Question

Conversion of glucose to glucose-6-phosphate, is catalyzed by _______________

• A. Aldolase
• B. Enolase
• C. Phosphofructokinase
• D. Hexokinase

Hint:

The prefix "hexo-" means six (referring to the six-carbon glucose), and "-kinase" refers to any enzyme that transfers a phosphate group from a high-energy molecule like ATP. This naming convention makes the correct option easy to identify.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Glycolysis is the first metabolic pathway of cellular respiration, occurring in the cytosol of almost all living cells.
The pathway begins with the activation of a stable glucose molecule, preparing it for subsequent cleavage into three-carbon sugars.

Step 2: Detailed Explanation:
The first step of glycolysis involves the phosphorylation of glucose to form glucose-6-phosphate:
- This reaction is an irreversible, rate-limiting step that traps the glucose molecule inside the cell because the charged phosphate group prevents it from crossing the plasma membrane.
- This phosphorylation is catalyzed by the enzyme hexokinase (or glucokinase in liver cells), which transfers a phosphate group from adenosine triphosphate (ATP) to the hydroxyl group on the sixth carbon of glucose:
\[ \text{Glucose} + \text{ATP} \xrightarrow{\text{Hexokinase, Mg}^{2+}} \text{Glucose-6-phosphate} + \text{ADP} + \text{H}^+ \] - Let us review the other enzymes listed:
Aldolase: Catalyzes the cleavage of fructose-1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
Enolase: Catalyzes the dehydration of 2-phosphoglycerate to form phosphoenolpyruvate (PEP) in the later stages of glycolysis.
Phosphofructokinase (PFK): Catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, which is the main regulatory step of glycolysis.
Thus, hexokinase is the correct enzyme for the initial phosphorylation step.

Step 3: Final Answer:
The correct option is (D).