Citric Acid Cycle

Question 1

Give the other two names for the citric acid cycle.

Answer 1

Krebs cycle

Tricarboxylic acid cycle

Question 2

Which compound is the most reduced and which is the most oxidised?

Answer 2

R = Citrate
O = Oxaloacetate

Question 3

Octet rule

Answer 3

atoms share e- to get 8 e- in there outer shell

Question 4

What reaction does citrate synthase catalyse?

Answer 4

1. oxaloacetate + acetyl –> Citryl CoA
2. Citryl CoA +nH2O –> Citrate + CoA
   ~ hydrolysis of energy-rich thioester bond drives the reaction forward

Question 5

Describe the structure of citrate synthase.

Answer 5

~ two identical subunits
~ binding of oxaloacetate (open form) = causes major structural changes (closed form) = binding site for acetyl CoA
–> prevents wasteful enzyme activity
e.g. hydrolysis of acetyl CoA

Question 6

What reaction does aconitase catalyse?

Answer 6

1. Citrate –> cis-Aconitate + H2O (dehydration)

2. cis-Aconitate + H2O –> Isocitrate (rehydration)

Question 7

What reaction does isocitrate dehydrogenase catalyse?

Answer 7

1. Isocitrate + NAD+ –> Oxalosuccinate + NADH + H+ (oxidation)
2. Oxalosuccinate + H+ –> alpha-ketoglutarate + CO2 (decarboxylation)

Question 8

What reaction does alpha-ketoglutarate dehydrogenase catalyse?

Answer 8

alpha-ketoglutarate + NAD+ + CoA –> Succinyl CoA + CO2 + NADH
(oxidative decarboxylation)
VERY SIMILAR TO PYRUVATE DEHYDROGENASE

Question 9

Which 5 cofactors affect a-ketoglutarate?

Answer 9

~ NAD
~ Coenzyme A
~ FAD
~ Lipoamide
~ Thiamine pyrophosphate

Question 10

What is the structure of a-ketoglutarate like?

Answer 10

3 different enzyme proteins:
E1 - alpha-ketoglutarate dehydrogenase (decarboxylation)
E2 - transsuccinylase (makes succinylcholine CoA)
E3 - dihydrolipoyl dehydrogenase (identical to PDH & reduces FAD/NAD)

Question 11

What reaction does succinylcholine CoA synthetase catalyse?

Answer 11

Succinyl CoA + Pi + GTP –> Succinate + GTP + CoA (energy transfer)

Question 12

What reaction does succinate dehydrogenase catalyse?

Answer 12

Succinate + FAD –> Fumarate + FADH2 (oxidation)

Question 13

What makes the succinate dehydrogenase different?

Answer 13

~ it is the only membrane-bound enzyme in the krebs cycle

~ contains 3 types of Fe-S clusters (2Fe-S; 3Fe-S & 4Fe-S)

Question 14

What reaction does fumarase catalyse?

Answer 14

Fumarate + H2O –> Malate (Hydration)

~ can occur via two intermediates either cation or anion depending on whether OH- or H+ gets added first

Question 15

What reaction does malate dehydrogenase catalyse?

Answer 15

Malate + NAD+ –> oxaloacetate + NADH + H+ (oxidation)

Question 16

The equilibrium is towards malate not oxaloacetate, so how does the towards oxaloacetate occur?

Answer 16

~ oxaloacetat is unstable & kept in low conc.s

~ NADH is rapidly oxidised by the e- transport chain

Question 17

Which molecules globally regulate metabolism demand?

Answer 17

ADP/ATP

NAD/NADH

Question 18

What is the energy yield of the krebs cycle?

Answer 18

2 GTP = 2 ATP
6 NADH = 18 ATP
2 FADH2 = 4 ATP
(everything doubled as 2 pyruvate produced from 1 glucose)

Question 19

What is the purpose of the krebs cycle?

Answer 19

~ aid energy release & electron transference/create ATP

~ provide intermediates for biosynthesis