Overview
The transcript explains hexokinase’s role in glycolysis, its induced fit mechanism, the requirement of magnesium ions, and contrasts hexokinase with glucokinase including Km concepts.
Hexokinase: Role and Reaction
- First enzyme of glycolysis; converts glucose to glucose-6-phosphate using ATP.
- Can phosphorylate other hexose sugars into their phosphorylated forms.
- Reaction involves glucose’s sixth carbon hydroxyl reacting with ATP’s phosphate.
- Final product forms after positioning glucose and ATP in the active site.
Induced Fit Model
- Hexokinase exemplifies induced fit; substrate binding changes enzyme conformation.
- Binding of glucose and ATP induces active site shape change.
- Two enzyme domains move closer upon substrate binding.
- Conformational change brings glucose and ATP into proximity for reaction.
Role of Magnesium Ions (Mg2+)
- Mg2+ is required for the hexokinase reaction to proceed.
- Positively charged Mg2+ shields negative charges on ATP’s phosphate oxygens.
- Shielding reduces electrostatic repulsion with glucose’s hydroxyl group.
- Without Mg2+, negative charges repel; the reaction does not occur.
Hexokinase vs Glucokinase
- Hexokinase is present in all body cells; glucokinase is present in liver cells only.
- Hexokinase acts on multiple hexose sugars; glucokinase acts only on glucose.
- Hexokinase has low Km; glucokinase has high Km.
- Low Km means effective function at low substrate concentration.
- High Km means requires higher substrate concentration to function.
Km Concept and Implications
- Km is the substrate concentration where 50% of enzyme sites are occupied.
- At Km, half of enzyme molecules form the enzyme-substrate (ES) complex.
- Hexokinase reaches 50% ES at lower substrate levels than glucokinase.
- Glucokinase requires a larger substrate amount to reach 50% ES.
Comparison Table
| Feature | Hexokinase | Glucokinase |
|---|
| Location | All body cells | Liver cells |
| Substrate specificity | Multiple hexose sugars | Glucose only |
| Role in pathway | First enzyme of glycolysis | Phosphorylates glucose in liver |
| Km value | Low Km | High Km |
| Substrate requirement | Functions at low substrate levels | Requires high substrate levels |
Key Terms & Definitions
- Induced fit: Substrate binding triggers active site and enzyme conformational change.
- Km: Substrate concentration at which 50% of enzyme active sites are occupied.
- Enzyme-substrate (ES) complex: Temporary complex formed when enzyme binds substrate.
Action Items / Next Steps
- Review glycolysis steps focusing on initial phosphorylation.
- Practice explaining how Mg2+ facilitates phosphoryl transfer.
- Compare low vs high Km with examples of substrate concentrations.