what is the purpose of the TCA cycle?
- generate energy (GTP, NADH, FADH2)
- provide intermediates for biosynthesis
- provide feedback regulator (citrate) to other pathways
what is the overall equation of the TCA cycle?
acetyl CoA + 3NAD+ + FAD + GDP + Pi→ CoA + 3NADH + FADH2 + GTP+ 2CO2
where does the TCA cycle take place?
mitochondria:
- succinate dehydrogenase on inner membrane
- rest of enzymes in matrix
pyruvate moves into mitochondria fr cytosol as it converts to acetyl-CoA
what is the rate limiting enzyme of TCA cycle?
isocitrate dehydrogenase
which steps produce NADH? (3)
- isocitrate→ a-KG
- a-KG→ succinyl CoA
- Malate→ oxaloacetate
“dehydrogenase”→ produces NADH/FADH2
which step produce FADH2? (1)
succinate→ fumarate
Fumarate→ produces FADH2
what stimulates/inhibits the TCA cycle?
stimulates: ADP (ie insufficient ATP, needs NADH & FADH2)
inhibits: ATP, NADH, FADH2 (dont need make more)
how is pyruvate converted to acetyl CoA?
pyruvate dehydrogenase complex
what substances are required for pyruvate dehydrogenase complex? (3)
made of 3 distinct enzymes (E1, 2, 3)
coenzymes:
1. thiamine pyrophosphate (fr vit B1)
2. lipoate, coenzyme A
3. FAD, NAD+
how is PDH regulated? (2 pathways)
- allosteric regulation (directly upregulates/inactivates PDH itself)
- phosphorylation of PDH (activate/inactivate kinase and phosphatase that phosphorylates/dephosphrylates PDH, inactivating/activating it)
how is PDH regulated during high energy need?(3)
- increased NAD+ and CoASH→ increase PDH activity
- increased Ca2+→ activate phosphatase to dephosphorylate/ activate PDH→ activates it
- ADP/pyruvate inhibit kinase to activate PDH
how is PDH regulated during low energy need?
- increased NADH→ inhibit PDH complex
- high NADH/acetyl CoA→ activate kinase to phosphorylate/inactivate PDH complex
what diseases affect PDH? (3)
- PDH deficiency
- arsenic/mercury poisoning
- thiamine/B1 deficiency
what is the pathophysiology of pyruvate dehydrogenase deficiency? (2)
- accumulation of pyruvate→ converted to lactate→ lactate acidosis
- depletion of ATP→ neurological dysfunction/organ failure
what is the pathophysiology of arsenic/mercury poisoning?
inhibits lipoic acid required for PDH/aKG DH→ decreased ATP production→ neurological dysfunction/organ failure
what is thiamine/B1 used for in TCA cycle?
required for PDH and a-KG DH function
what is the pathophysiology of thiamine deficiency?
decreased ATP production→ affects high O2 requiring organs→ beriberi
how does high energy need affect TCA cycle regulation? (3)
e.g. exercise
increase ADP, increase Ca2+, decrease NADH→ activate:
1. isocitrate DH
2. aKG DH
3. malate DH
how does low energy need affect TCA cycle regulation? (4)
e.g. resting
decrease ADP/increase ATP, increase NADH→ inhbits
1. isocitrate DH
2. aKG DH
3. malate DH
4. citrate synthase!!
how does TCA cycle get replenished?
via OXALOACETATE!! (acetyl coa insufficient)
by pyruvate carboxylase + biotin (pyruvate→ oxaloacetate)
which organ contains a high concentration of pyruvate carboxylase?
liver!
TCA intermediates are rapidly depleted by gluconeogenesis (to generate glucose while fasting) → need regenerate oxaloacetate
what is the pathophysiology of TCA enzyme deficiency?
lack of TCA enzymes→ TCA cycle disrupted→ acetyl CoA not processed→ backward accumulation→ accumulate pyruvate & lactic acid→ lactic acidosis, neurological dysfunction
