Hexokinase

Definition

Hexokinase is a crucial enzyme involved in the initial step of glycolysis, the metabolic pathway that breaks down glucose to generate energy in the form of ATP. It catalyzes the phosphorylation of glucose to glucose-6-phosphate, which is the first committed step in glucose metabolism.

5 Must Know Facts For Your Next Test

1. Hexokinase is responsible for the first committed step of glycolysis, the phosphorylation of glucose to glucose-6-phosphate.
2. There are four different isoforms of hexokinase (I, II, III, and IV) that are expressed in different tissues and have varying affinities for glucose.
3. Hexokinase IV, also known as glucokinase, is the predominant isoform found in the liver and has a lower affinity for glucose compared to the other hexokinase isoforms.
4. The phosphorylation of glucose by hexokinase traps the glucose inside the cell, preventing it from diffusing back out and allowing it to enter further metabolic pathways.
5. Hexokinase activity is regulated by the availability of its substrate, glucose, as well as by allosteric inhibition by its product, glucose-6-phosphate.

Review Questions

• Explain the role of hexokinase in the glycolytic pathway and its significance in glucose metabolism.
  • Hexokinase catalyzes the first committed step of glycolysis by phosphorylating glucose to glucose-6-phosphate. This reaction traps the glucose inside the cell, preventing it from diffusing back out, and allows it to enter further metabolic pathways. The phosphorylation of glucose by hexokinase is a crucial control point in glucose metabolism, as it commits the glucose to be broken down for energy production through glycolysis or redirected to other metabolic processes, such as glycogen synthesis or the pentose phosphate pathway.
• Describe the different isoforms of hexokinase and their tissue-specific expression and regulation.
  • There are four main isoforms of hexokinase (I, II, III, and IV) that are expressed in different tissues and have varying affinities for glucose. Hexokinase I, II, and III have a high affinity for glucose and are found in tissues with high glucose utilization, such as the brain, heart, and skeletal muscle. Hexokinase IV, also known as glucokinase, is the predominant isoform in the liver and has a lower affinity for glucose, allowing it to respond more effectively to changes in blood glucose levels. The activity of these hexokinase isoforms is regulated by allosteric inhibition by their product, glucose-6-phosphate, as well as by the availability of their substrate, glucose.
• Analyze the significance of hexokinase's role in the context of both glycolysis and gluconeogenesis, and explain how it contributes to the overall regulation of glucose homeostasis.
  • Hexokinase plays a crucial role in the regulation of glucose metabolism, as it is involved in both the catabolic process of glycolysis and the anabolic process of gluconeogenesis. In glycolysis, hexokinase catalyzes the first committed step, phosphorylating glucose to glucose-6-phosphate, which allows the glucose to be broken down for energy production. Conversely, in gluconeogenesis, the reverse reaction, where glucose-6-phosphate is dephosphorylated to glucose, is catalyzed by the enzyme glucose-6-phosphatase. The balance between the activities of hexokinase and glucose-6-phosphatase is a key factor in maintaining glucose homeostasis, as it regulates the partitioning of glucose between energy-producing pathways and glucose-producing pathways. This fine-tuned regulation of glucose metabolism by hexokinase is essential for maintaining proper blood glucose levels and ensuring that the body's energy needs are met.

Related terms

Glycolysis:

Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating a small amount of ATP in the process. It is the first stage of carbohydrate catabolism.

Gluconeogenesis:

Gluconeogenesis is the metabolic pathway that synthesizes glucose from non-carbohydrate precursors, such as amino acids, lactate, and glycerol. It is the reverse of glycolysis and plays a crucial role in maintaining blood glucose levels.

Glucose-6-phosphate: Glucose-6-phosphate is the product of the hexokinase-catalyzed reaction, where glucose is phosphorylated to form this key intermediate in glucose metabolism.