Question 5: 1) In the citric acid cycle, a five-carbon compound is decarboxylate

Question

Question 5: 1) In the citric acid cycle, a five-carbon compound is decarboxylated to yield an activated four- carbon compound. Show the substrate and product in this step, and indicate where any cofactor(s) participate(s) 2 CO is produced in two reactions in the citric acid cycle. For each of these reactions, name the reactant and product, name the enzyme, and show how any cofactors participate 30 In which reaction of the citric acid cycle does substrate-level phosphorylation occur? 4) Explain in quantitative terms the circumstances under which the following reaction can proceed L-Malate NAD oxaloacetate NADH H AG +29.7 kJ/mol 5) You are in charge of genetically engineering a new bacterium that will derive all of its ATP from sunlight by photosynthesis. Will you put the enzymes of the citric acid cycle in this organism? Briefly explain why or why not. 6 Germinating plant seeds can convert stored fatty acids into oxaloacetate and a variety of carbohydrates. Animals cannot synthesize significant quantities of oxaloacetate or glucose from fatty acids. What accounts for this difference?

Explanation / Answer

1)

In citric acid cycle the 5 carbon compound malate ( substrate) regenerates the activated four carbon compound oxaloacetate (product). The cofactor involved is NAD+ and it gets reduced to NADH + H+.

2)

In citric acid cycle CO2 is generated in two steps. First CO2 molecules is generated by oxidative decarboxylation of Isocitrate (substrate) to alpha - ketoglutrate. Second CO2 molecule is generated by conversion of alpha - ketoglutarate (substrate) to succinyl CoA (product) by oxidative decarboxylation.

3)

Substrate level phosphorylation is the process of transfer of phospahte group through the help of energy rich compound like ATP or GTP. In citric acid cycle, conversion of succinyl CoA to succinate is the reaction that required the substarte level phosphorylation and GTP is converted to GDP and Pi.

4)

The last reaction of the citric acid cycle is the conversion of malate to oxaloacetate. In this reaction NAD+ is reduced to NADH+H+. This reaction is catalyzed by malate dehydrogenase enzyme. The equilibrium of the reaction lies towards the left under standard thermodynamic conditions. In intact cells the OAA is continuously removed through the highly exergonic citrate synthase reaction. Due to this the concentration of OAA in the cell is very low and is about < 10-6 M and this concentration forces the malate dehydrogenase reaction forward to form OAA.

5

Photosynthetic bacterium is an aerobic organism and metabolize the carbons to reduce them to CO2. In this organism the enzymes of citric acid cycles already exists and useful for the reduction of carbons to CO2. Thus it is not necessary to put the enzymes of citric acid cycles in this organisms.

6

Animals cannot convert fatty acids or acetate derived from fatty acids into glucose this is becuase the conversion of phosphoenolpyruvate to pyruvate and pyruvate to acetyl CoA is an essentially irreversible reaction. In plants the acetate serves as energy rich fuel and also as the source of phospoenolpyruvate for carbohydrate synthesis. The plants posses enzymes of glyoxylate cycle to convert acetate into succinate and other intermediates of citric acid cycle. The enzymes of glyoxylate cycle occur in specialized peroxisomes called glyoxysomes. These enzymes are common in both glyoxylate cycle and citric acid cycle and are isozymes specific to mitochondria and glyoxysomes respectively.

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