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Biochemistry

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Carbohydrates

Question
37 out of 40
 

First substrate of Kreb's cycle is(AIIMS May 2007)

 

 
A Pyruvate
B Lysine
C Hydrochloric Acid
D Corticosteroids
Ans. A Pyruvate

Jet saebhat for kecbeycliareoxaloaectak&aeclyle COA

Mot given in the option you can mark purcualtear Ans. But you mens now that pyruvate in not a subotrh for krebcycle.

Krebs cycle or TCA cycle is process by which the acetyl CoA, gets completely oxidized to give CO2 and water.

There are three sources of acetyl CoA:

i. From glucose pyruvate acetyl CoA

ii. Fatty acid oxidation

iii. Ketogenic amino acid oxidation

Thus citric acid cycle acts as a common pathway for all three metabolites.

Hence the apt answer is A.

Some other relevant points regarding TCA cycle:

  1. Occurs in mitochondria
  2. Truly an aerobic pathway
  3. Amphibolic pathway
  4. Catabolic because generates ATP
  5. One whole cycle from acetyl CoA generates 10 A TP.
  6. Anabolic because the various intermediates of the cycle forms various compounds

a. Transaminases help in the formation of glutamate a nonessential amino acid from alpha ketoglutarate.

b. AST / SGOT forms aspartate from oxaloacetate.

c. Acetyl coA forms fatty acids. The building block of fatty acid is acetyl CoA.

 

How is it regulated?

a. It depends on the type of cell. In skeletal muscle, where the main purpose of TCA cycle is energy pro­duction, the cycle goes in a clockwise direction to gen­erate ATP.

b. The dehydrogenases are all activated by Calcium ions and the A TP / ADP ratio will be very low, so the inhibition on PDH is overruled, thus the cycle goes in a clockwise direction

c. In liver, where it is flooded with metabolic fuels, the main purpose is anabolic.

d. Citrate synthase is inhibited by high energy level, so oxaloacetate accumulates, which can be utilized for aspartate synthesis. Similarly, all dehydrogenases are allosterically inhibited by ATP, thus alpha ketoglutarate accumulates, which can be utilized for glutamate synthesis, Succinyl CoA can be utilized for heme synthesis.

- In adipose tissue, aconitase is inhibited and hence ci­trate accumulates which enters the cytoplasm, where it can be utilized for fatty acid synthesis.

 

 

Energetics of TCA cycle:

  1. Isocitrate dehydrogenase - 1 NADH – 2.5 A TP
  2. Alpha ketoglutarate dehydrogenase - 1 NADH – 2.5 A TP
  3. Succinylthiokinase - 1 A TP / 1 GTP
  4. Malate dehydrogenase - 1 NADH - 2.5 A TP - Totally 10 ATP

Carbohydrates Flashcard List

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