Question

Pick one mechanism from List B. In words, describe how that enzyme carries out its catalysis in detail. You may use pictures, and you may also choose to use curved arrows. Your description should include words, not just images.

List B:

• PDHC

• Citrate synthase

• ATP synthase

Answer 1

PDHC (pyruvate dehydrogenase complex)..

Metabolic fate of pyruvate :

In aerobic conditions, pyruvate is converted into acetyl CoA . It enters the TCA cycle and further , it is Oxidized to carbon dioxide. ATP is generated.

Glycolysis takes place in the cytoplasm so, pyruvate is also generated in the cytoplasm. This is transported into the mitochondria through pyruvate transporter.

Pyruvate dehydrogenase complex :

Inside mitochondria, pyruvate is oxidatively decarboxylated into acetyl CoA by pyruvate dehydrogenase complex.

It is a multienzyme complex. It contains 5 coenzymes and 3 apoenzymes.

The enzyme part of PDH complex is made up of three component enzymes.

• Pyruvate dehydrogenase - It catalyzes oxidative decarboxylation. It requires TPP. Thiamine , B complex vitamin is essential for utilization of pyruvate.
• Dihydrolipoyl transacetylase
• Dihydrolipoyl dehydrogenase

The pyruvate dehydrogenase complex consists of 5 coenzymes that acts as carriers for the intermediate reaction. They are-

• Thiamine pyrophosphate(TPP)
• Coenzyme A
• FAD
• NAD+
• Lipoamide-it is lipoic acid or thioctic acid. It has 2 sulphur atoms and 8 carbon atoms. It can donate and accept hydrogen atoms.

Pyruvate dehydrogenase complex reaction :

Pyruvate Dehydrogenase Reaction PDHC Acetyl CoA Pyruvate T A Co ASH CO2 NAD- NADH+H+ HSCOA CH3-cous.L-SH - Thiamine - P.P Thiamine -P.P a CH3-Co-cool - Pyruvate CHLO- SCOA CO2 » C), CH-01 - TPP s-s SH-SH Reduced Lipoicaud r oxidized] Lipoic acid Enzymes 3 . FAD K 0 - Gruvate dehydrogenase (Enzyme 1)/E, NAD #+ FADH2 ^ NADT ② - Dihydrolipoll transacetylase /Ę (Enzyme 2) ③ - Dihydrolipays dehydrogenase (Enzyme 3)/E3

Steps involved in pyruvate dehydrogenase reaction:

• Pyruvate is decarboxylated to form hydroxyethyl derivative bound to Thiamine Pyrophosphate.
• It is then oxidized by transfer to the disulfide form of lipoic acid that is bound to E2.
• Acetyl group is then transferred to the CoA.
• The sulfhydryl form of lipoic acid is oxidized by dihydrolipoyl dehydrogenase enzyme.
• FADH​​​2  on E3 is reoxidized to FAD as NAD+ is reduced to NADH+H+ .

Regulation :

PDH kinase phosphorylates and inactivates E1 and PDH phosphatase dephosphorylates and activates E1 .

Other allosteric inhibitors are Acetyl CoA and NADH.

Importance of pyruvate dehydrogenase :

It is a completely irreversible process. PDH reaction is the committed step towards oxidation of glucose. The glucose is converted into acetyl CoA from which fatty acids can be synthesized. But, the backward reaction is not possible. There is no synthesis of glucose from fat.

Pyruvate is junction between various metabolic pathways. It may be converted into acetyl CoA which enters the citric acid cycle or it may be used for fatty acid synthesis or carboxylated to oxaloacetate which is used for gluconeogenesis.

Deficiency :

Deficiency of alpha subunit of E 1 component of pyruvate dehydrogenase complex leads to congenital lactic acidosis. The symptoms include neurodegeneration , muscular spasticity, In neonatal onset- there is early death.