Hexokinase is a:
**Core Concept**
Hexokinase is a crucial enzyme in glucose metabolism, specifically involved in the first step of glycolysis. It catalyzes the transfer of a phosphate group from ATP to glucose, resulting in the formation of glucose-6-phosphate (G6P). This reaction is essential for the regulation of glucose levels within the cell.
**Why the Correct Answer is Right**
Hexokinase is a type of hexokinase-1 (HK1), which is the most abundant form of hexokinase in the liver and brain. This enzyme is responsible for phosphorylating glucose to G6P, thereby trapping glucose within the cell and facilitating its subsequent breakdown through glycolysis. The reaction is also reversible, but under physiological conditions, the forward reaction is favored due to the high concentration of ATP. The enzyme is inhibited by its product, G6P, providing a negative feedback mechanism to regulate glucose metabolism.
**Why Each Wrong Option is Incorrect**
**Option A:** Glucokinase is another enzyme that phosphorylates glucose, but it is primarily found in the liver and pancreas, and has a higher Km for glucose compared to hexokinase.
**Option B:** Phosphofructokinase is an enzyme involved in the glycolytic pathway, but it catalyzes a different reaction, phosphorylating fructose-6-phosphate to fructose-1,6-bisphosphate.
**Option C:** Pyruvate kinase is another enzyme in the glycolytic pathway, but it catalyzes the final step, converting phosphoenolpyruvate (PEP) to pyruvate.
**Clinical Pearl / High-Yield Fact**
Hexokinase inhibitors, such as 2-deoxyglucose, can be used to inhibit glycolysis and induce cell death in cancer cells. This is because cancer cells often have high rates of glycolysis, even in the presence of oxygen, a phenomenon known as the Warburg effect.
**Correct Answer:** C.