At what steps in the Krebs cycle are coenzymes reduced? How would you classify t

Question

At what steps in the Krebs cycle are coenzymes reduced? How would you classify the enzymes that catalyze those reactions? What occurs at step 5 in the Krebs Cycle? Summarize the products for each turn of the Krebs Cycle? As with the other stages of cellular respiration, regulation of the citric acid cycle is important in maintaining the needs of the cell. Where are the key sites of allosteric regulation in the Krebs Cycle? What enzymes catalyze those reactions? How do NADH levels affect activity of those enzymes?

Explanation / Answer

5. Steps in Kreb Cycle where coenzymes are reduced-

a) At Step 3- Isocitrate dehydrogenase complex converts isocitrate to alpha keto glutarate, reducing NAD+ to NADH

b)At Step 4- Alpha ketoglutarate dehydrogenase complex converts alpha Alpha ketoglutarate to Succinyl-S-CoA reducing NAD+ to NADH

c) Step 6- Succinate Dehydrogenase converts Succinate to Fumarate reducing FAD to FADH2

d) Step 8- Malate Dehydrogenase converts Malate to Oxaloacetate reducing NAD+ to NADH

All those enzymes come under dehydrogenases which is a sub class of enzyme Oxidoreductase. Oxidoreductase catalyze oxidation reduction reaction.These enzymes fall into six categories-Oxygenases,reductases,peroxidases,oxidases,hydroxylases and dehydrogenases. Dehydrogenases in particular oxidize a substrate by transferring a hydrogen to an electron acceptor mostly NAD+ or FAD

6) At step 5 of the Kreb Cycle

Succinyl CoA gets converted into Succinate by the enzyme SuccinylCoA Synthetase regenerating CoA-SH and producing a GTP molecule. This is a Substrate level Phosphorylation where a substrate is utilized for production of energy molecule in the form of GTP which is susbsequentlt converted to ATP.

7) Each turn of the Kreb Cycle yields 2 Carbon Dioxide ,3 NADH, 1FADH2, 1ATP or GTP

8) Key site of allosteric regulation in kreb Cycle are-

Step-1 Citrate synthase enzyme allosterically inhibited by ATP and long chain fatty acyl CoA

Step-3 Isocitrate dehydrogenase enzyme inhibited by NADH

Step 4-Alpha ketoglutarate Dehydrogenase enzyme inhibited by succinyl CoA and NADH

High Concentration of NADH inhibits these enzyme as high concentration of NADH ultimately signals energy rich state of the cell inhibiting these key enzymes.

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