Module 8: Citric Acid Cycle

Question 1

Stage 1&2 of pyruvate oxidation

Answer 1

1.Three-carbon pyruvate converted to a two- carbon molecule of acetylCoA
Oxidation catalyzed by Pyruvate Dehydrogenase (PDH)
2.Oxidation of acetylCoA to carbon dioxide via CAC

Question 2

CAC products

Answer 2

2C acetylCoA into two 1C molecule of CO2, and 3 NADH, 1 FADH2 and 1 GTP

Question 3

Mitochondrion

Answer 3

specializes in energy production
encapsulated by a double membrane
Invaginations=cristae
Inner compartment=matrix
Space b/w inner & outer membrane=inter-membrane space

Question 4

Describe the permeability of the inner and outer mitochondrial membrane.

Answer 4

Outer mitochondrial membrane: highly permeable with large channels that span the membrane (porins)
Inner membrane: relatively impermeable, allowing only small uncharged compounds like CO2 and water to cross

Question 5

Coenzyme A (CoASH)

Answer 5

free thiol group is the reactive part of coenzyme A which will form energy-rich thioester bond with the 2C acetyl group that will be derived from pyruvate

Question 6

Pyruvate dehydrogenase (PDH)
(subunits & regulatory enzymes)

Answer 6

multi-enzyme complex, made up of three core enzyme subunits, named E1, E2 and E3
also composed of two regulatory enzymes:
PDH kinase -inactivates PDH by phosphorylation
PDH phosphatase -activates PDH by dephosphorylation

catalyzes the irreversible oxidative decarboxylation of pyruvate to acetylCoA

Question 7

What 5 co-factors does PDH require?

Answer 7

NAD+
TPP
Lipoic acid
CoA
FAD

Question 8

What are the advantages of multienzyme complexes?

Answer 8

1. Increased rate/efficiency of reaction due to a reduction in the diffusion distance for intermediates between consecutive enzymes
2. The complex channels intermediates between successive enzymes in a pathway, minimizing side reactions (run goes to completion)
3. Reactions coordinately regulated

Question 9

PDH phosphorylation/dephosphorylation

Answer 9

phosphorylated by PDH kinase= inactivated

dephosphorylated by PDH phosphatase=activated

Question 10

PDH kinase allosterically regulated by:

Answer 10

Elevated conc. of acetylCoA, ATP and NADH activate kinase and promote phosphorylation
Elevated conc. of pyruvate and ADP inhibit kinase

Question 11

PDH phosphatase allosterically regulated by:

Answer 11

activated by insulin and Ca2+ and promote dephosphorylation
Insulin: signal glucose abundant and can be converted to acetyl CoA
Ca2+: increase flux of pyruvate into CAC for cont’d generation of ATP to support the exercising muscle

Question 12

How is PDH regulated by competitive inhibition?

Answer 12

Inhibited by elevated conc. of acetylCoA & NADH

Question 13

AcetylCoA Enters The Citric Acid Cycle

products and characteristics of CAC enzymes

Answer 13

complete the oxidation of glucose to carbon dioxide
Products: 2 CO2, 3 NADH, 1 FADH2, 1 GTP, free CoA

CAC enzymes compartmentalized in mitochondria
All CAC enzymes soluble within matrix, with exception of succinate dehydrogenase (membrane pr-)

Question 14

How ddi the cycle get its 3 names?

Answer 14

Krebs cycle: scientist who discovered the cycle in 1937
Citric Acid Cycle: the product of the first reaction, citrate
Tricarboxylic acid Cycle: citrate is a tricarboxylic acid
Cycle b/c begins and ends with oxaloacetate

Question 15

What happens to each of the products in CAC?

Answer 15

NADH & FADH2=reducing equivalents
CO2=waste (enter bloodstream and exhaled from our lungs)
CoA=regenerated

Question 16

Regulation of Citric Acid Cycle (3)

Answer 16

1.Availability of substrates (oxaloacetate and acetylCoA must be 1:1 ratio, adequate supply of free NAD+)
2.Competitive inhibition by products
(succinyl CoA, NADH-inhibit enzymes)
3.Allosteric regulation
-Ca2+ & ADP: positive allosteric regulation
-ATP: negative allosteric regulator
-NADH

Question 17

Common Themes In Metabolic Regulation

3

Answer 17

1. Energy Status of cell (conc. of ADP/ATP)
2. Redox State of cell (conc. of NADH/NAD+)
3. Feedback inhibitor by products

Question 18

How is the CAC both catabolic and anabolic? (called amphibolic)

Answer 18

Catabolic: oxidatively breaks down glucose, fat, aa producing energy
Anabolic: CAC intermediates can also be starting materials for biosynthetic pathways

Question 19

Anaplerosis

Answer 19

process of replenishing citric acid cycle intermediates
ex: synthesis of oxaloacetate from pyruvate
catalyzed by pyruvate carboxylase

Question 20

What occurs if supply of acetylCoA is greater than the supply of oxaloacetate (should be 1:1 ratio)?

Answer 20

citrate synthase will not function maximally due to substrate limitation; therefore, excess acetylCoA will inhibit PDH, diverting pyruvate to the pyruvate carboxylase reaction