Characteristics of Hexokinase
Hexokinase is characterized by having a low Km (high affinity for glucose), which is the correct answer to this question.
Key Features of Hexokinase
Hexokinase is a critical enzyme in glucose metabolism that catalyzes the first step of glycolysis by phosphorylating glucose to glucose-6-phosphate using ATP as a phosphoryl donor. Understanding its properties is essential for comprehending cellular energy metabolism.
Affinity for Glucose (Km)
- Low Km value: Hexokinase has a high affinity for glucose, allowing it to function efficiently even at low glucose concentrations 1
- This property enables cells to effectively utilize glucose for energy production even when glucose levels are relatively low
- The low Km is a critical adaptation that ensures priority access to glucose for vital tissues
Regulatory Properties
- Product inhibition: Hexokinase is inhibited by its product, glucose-6-phosphate 2
- This feedback inhibition serves as an important regulatory mechanism to prevent excessive phosphorylation of glucose when glucose-6-phosphate accumulates
- Unlike glucokinase (hexokinase IV), hexokinase is not directly regulated by insulin 3
Tissue Distribution
- Hexokinase is widely distributed throughout body tissues, not limited to liver and pancreatic islet cells
- It is particularly abundant in tissues with high energy demands such as brain, muscle, and heart
- This contrasts with glucokinase (hexokinase IV), which is primarily found in liver and pancreatic β-cells 3
Catalytic Efficiency
- While hexokinase has a high affinity for glucose (low Km), it has a relatively low Vmax compared to glucokinase
- This means that while it can function at low glucose concentrations, it reaches saturation quickly and cannot increase its activity proportionally at high glucose concentrations 1
Substrate Specificity
- Hexokinase can phosphorylate several hexoses beyond glucose, including mannose, fructose, and 2-deoxyglucose 4
- This broader substrate specificity distinguishes it from more selective kinases
Clinical Significance
Understanding hexokinase properties is important in several clinical contexts:
- Diabetes management: The different kinetic properties of hexokinase versus glucokinase help explain tissue-specific responses to glucose
- Cancer metabolism: Many cancer cells show upregulation of hexokinase activity as part of the Warburg effect
- Rare genetic disorders: Deficiencies in hexokinase can lead to hemolytic anemia due to its importance in red blood cell metabolism 1
The low Km value of hexokinase ensures that tissues with high energy demands can efficiently utilize glucose even when blood glucose levels are relatively low, making it a crucial enzyme for maintaining cellular energy homeostasis.