Difference Between Hexokinase and Glucokinase

The main difference between hexokinase and glucokinase is that the hexokinase is an enzyme present in all cells whereas the glucokinase is an enzyme only present in the liver. Furthermore, hexokinase has a high affinity towards glucose while glucokinase has a low affinity towards glucose.

Hexokinase and glucokinase are two enzymes involved in the conversion of glucose into glucose 6-phosphate (G6P) during glycolysis.

Key Areas Covered

1. What is Hexokinase
     – Definition, Types, Role
2. What is Glucokinase
     – Definition, Properties, Role
3. What are the Similarities Between Hexokinase and Glucokinase
     – Outline of Common Features
4. What is the Difference Between Hexokinase and Glucokinase
     – Comparison of Key Differences

Key Terms

Affinity, Cellular Localization,  Glucokinase, Glucose, Glucose 6-phosphate (G6P), Hexokinase

What is Hexokinase

Hexokinase is the enzyme responsible for the conversion of glucose into glucose 6-phosphate. It can phosphorylate other hexoses apart from D-glucose such as D-fructose, 5-keto-D-fructose, 2-deoxy-D-glucose, D-mannose, and D-glucosamine. The four main types of hexokinases are hexokinase I, II, III, and IV. The first three hexokinases, hexokinase I-III, can be found in all cells of the body while hexokinase IV or glucokinase is only found in the liver cells and β-pancreatic cells.

Figure 1: Hexokinase Action

G6P inside the cells are used to maintain the diffusion gradient towards the cells from the blood, maintaining a continuous glucose uptake by the cells. The G6P is not freely diffusable out of the cell. Also, the Km value and the Vmax of hexokinase I-III is low, giving a high affinity towards glucose and low capacity, respectively. The low capacity of the enzyme helps to saturate the enzyme even in the presence of low levels of glucose. Hexokinase I-III are allosteric enzymes, which are inhibited by the high amounts of G6P. This keeps glucose in the bloodstream and the glucose can be then taken up by the liver.

What is Glucokinase

Glucokinase is a type of hexokinase only found inside the liver cells and β-pancreatic cells. They have a vital role in carbohydrate metabolism. It is also called hexokinase IV or hexokinase D and encoded by the GCK gene on the chromosome 7 in humans. The only substrate of glucokinase is D-glucose and the enzyme is only functional at high glucose levels since it has a low affinity towards glucose. Glycolysis slows down in the liver and glucose in the liver is mainly used to synthesize glycogen in a process known as glycogenesis. However, glucokinase is stimulated by insulin in the presence of high blood glucose levels to undergo glycolysis.

Figure 2: Effect of Insulin on Glucokinase

During starvation, under low blood glucose concentrations, the liver does not take up glucose; hence, glucokinase is not functional due to low glucose levels.

Similarities Between Hexokinase and Glucokinase

• Hexokinase and glucokinase are two types of enzymes involved in glycolysis.
• They are isoenzymes, enzymes with different structure and the same function.
• Both convert glucose into glucose 6-phosphate.
• They are rate-limiting enzymes in the glycolysis.
• These enzymes trap glucose inside the cell since G6P cannot freely diffuse out of the cell.

Difference Between Hexokinase and Glucokinase

Definition

Hexokinase refers to any of a group of enzymes that accelerate the phosphorylation of hexoses (as in the formation of glucose-6-phosphate from glucose and ATP) in carbohydrate metabolism while glucokinase refers to a hexokinase found especially in the liver, which catalyzes the phosphorylation of glucose in metabolic processes.

Found in

Hexokinase is found in each metabolizing cell in the body while glucokinase is only found in liver cells and β-pancreatic cells.

Type of Substrate

Furthermore, hexokinase acts upon hexose sugars including glucose while glucokinase acts only on glucose.

Km Value and Affinity

Also, hexokinase has a low Km value; hence, it has a higher affinity towards glucose while glucokinase has a high Km value; hence, it has a lower affinity towards glucose.

Vmax

Moreover, hexokinase has a low Vmax; hence, it is saturated in low glucose levels as well while glucokinase has a high Vmax; hence, it has a high capacity towards glucose.

Importance

Besides, hexokinase is involved in the production of energy in normal cells while glucokinase reduces glycolysis in liver cells.

Feedback Control

Additionally, the amount of G6P regulates the hexokinase while no feedback effect acts on glucokinase.

Effect of Insulin

Further on, the insulin has no effect on hexokinase while insulin stimulates glucokinase.

Conclusion

Hexokinase is an enzyme found in all living cells, involved in the phosphorylation of glucose. On the other hand, glucokinase is a type of hexokinase only found in the liver. Hexokinase has a high affinity towards glucose while glucokinase has a low affinity towards glucose. Therefore, hexokinase helps in glycolysis in normal cells while glucokinase facilitates glycogenesis in the liver. The main difference between hexokinase and glucokinase is the cellular localization and the role.

Reference:

1. “Glycolysis.” BiologyGuide.net, Available Here

Image Courtesy:

1. “Hexokinase-glucose” By Jmun7616 – Own work (Public Domain) via Commons Wikimedia 
2. “Glucose Insulin Release Pancreas” By Aydintay – Own work (Public Domain) via Commons Wikimedia