SN5
SHORT NOTE
5M
★ High-yield
Define Km. Describe its significance with the example of hexokinase and glucokinase.
Definition
Michaelis constant (Km) = the substrate concentration at which the reaction velocity is half of Vmax.
- Units are those of concentration (mol/L or mmol/L)
- Derived from the Michaelis–Menten equation: v = Vmax[S] / (Km + [S])
Significance
- Km is inversely related to the affinity of the enzyme for its substrate
– Low Km → high affinity
– High Km → low affinity
- It is a constant for a given enzyme–substrate pair under stated conditions, and is independent of enzyme concentration — unlike Vmax
- When a single enzyme acts on several substrates, the one with the lowest Km is the preferred (physiological) substrate
- It indicates whether an enzyme is operating near saturation at physiological substrate levels
- It is used to identify isoenzymes and to detect inhibitors
Physiological Illustration — Hexokinase VS Glucokinase
| Feature | Hexokinase | Glucokinase |
|---|---|---|
| Km | 0.05 mM (low) | 10 mM (high) |
| Affinity | High | Low |
| Tissue | All tissues, including brain and RBC | Liver and pancreatic β cell |
| Inhibited by G6P | Yes | No |
| Induced by insulin | No | Yes |
| Role | Ensures a constant supply to vital tissues | Removes excess glucose after a meal; acts as the glucose sensor |
CLINICAL PEARL
The consequence: the brain, with its low-Km hexokinase, takes glucose at all times; the liver, with its high-Km glucokinase, takes it only when there is plenty. The brain is therefore never starved by the liver.
Effect of Inhibitors on KM
| Inhibition | Km | Vmax |
|---|---|---|
| Competitive | ↑ | Unchanged |
| Non-competitive | Unchanged | ↓ |
| Uncompetitive | ↓ | ↓ |
Applied Aspects
- MODY-2 — a glucokinase mutation raises its Km so the β cell senses glucose at a higher threshold → mild stable hyperglycaemia
- Alcohol dehydrogenase has a low Km and is saturated at low blood alcohol → ethanol is eliminated at a constant rate (zero order)
- Drug design — a competitive inhibitor is effective only if its affinity exceeds that of the natural substrate
All questions in Enzymes
- Q1Define enzymes. Classify them according to the IUBMB system and describe the mechanism of enzyme action. Add a note on enzyme specificity. ★
- Q2Describe the effect of substrate concentration on enzyme activity. Explain the Michaelis–Menten equation, the significance of Km and the Lineweaver–Burk plot. ★
- Q3Classify enzyme inhibition. Describe competitive, non-competitive and uncompetitive inhibition with their effects on Km and Vmax, and give examples. ★
- Q4Describe the mechanisms regulating enzyme activity. Explain allosteric regulation, covalent modification and enzyme induction with examples. ★
- Q5Describe the diagnostic importance of serum enzymes. Give the enzyme profile in myocardial infarction and liver disease. Add a note on isoenzymes. ★
- SN1Define coenzymes. Enumerate the coenzymes derived from vitamins with their functions. ★
- SN2Define isoenzymes. Describe the isoenzymes of lactate dehydrogenase and creatine kinase with their clinical significance. ★
- SN3Describe allosteric enzymes with their characteristics and examples. Add a note on feedback inhibition. ★
- SN4Define zymogens. Give examples and explain their physiological importance. ★
- SN5Define Km. Describe its significance with the example of hexokinase and glucokinase. ★
- SN6Describe competitive inhibition with its characteristics and medically important examples. ★
- SN7Describe the types of enzyme specificity with examples.