation
- glucagon secretion -> decrease F-2,6-P -> decrease inhibition of F-1,6-BP
- glucagon secretion -> increase transcription of G-6-phosphatase, fructose-1,6-biphosphatase, PEP carboxykinase
69
genetic diseases of gluconeogenesis
- glucose-6-phosphatase deficiency
- pyruvate carboxlase deficiency
70
glucose-6-phosphate deficiency pathogenesis
- glucose cannot be formed in the last step -> glucose-6-P diverted to glycogen formation (ORGANOMEGALY) and HMT shunt (nucleotide metabolism -> INCREASE URIC ACID)
- backpressure on gluconeogenesis -> accumulation of pyruvate -> form lactate
presentations: organomegaly, hyperuricaemia, lactic acidosis
71
pyruvate carboxylase deficiency pathogenesis
- pyruvate accumulates (cannot be converted to oxaloacetate) -> reduced ATP pdn from TCA; increase acetyl-CoA via PDH catalysis; increase lactate buildup
*short life span (6months)
72
acquired disease of gluconeogenesis
- diabetes mellitus
- relative/ absolute insulin deficiency -> unopposed glucagon action even in fed state -> stimulates gluconeogenesis
73
can ethanol contribute to maintaining blood glucose levels if a person just drinks alcohol w/o carbohydrates/ protein
- NO. ethanol is converted to acetyl-CoA, which enters TCA cycle for energy pdn -> DOES NOT contribute to gluconeogenesis
74
where does glycogen formation/ metabolism occur
- cytoplasm, present in most cell types
*glucose-6-phosphatase only present in liver (required for glucose export)
75
how is glycogen metabolism regulated
tissue response:
- allosteric/ phosphorylation -> regulate glycogen breakdown/ synthesis
systemic response:
- hormonal -> directly cause glycogen breakdown/ synthesis + stimulate allostery/ phosphorylation in tissues
76
regulation in liver in fed state (high glucose, ATP)
allosteric regulation
- increase amt of glucose-6-P activates glycogen synthase -> increase pdn of glycogen from UDP-glucose
- glucose-6-P, glucose & ATP inhibits glycogen phosphorylase (liver isoform) -> prevent breakdown of glycogen to glucose
77
regulation in liver in fasted state (low glucose, ATP)
allosteric regulation
- low glucose, glc-6-P, ATP -> no more activation of glycogen synthase
- glycogen phosphorylase no longer inhibited -> glucose produced
78
regulation in muscles at high energy needs (high AMP, Ca)
allosteric regulation
- AMP directly activates glycogen phosphorylase (muscle isoform)
- Ca -> binds to calmodulin -> activates kinase -> phosphorylate & activates glycogen phosphorylase
79
regulation of muscles at low energy needs (high ATP, glc-6-P)
- increase ATP, glc-6-P -> activates glycogen synthase -> increase conversion of UDP-glucose to glycogen
- ATP, glc-6-P -> inhibit glycogen phosphorylase
80
systemic regulation of glycogen metab at high glucose vs low glucose
HORMONES
high glucose
- insulin signalling -> insulin binds to RTK -> phosphorylate IRS (insulin receptor substrate) -> activate PP1 (protein phosphatase 1)
- PP1 dephosphorylate (ACTIVATES) glycogen synthase -> glycogen synthesis
- PP1 dephosphorylate (INACTIVATE) glycogen phosphorylase -> inhibit glycogen breakdown
low glucose
- glucagon & epinephrine signalling -> glucagon (liver) & epinephrine (liver/ muscle) binds to GPCR -> dissociation of heterotrimeric G protein -> a subunit activate adenylate cyclase -> ATP convert to cAMP -> activate PKA
- PKA phosphorylate (INACTIVATE) glycogen synthase
- PKA phosphorylate phosphorylase kinase -> phosphorylates (ACTIVATE) glycogen phosphorylase
