Question

Explain how cellular ATP is replenished by phosphagens (phosphoguanidines).

What are the cellular roles of nucleoside diphosphate kinase and adenylate kinase?

Why is a thioester bond a “high energy” bond?

What are the metabolic roles of NAD⁺ and FAD?

Answer 1

Explain how cellular ATP is replenished by phosphagens?

The ATP reserves in the body are very less and can provide energy for few only few moments. Hence, body made unique systems to regenerate ATP so that it can be replenished as when it is used by the metabolism.

One such system is Phosphagene system. For example muscle utilizes the Phosphagene system, to generate ATP as it cannot obtain ATP from blood. Phosphagene system consists of two reactions with which ATP is regenerated, namely

i. Creatinine Kinase reaction: The overall reaction is as follows:

Creatinine Phosphate + ADP + Inorganic Phosphate ---------Creatine kinase----------> ATP + Creatinine

Thus ADP is phosphorylated to produce the ATP. This reaction is the most immediate way to regenerate ATP. The creatinine kinase reaction involves two reactions that are coupled together to make them energetically favorable

Creatine Phosphate <----------> Creatine + inorganic phosphate (Exergonic reaction)

ADP + H+ + Inorganic phosphate <----------> ATP (Endergonic reaction)

Creatine kinase essential for this reaction is present in the mitochondrial membrane (4%), bound to myofibrillar proteins of sarcomere (3%) and remainder is present in the cytosolic environment of skeletal muscles

ii. Adenylate kinase reaction:

It involves regeneration of ATP from ADP through the reaction

ADP + ADP <---------adenylate kinase------> ATP + AMP

AMP serves as the allosteric activator for the above reaction during vigorous exercise. Continued increase in AMP leads to reduction of phosphorylation potential of the cell which is detrimental to the cell.

2. What are the cellular roles of nucleoside diphosphate kinase and adenylate kinase?

Ans. Nucleoside diphosphate kinase is resposible for regeneration of nucleoside triphosphates through the reversible reactions like:

UDP + ATP <------------> UTP + ADP (used for polysaccharide synthesis)

CDP + ATP <------------> CTP + ADP (used for lipid synthesis)

GDP + ATP <------------> GTP + ADP (used for protein elongation)

Role of adenylate kinase:

As explained in the first question, adenylate kinase is a myokinase which is found in muscle and responsible for catalysing the reactions of :

ADP + ADP <---------adenylate kinase------> ATP + AMP

During metabolism ATP is converted to ADP, and when ADP levels rise, it gets converted to ATP + AMP by Adenylate kinase. Thus produced ATP can provide energy for work. During rest AMP and ADP convert to ATP again

Why is a thioester bond a “high energy” bond?

Apart from ATP conversion to ADP and AMP, thioester bond is also a high energy bond. For example:

Acetyl CoA --------------H2O-----------> CoA SH + Acetate (Delta G⁰) = -31.5kJ /mole

This is a high energy bond which drives the TCA cycle in forward direction, The energy released in cleaving of Succinyl CoA thioester bond is used for the synthesis of high energy phosphate bond of GTP substrate level phosphorylation

Thus, the large negative Delata G value makes the thioster bond, a high energy bond

What are the metabolic roles of NAD+ and FAD?

Ans. NAD and FAD act as electron carriers. NAD is reduced to NADH in citric acid cycle and glycolysis, transfers electrons to electron transport. One molecule of NADH gives 3 molecules of ATP.

On the other hand, FAD is reduced to FADH2 in citric acid cycle and enter the electron transport chain. One molecule of FADH2 produces 2 molecules of ATP