Citric Acid Cycle - Fate of Pyruvate Flashcards
What happens to pyruvate upon aerobic respiration?
It is transformed into acetyl-CoA and enters the citric acid cycle
What catalyzes the conversion of pyruvate to acetyl-CoA?
Pyruvate dehydrogenase complex
Where in the cell does the formation of acetyl-CoA occur?
In the mitochondrial matrix (eukaryotes)
In the cytosol (prokaryotes)
What are the three enzymes that make up the pyruvate dehydrogenase complex?
- Pyruvate dehydrogenase (E1)
- Dihydrolipoyl transacetylase (E2)
- Dihydrolipoyl dehydrogenase (E3)
What are the 5 different coenzymes required for the PDC?
- NAD+
- FAD
- TPP
- Lipoic acid
- Coenzyme A (CoA)
How is coenzyme A activated?
Addition of acetyl or acyl group via a thioester linkage
From what compound is coenzyme A derived?
Pantothenic acid (Vitamin B5)
What is the business end of coenzyme A?
The SH group
What is the structure of lipoic acid in its oxidized form?
What is the structure of lipoic acid in its reduced or thiol form?
How is lipoic acid linked to an enzyme?
To a lysine residue via an amide bond, forming a lipoamide bond
What is the function of lipoic acid?
Acts like a flexible linker between two active sites and carries acyl groups (similar to biotin)
What is the overall reaction of the pyruvate dehydrogenase complex?
Oxidative decarboxylation
Does the pyruvate dehydrogenase complex catalyze reversible reactions?
No; it is irreversible
What is the first step in the pyruvate dehydrogenase complex?
Decarboxylation of pyruvate via pyruvate dehydrogenase
What coenzyme does pyruvate dehydrogenase require?
TPP
What is the mechanism of pyruvate dehydrogenase?
- TPP carbanion attacks keto group
- Electrons on terminal O kick back and expel carbon dioxide
- Lone pair on N kicks back and a new carbanion/hydroxylethyl-TPP forms
- Hydroxyethyl-TPP on E1 attacks lipoamide disulfide on E2
- TPP is eliminated, and hydroxyethyl group now on E2
What does E2 of the pyruvate dehydrogenase complex catalyze?
A transesterification reaction as the acetyl group is transferred to CoA and the lipoamide group is regenerated via oxidation by E3
What is the mechanism of E2 or dihydrolipoyl transacetylase?
During the reaction, the sulfhydryl group of coenzyme A carries out a nucleophilic attack on the carbonyl carbon of the acetyl group of acetyl dihydrolipoamide-dihydrolipoyl transacetylase to form a transient tetrahedral intermediate, that “decomposes” to dihydrolipoamide-dihydrolipoyl transacetylase and acetyl-CoA
What does E3 or dihydrolipoyl dehydrogenase catalyze?
E3 regenerates the lipoamide group via its own oxidation; E3 contains FADH2, and it transfers a hydride The enzyme catalyzes electron transfers needed to regenerate the disulfide bridge of the lipoyl group of dihydrolipoyl transacetylase, that is, to regenerate the oxidized form of the prosthetic group, and thus completing the catalytic cycle of the transacetylase.
What is the mechanism of E3 or dihydrolipoyl dehydrogenase (specifics not required)?
The enzyme catalyzes electron transfers needed to regenerate the disulfide bridge of the lipoyl group of dihydrolipoyl transacetylase, that is, to regenerate the oxidized form of the prosthetic group, and thus completing the catalytic cycle of the transacetylase.
The reaction has a ping-pong catalytic mechanism: it occurs in two successive half-reaction. In the first half-reaction the oxidation of dihydrolipoamide to lipoamide occurs; in the second half-reaction the reduction of NAD+ to NADH occurs.
Is FAD tightly bound to E3 or dihydrolipoyl dehydrogenase?
Yes
Is NAD+ tightly bound to E3 or dihydrolipoyl dehydrogenase?
No; it is considered a cosubstrate as it is reversibly bound. The electrons transferred to NADH are routed to the electron transport chain
The pyruvate dehydrogenase complex mechanism is irreversible and thus highly regulated. It commits carbons to what part of respiration?
Citric acid cycle or fatty acid synthesis
What are three allosteric activators of pyruvate dehydrogenase complex?
- AMP
- NAD+
- CoASH
What are four allosteric inhibitors of pyruvate dehydrogenase complex?
- ATP
- NADH
- Acetyl-CoA
- Fatty acids
Can the pyruvate dehydrogenase complex be covalently modified? If yes, how?
Yes via phosphorylation by pyruvate dehydrogenase kinase or dephosphorylation via pyruvate dehydrogenase phosphtase
When is the PDC activated?
When it is dephosphorylated via pyruvate dehydrogenase phosphatase
When is the PDC inactivated?
When it is phosphorylated by pyruvate dehydrogenase kinase
