Lecture 12 - Coenzymes

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30 Terms

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Coenzyme

  • Non-protein component often bound to an enzyme

  • Usually vitamin-derived

  • Function in catalysis or carrier

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Prosthetic group

Subset of coenzymes tightly bound to enzymes

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All prosthetic groups…

Are coenzymes, BUT not all coenzymes are prosthetic groups

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Coenzymes in PDH

  1. TPP

  2. Lipoamide

  3. FAD/FADH2

  4. NAD+/NADH

  5. Coenzyme A

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NAD+/NADH Vitamin

Niacin (B3)

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NAD+/NADH Function

Redox (Resets PDH cycle by oxidizing FADH2 → FAD)

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Is NAD+/NADH a prosthetic group?

No, it’s only a coenzyme (not always bound)

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FAD/FADH2 Vitamin

Riboflavin (B2)

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FAD/FADH2 Function

Redox (Oxidizes lipoamide to reset PDH)

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Is FAD/FADH2 a prosthetic group?

YES, it is always bound to E3

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Special property of FAD/FADH2

It can be detected in Nickel affinity column (yellow/orange color due to flavin)

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TPP Vitamin

Thiamine (B1)

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TPP Function

Decarboxylation, its carbon can lose a hydrogen, become negative REACTIVE

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Is TPP a prosthetic group?

Yes, it is always bound to E1

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Coenzyme A (CoA) Vitamin

Pantothenic acid (B5)

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CoA Function

Acyl group transfers (gains acetyl from Lipoamide)

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Is CoA a prosthetic group?

No, it is not always bound to an enzyme.

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Unique property of CoA

Sulfur, where acyl group attaches

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Lipoamide Vitamin

Lipoic acid

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Lipoamide Functions

  • Redox (disulfide bond readily reduced)

  • Acyl group transfers (reduced sulfhydryl)

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Is lipoamide a prosthetic group?

Yes, it’s always attached to E2

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PDH Reaction

Glucose (C6) → 2 Pyruvate → 2 Acetyl-CoA + 2CO2

Acetyl-CoA goes to TCA cycle to generate ATP via oxidative phosphorylation.

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E1 Coenzyme

TPP

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E2 Coenzyme

Lipoamide

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E3 coenzyme

FAD/FADH2

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PDH Steps

  1. Decarboxylation (E1/TPP)

  2. Reduction and transacetylation (E1/E2 and TPP/LA)

  3. Reposition acetyl group (E2/LA)

  4. Transthiolation (E2/LA and CoA)

  5. Re-oxidation of LA (E2/E3 and LA/FAD)

  6. Re-oxidation of FAD (E3/FAD and NAD)

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PDH Complex

Supercomplex of 3 enzymes and their coenzymes

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Unique feature of Lipoamide

“Lipoamide arms” and multiple functions

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Steps 1-4 of PDH

Form Acetyl-CoA

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Steps 5 and 6 of PDH

Reset the cycle by re-oxidation reactions