Which of the following best explains why an enzyme hasn\'t evolved to catalyze t

Question

Which of the following best explains why an enzyme hasn't evolved to catalyze the following reaction as the first step of glycolysis?

glucose + inorganic phosphate glucose-6-phosphate

a. The use of ATP is required for this step to serve as a regulatory control point.

b. The use of ATP is important because it helps keep the concentration low. Otherwise the high [ATP] would slow glycolysis too much through feedback inhibition.

c. The large positive ?G°' of this reaction would need to be overcome by an exceedingly high [Pi].

d. What are you talking about? That's exactly what does happen.

e. None of the above provide a reasonable explanation.

I know the answer is not d, but I'm not sure about the rest. Please help : )

Explanation / Answer

Regulation of Glycolysis

a metabolic pathway can be regulated in several ways:

1. Availability of substrate

2. Concentration of enzymes responsible for rate-limiting steps

3. Allosteric regulation of enzymes

4. Covalent modification of enzymes (e.g. phosphorylation)

Of the 10 steps in the glycolytic pathway, three involve large negative DG and are essentially irreversible. These are steps 1 (phosphorylation of glucose), 3 (phosphorylation of fructose-6-phosphate) and 10 (transfer of phosphate from phosphoenolpyruvate to ADP).

The enzymes responsible for catalyzing these three steps, hexokinase (or glucokinase) for step 1, phosphofructokinase for step 3, and pyruvate kinase for step 10, are the primary steps for allosteric enzyme regulation. Generally, enzymes that catalyze essentially irreversible steps in metabolic pathways are potential sites for regulatory control. Availability of substrate (in this case, glucose), is another general point for regulation.

The concentration of these three enzymes in the cell is regulated by hormones that affect their rates of transcription. Insulin is a peptide hormone secreted by pancreatic b cells in response to sudden increases in blood glucose levels, such as after a carbohydrate-rich meal. It is also secreted in response to increases in amino acid levels in the blood, as well as in response to gastrointestinal and other hormones (insulin will be revisited in future discussions of lipid metabolism and diabetes). The general effect of insulin is to promote the storage of energy when food is available in abundance. Glucagon is a different peptide hormone secreted by the pancreatic a cells. Its secretion is stimulated by low blood glucose levels, and its general effect is to oppose the action of insulin. Insulin upregulates the transcription of glucokinase, phosphofructokinase, and pyruvate kinase, while glucagon downregulates their transcription. These effects take place over a period of hours to days, and generally reflect whether a person is well-fed or starving

Hence a. The use of ATP is required for this step to serve as a regulatory control point.

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