Agbas (Citric Acid Cycle)

Question 1

Citric Acid Cycle Overview

Answer 1

• oxidizing carbon fuels for harvesting high energy electrons (2 carbon unit)
• source of precursors for biosynthesis (in mitochondria)
• TCA, Krebs Cycle
• generates 2 CO2, 1 GTP, electrons (NADH, FADH2)

Question 2

Acetyl CoA

Answer 2

• activated form of acetate
• from
  1) carbohydrates: glucose –> 2 pyruvate –> 2 ACoA
  2) lipids: TAG –> FA –> ACoA
  3) Proteins: AA –> ACoA

Question 3

Acetyl CoA from Pyruvate

Answer 3

Decarboxylation, Oxidation, Acetly group transfer to coenzyme A

• rxns coupled to preserve energy to drive formation of NADH and acetyl CoA

Question 4

Aerobic Glycolysis and OxPhos

Answer 4

• pyruvate (glycolysis) enters mitochondria via mitochondrial pyruvate carrier (MPC)
• pyruvate dehydrogenase complex (PDC) catalyzes decarboxylation of pyruvate
• mitochondria VERY selective

Question 5

Pyruvate Dehydrogenase Complex

Answer 5

• 3 enzymes: E1 (TPP coenzyme), E2 (lipoic acid, CoA coenzymes), E3 (FAD, NAD coenzyme)
• E1 phosphorylation inactivates PDC

PDK inactivates: products activate, reactants inhibit
PDP activates: Ca, Mg

• regulated by allosteric interactions, reverse phosphorylation
• high ACoA inhibits E2
• ADP/pyruvate activate phosphatases

Question 6

What is the currency of TCA cycle?

Answer 6

Acetyl-CoA

Question 7

CAC Step 1

Answer 7

Oxaloacetate + Acetyl CoA –> citrate

enzyme: citrate synthase

condensation of 4 carbon and 2 carbon

Question 8

CAC Step 2

Answer 8

Citrate –> Isocitrate

enzyme: aconitase –> moves OH atoms

hydroxyl group of citrate NOT in proper location for oxidative decarboxylation

intermediate: cis-Aconitate

Question 9

CAC Step 3

Answer 9

Isocitrate –> Oxalosuccinate –> a-ketoglutarate

enzyme: isocitrate dehydrogenase

1st OXIDATION?REDUCTION REACTION

oxalosuccinate loses CO2 while enzyme bound

generate NADH

Question 10

What is the rate limiting step of the Citric Acid Cycle?

Answer 10

Isocitrate –> a-ketoglutarate

Question 11

CAC Step 4

Answer 11

a-ketoglutarate –> succinyl CoA

enzyme: a-ketoglutarate dehydrogenase

creates thioester linkage with CoA (high energy bond)

generate NADH

Question 12

CAC Step 5

Answer 12

succinyl CoA –> succinate

enzyme: succinyl CoA synthetase

ONLY step that directly yields a high energy phospho-transfer compound (ATP/GTP)

GDP (liver) –> anabolic reactions
ADP (skeletal/heart muscle) –> large amounts of cellular respiration

Question 13

What is the only step in the Citric Acid Cycle that generates ATP/GTP?

Answer 13

Succinyl CoA –> Succinate

Question 14

CAC Step 6

Answer 14

succinate –> fumarate

enzyme: succinate dehydrogenase

in inner mitochondrial membrane (directly associated with ETC through Complex II)

FADH2 generation (pass electrons to Co-Q)

Question 15

What step in the Citric Acid Cycle links the citric acid cycle to electron transport chain?

Answer 15

succinate –> fumarate

Question 16

CAC Step 7

Answer 16

fumarate –> L-Malate

enzyme: fumarase

can only generate D-Malate in LABORATORY

Question 17

CAC Step 8

Answer 17

malate –> oxaloacetate

enzyme: malate dehydrogenase

reactions drive by use of products

generates NADH

Question 18

Citric Acid Cycle Regulation (PDH)

Answer 18

High Energy Charge
- NADH, ACoA, ATP (-) regulates PDH

Low Energy Charge
- Pyruvate, ADP (+) regulates PDH

Phosphatases (activators) stimulated by Ca, insulin

Question 19

Citric Acid Cycle Regulation Control Sites (3)

Answer 19

1. citrate synthase prevents wasteful hydrolysis of ACoA
   - oxaloacetate binds to enzyme 1st, then enzyme changes to accept ACoA
   - stim: ADP/pyruvate, inhib: ATP, ACoA, NADH
2. isocitrate dehydrogenase
   - allosterically stimulated by ADP, inactivated by NADH/ATP
3. a-ketoglutarate dehydrogenase
   - similar to PDH, allosterically inhibited by products (ATP, succinyl CoA, NADH)

Question 20

TCA anaplerotic

Answer 20

• fill up reactions provide intermediates for replenishment of TCA cycle

1. AA degradation
2. pyruvate carboxylation

when energy needs met, intermediates drawn for biosynthesis of other molecules (replenished by oxaloacetate formation from pyruvate)