Citric acid cycle

Question 1

Location, function, condition

Answer 1

• mitochondrial matrix
• reoxidize electron carriers to generate ATP
• aerobic
• amphibolic:oxidation, carbon skeletons for gluconeogenesis, acetyl CoA for FA synthesis, interconversion of AAs

Question 2

Acetyl-CoA -> Citrate

Answer 2

Acety-CoA/Oxaloacetate -> Citrate + SH-CoA

• IRREVERSIBLE
• enzyme: CITRATE SYNTHASE
• condensation
• overall deltaG=-31,5kJ/mol
• first committed step

Question 3

Allosteric regulation of citrate synthase

Answer 3

inhibited by:
high ATP
high NADH
high succinyl-CoA
high citrate conc.

Question 4

Citrate -> Isocitrate

Answer 4

Citrate->cis-Aconitate->Isocitrate

• dehydration- hydration
• reversible
• enzyme: ACONITASE (Fe-S protein, inhibited by fluoroacetate, serves as enzyme-bound intermediate)
• channeling
• citrate prochiral

Question 5

Isocitrate -> alpha-ketoglutarate

Answer 5

Isocitrate + NAD+ -> alpha-ketoglutarate + NADH + CO2

• IRREVERSIBLE
• oxidative decarboxylation (requires Mg2+ or Mn2+)
• enzyme: ISOCITRATE DEHYDROGENASE (3 isoenzymes)
• rate limiting step

Question 6

Allosteric regulation of Isocitrate DH

Answer 6

-inhibited by:
high ATP
high NADH
-activated by:
high ADP
high NAD+

Question 7

Alpha-ketoglutarate -> Succinyl CoA

Answer 7

Alpha-ketoglutarate + NAD+ + SH-CoA -> Succinyl CoA + NADH + CO2

• IRREVERSIBLE
• oxidative decarboxylation
• enzyme: ALPHA-KETOGLUTARATE DEHYDROGENSE
  (cofactors: TPP, lipoic acid, FAD, NAD+, CoA)

Question 8

Allosteric regulation of alpha-ketoglutarate DH

Answer 8

inhibited by:
high ATP
high NADH
high succinyl-CoA
(also inhibited by arsenate and NH3)

Question 9

Succinyl CoA -> Succinate

Answer 9

Succinyl CoA + GDP + Pi -> Succinate + SH-CoA + GTP

• reversible
• substrate-level phosphorylation
• enzyme: SUCCINATE THIOKINASE/ SUCCINATE CoA SYNTHASE

GTP used by PEP carboxykinase (OAA –> PEP)

Question 10

Succinate -> Fumarate

Answer 10

Succinate + FAD -> Fumarate + FADH2

• reversible
• dehydrogenation
• enzyme: SUCCINATE DEHYDROGENASE (present in inner membrane, FAD as coenzyme, malonate competitively inhibits)

Question 11

Fumarate -> Malate

Answer 11

Fumarate + H2O -> (L) Malate

• reversible
• hydration
• enzyme: FUMARASE

Question 12

Malate -> Oxaloacetate

Answer 12

Malate + NAD+ -> Oxaloacetate + NADH

• reversible
• dehydrogenation
• enzyme: MALATE DEHYDROGENASE (not spontaneous in normal condition, pushed forward by high malate/OAA ratio and NAD+/NADH ratio)

Question 13

Intermediates of cycle

Answer 13

• not produced/consumed
• becomes catalytic cofactors
• consumed in other processes -> resynthesized (anaplerosis)

Question 14

Vitamins in TCA

Answer 14

Riboflavin (FAD, cofactor in succinate DH)
Niacin (NAD+, electron acceptor for isocitrate DH, alpha-ketoglutarate dh, malate dh)
Thiamin (thiamin diphosphate, coenzyme for decarboxylation in alpha-ketogluterate dh rxn)
Pantothenic acid (coenzyme A)