W5- Lecture 21- citrate cycle

Question 1

describe citrate cycle reaction 1

Answer 1

oxaloacetate + Acetyl-CoA + H2O
catalysed by citrate synthase
Citrate + CoA
Extremely favourable under standard conditions
reaction is driven by the formation of a citryl-coA intermediate which undergoes rapid hydrolysis

Question 2

describe citrate cycle reaction 2

Answer 2

Reversible, 2-step isomerisation reaction

Step 1 – dehydration and formation of intermediate

Step 2 – rehydration to form isocitrate
both step done by active site 4Fe-4S cluster
in mitochondria

Question 3

describe citrate cycle reaction 3

Answer 3

2-step de-carboxylation reaction and formation of NADH

Step 1 – oxidation of isocitrate and formation of intermediate

Step 2 – decarboxylation to form alpha-ketoglutarate
Metabolic
nodal point – other pathways produce alpha-ketoglutarate

Question 4

describe citrate cycle reaction 4

Answer 4

Oxidative de-carboxylation with formation of NADH
Enzymatic mechanism involves 3 different subunits that catalyse different stages of the reaction

1) Decarboxylation of alpha-ketoglutarate and binding to TPP to form an intermediate
2) Transfer of the intermediate to a lipoamide group of dihydrolipoyl succinyltransferase (E2) which catalyses the formation of succinyl-coA upon addition of coA
3) Oxidation of the lipoamide group of the E2 subunit (to regenerate it for reaction step 2) by transfer of 2 electrons from FADH2 to NADH

Question 5

describe citrate cycle reaction 5

Answer 5

Substrate-phosphorylation of GDP to form GTP, via the production of succinate catalysed by succinyl-CoA synthetase

Phosphorylation driven by the formation of multiple intermediates (pi in CoA out)

Resulting GTP + ADP can be converted into ATP+ GDP via a phosphoryl transfer reaction

Question 6

describe citrate cycle reaction 6

Answer 6

Reduction of FAD to FADH2
This FADH2 can enter the electron transport chain via reduction of coenzyme Q10 in the inner membrane of the mitochondria

Succinate dehydrogenase catalyses reaction + is is associated with the membrane, and covalently-linked to the FAD/FADH2 molecule

Question 7

describe citrate cycle reaction 7

Answer 7

2 step reaction
1)Hydration of the C=C in fumarate catalysed by a highly stereospecific enzyme fumerase to produce a carbanion intermediate

2)carbanion intermediate catalysed by fumarase with addition of a proton
Formation of the L-isomer of malate
Malate important for metabolite transporters across mitochondrial membranes

Question 8

describe citrate cycle reaction 8

Answer 8

malate ,Oxidation reaction generating oxaloacetate

Oxidation of the hydroxyl group and reduction of NAD+ to NADH

catalysed by malate dehydrogenase

Question 9

draw the citrate cycle

Answer 9

please

Question 10

what is the fate of Acetyl-coA carbons

Answer 10

The Acetyl-coA carbons become incorporated into oxaloacetate at the end of the cycle

Thus they are lost as CO2 in subsequent turns of the cycle