Day 36

Question 1

Thioesters

Answer 1

• High energy compounds
• Phosphates rare in primitive world so the sulfate in thioesters were probably used because they are found in the metabolisms of all organisms
• Acetyl CoA is a common product of carbohydrate, fatty acid, and a.a. degradation
• CoA (co enzyme A) carries these high energy thioesters (dG*of acetyl CoA -31.5kJ/mol)
• Energy comes from thioesters being LESS stabilized by resonance

Question 2

Multienzyme Complex

Answer 2

• noncovalently associated and will catalyze several sequential steps
• channeling of intermediates keeps them “on track”
• coordinately controlled

Question 3

Pyruvate Dehydrogenase Complex

Answer 3

Dehydrogenase means redox reaction

• over all pyruvate + CoA + NAD+ = acetyl-CoA + CO2 + NADH
• E1: pyruvate dehyrogenase
• E2: dihydrolipoyl transacetylase (make acetyl-CoA)
• E3: dihydrolipoyl dehydrogenase (reset system)
• cofactors/coenzymes are tiamine pyrophosphate (TPP), lipoamide, CoA, FAD, NAD+

Question 4

Reactions of the Pyruvate Dehydrogenase Complex

Answer 4

1. Formation of Hydroxyethyl TPP intermediate from pyruvate (release CO2)
2. Transfer of hydroxyethyl group
3. Acetyl group transferred to CoA
4. Regeneration of lipoamide group in E2
5. Reoxidation of reduced E3

Question 5

TPP

Answer 5

bound to E1 and decarboxylates pyruvate, yielding a hydroxylethyl-TPP carbanion

Question 6

Lipoic Acid

Answer 6

Covalently linked to Lys on E2 (lipoamide) and it accepts the hydroxyethyl carbanion from TPP as an acetyl group

Question 7

CoA

Answer 7

Substrate for E2 (not bound to enzyme) and it accepts the acetyl group from lipoamide

Question 8

Flavin adenin dinucleotide (FAD)

Answer 8

Bound to E3 and its reduced by lipoamide

Question 9

Nicotinamide adenine dinucleotide (NAD+)

Answer 9

Substrate for E3 (not bound to enzyme) and it is reduced by FADH2

Question 10

Reaction 1: Formation of hydroxyethyl-TPP

Answer 10

Pyruvate + TPP-E1 + (H+) = (CO2) + Hydroxyethyl-TPP

Question 11

Reaction 2: Transfer of Hydroxyethyl group to E2

Answer 11

• (lipoic acid bound to lysine make lipoamide covalently to E2)
• Lipoic acid has a cyclic disulfide that is reduced to make dihydrolipoamide
• hydoxyethyl group moves off TPP-E1 and onto the lipoamide. The lipoamide is reduced and the hydroxyethyl group is reduced to acetyl group.
• now we have acetyl-dihydrolipoamide-E2

Question 12

Reaction 3: Acetyl Group Trasferred to CoA

Answer 12

• E2 catalyzes transesterification so acetyl group is trasferred from dihydrolipoamide to CoA
• leaves a reduced dihydrolipoamide-E2
  (CoA-SH) + Acetyl-dihydrolipoamide-E2 = acetyl-CoA + dihydrolipoamide-E2

Question 13

Reaction 4: Regeneration of Lipoamide Group on E2

Answer 13

E3 reoxidizes dihydrolipoamide with disulfide interchange reaction
E3(oxidized) + dihydrolipoamide (reduced) = E3 (reduced) + dihydrolipoamide (oxidized)

Question 14

Reaction 5: Reoxidation of Reduced E3

Answer 14

• E3 is reoxidized by funneling electrons from reduced cysteine through FAS cofactor to NAD+ to form FADH2
• But really FADH2 not really formed because electrons transiently passed from FAD to NAD+
  FAD FAHD2 (NAD+ - - NADH + (H+) —-E3 oxidized
  SH s
  SH s

Question 15

Intermediate Transport between Enzymes

Answer 15

1) lipoamide of E2 has a very long arm that stretches from one active site to the next.
2) “flexible loop” for Pro- and Ala- rich sequence

Question 16

Arsenic Poisoning

Answer 16

Binds to sulfates in the lipoamide so the resetting of dihydrolipoamide is impossible.