Hexokinase is –
**Core Concept**
Hexokinase is a key enzyme involved in the first step of glycolysis, catalyzing the transfer of a phosphate group from ATP to glucose, resulting in the formation of glucose-6-phosphate (G6P). This irreversible reaction commits glucose to the glycolytic pathway, ensuring its utilization for energy production. The enzyme plays a crucial role in glucose metabolism in various cells, including red blood cells, liver cells, and muscle cells.
**Why the Correct Answer is Right**
Hexokinase is responsible for phosphorylating glucose to G6P, which is then further metabolized through glycolysis. The enzyme has a high affinity for glucose, allowing it to efficiently phosphorylate glucose at low concentrations. The reaction is facilitated by the presence of magnesium ions, which stabilize the enzyme-substrate complex. The product, G6P, is then either further metabolized through glycolysis or stored in the liver as glycogen.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect as hexokinase is not directly involved in the hexose monophosphate shunt (HMP shunt), which is a separate pathway that generates NADPH and pentoses from glucose-6-phosphate.
**Option B:** This is incorrect as phosphofructokinase (PFK) is a different enzyme that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate in the glycolytic pathway.
**Option C:** This is incorrect as pyruvate kinase is an enzyme involved in the final step of glycolysis, catalyzing the conversion of phosphoenolpyruvate (PEP) to pyruvate.
**Clinical Pearl / High-Yield Fact**
In the absence of glucose, hexokinase can phosphorylate other sugars, such as fructose, mannose, and glucosamine, highlighting its broad substrate specificity. This property is essential for the enzyme's role in various cellular processes, including the metabolism of dietary sugars.
**Correct Answer: A. Hexokinase is a crucial enzyme involved in glucose metabolism, catalyzing the first step of glycolysis by phosphorylating glucose to glucose-6-phosphate.**