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Biochemistry

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Carbohydrates

Question
28 out of 78
 

Which one of the following is the correct sequential order in which the given enzymes of Kreb’s cycle are formed after a molecule of acetyl CoA enters the cycle?



A Citrate, Oxaloacetate, Ketoglutarate

B Ketoglutarate, Oxaloacetate, Citrate

C Citrate, Ketoglutarate, Oxaloacetate

D Oxaloacetate, Ketoglutarate, Citrate

Ans. C Citrate, Ketoglutarate, oxaloacetate

See figure below-

* Citrate (C6) is the first product and oxaloacetate (C4) is the last product of TCA cycle Fig- The citric acid (Krebs) cycle. Oxidation of NADH and FADH2 in the respiratory chain leads to the generation of ATP via oxidative phosphorylation. In order to follow the passage of acetyl-CoA through the cycle, the 2 carbon atoms of the acetyl radical are shown labeled on the carboxyl carbon (using the designation [*] and on the methyl carbon (using the designation Although 2 carbon atoms are lost as CO2 in one revolution of the cycle, these atoms are not derived from the acetyl-CoA that has immediately entered the cycle but from that portion of the citrate molecule which derived from oxaloacetate. However, on completion of a single turn of the cycle. Because succinate is a symmetric compound and because succinate dehydrogenase does not differentiate between its 2 carboxyl groups, “randomization” of label occurs at this step such that all 4 carbon atoms of oxaloacetate appear to be labelled after one turn of the cycle. During glucogenesis, some of the label in oxaloacetate is incorporated into glucose and glycogen. The sites of inhibition (-) by fluoroacetate, malonate, and arsenite are indicated.

Table: Generation of high — energy phosphate in the catabolism of glucose

Pathway

Reaction catalyzed by

Method of (P) Production

Number of —(P) formed per mol of glucose (ATP)

Glycolysis Q

Glyceraldehyde-3- phosphate dehydrogenase

Respiratory chain oxidation of 2NADH

5

Phosphoglycerate kinase

Oxidation at substrate level

2

Pyruvate kinase

Oxidation at substrate level

2

9 (Total)

Allow for consumption of ATP by reactions catalyzed by hexokinase and phosphofructokinase

-2

Net

7

Citric Acid cycle Q

Pyruvate dehydrogenase

Respiratory chain oxidation of 2 NADH

5

Isocitrate dehydrogenase

Respiratory chain oxidation of 2 NADH

5

α-ketoglutarate dehydrogenase

Respiratory chain oxidation of 2NADH

5

Succinate thiokinase

Oxidation at substrate level

2

Succinate dehydrogenase

Respiratory chain oxidation of 2FADH2

3

Malate dehydrogenase

Respiratory chain oxidation of 2NADH

5

Net

25

Total per mol of glucose under aerobic conditions 32

Total per mol of glucose under anaerobic conditions 2

a. It is assumed that NADH formed in glycolysis is transported into mitochondria via the malate shuttle.

b. If the glycerophosphate shuttle is used, only 15 ATP would be formed per mole of NADH, the total net production being 30 instead of 32.

c. The calculation ignores the small loss of a ATP due to a transport of H into the mitochondria with pyruvate and a similar transport of H in the operation of the malate shuttle, totaling about I mol of ATP Note that there is a substantial benefit under anaerobic condition if glycogen is the starting point, since the net production of high-energy phosphate is increased from 2 to 3, as ATP is no longer required by the hexokinase reaction.

Carbohydrates Flashcard List

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