The Citric Acid Cycle

Question 1

What is the purpose of the CAC?

Answer 1

cellular respiration, and produces NADH and FADH2 for the electron transport chain

Question 2

How many enzymes work in the CAC?

Answer 2

8

Question 3

Name the 8 enzymes in order

Answer 3

citrate synthase
 aconitase
 isocitrate dehydrogenase
 alpha-ketoglutarate dehydrogenase
succinyl-coA synthetase
 succinate dehydrogenase
 fumarate
 malate dehydrogenase

Question 4

what is the function of citrate synthase?

Answer 4

the condensation of acetyl-coA and oxaloacetate into citrate

Question 5

what is the function of aconitase?

Answer 5

isomerization of citrate (OH on C3) into isocitrate (an alcohol). There is an intermediate formed (cis-aconitate). This turns into isocitrate after a hydration (moves the OH to C2)

Question 6

what is the function isocitrate dehydrogenase?

Answer 6

oxidation of isocitrate into an oxalosuccinate (this is a beta-keto acid intermediate).Oxalosuccinate is decarboxylated into alpha-ketoglutarate. Here is the first generation of NADH and CO2.

Question 7

what is the function of Alpha-ketoglutarate dehydrogenase?

Answer 7

the oxidative carboxylation of alpha-ketoglutarate into succinyl-coA. Here is the second CO2 and NADH production.

Question 8

what is the function of succinyl-coA synthetase?

Answer 8

it converts succinyl-coA into succinate. Here GTP is formed. (Using the free energy from the broken thio-ester bond)

Question 9

what is the function of succinate dehydrogenase?

Answer 9

oxidation of succinates’ central single bond into a trans-double bond of Fumarate. Here FAD is reduced into FADH2

Question 10

what is the function of fumarase?

Answer 10

the hydration of fumarates’ double bond forming malate

Question 11

what is the function of malate dehydrogenase?

Answer 11

it reforms oxaloacetate by oxidizing malates’ 2* alcohol to the corresponding ketone. Here NADH is produced, and all acetyl groups have been oxidized to CO2

Question 12

name the enzymes involved in the steps that produce NADH?

Answer 12

isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, malate dehydrogenase

Question 13

explain how Citrate synthase works?

Answer 13

an ordered sequential kinetic mechanism, oxaloacetate adds to the enzyme before acetyl-coA. The enzyme is a free dimeric enzyme, it has an open form where the 2 subunits form a deep cleft containing the oxaloacetate binding site, when it binds the smaller subunit turns 18* and the cleft is closed

Question 14

does citrate have a plane of symmetry?

Answer 14

no, it is prochiral.

Question 15

how does aconitase work?

Answer 15

it can differ between the pro-S and pro-R carboxymethyl groups of citrate. There is a dehydration, transelimination, rehydration.

Question 16

how is cis-aconitate formed?

Answer 16

there must be a dehydration reaction in ser 642 alkoxide and takes the pro-R proton of C2 of citrates’ pro-R carboxymethyl group.
there is the loss of OH from C3 by a transelimination of H2O to form cis-aconitate

Question 17

whats a characteristic of aconitase enzyme?

Answer 17

it has 4Fe-4S redox center clusters.

Question 18

how does isocitrate dehydrogenase work?

Answer 18

is NAD+ dependant, needs Mn+/Mg+ cofactors. The keto-B acts as an electron sink. First there is the oxidation of NAD+ a its Re-Face.

Question 19

how does alpha-ketoglutarate work?

Answer 19

it is mediated by a homologous multienzyme complex made of Eo1, Eo2…

Question 20

what inhibits succinate dehydrogenase?

Answer 20

malonate

Question 21

is the malate dehydrogenase catalyzed reaction favorable?

Answer 21

no it is very endergonic, however there is a lot of free energy release by the citrate synthase path

Question 22

for every acetyl-coA entering the cycle how many ATP are made?

Answer 22

10 ATP

Question 23

name the enzyme that work endergonically (far from equilibrium)

Answer 23

citrate synthase, isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase.

Question 24

what regulates the CAC

Answer 24

allosteric inhibition, competitive feed back inhibition by intermediates, substrate availability, product inhibition

Question 25

go to notes for amphibolic path way

Answer 25

ehhhhh laziness