U01.05.001 Enzyme kinetics

Enzyme kinetics is the study of the rates of enzyme-catalyzed reactions and how these rates change in response to various conditions, including substrate concentration, enzyme concentration, and the presence of inhibitors. Understanding enzyme kinetics is crucial for various applications in biochemistry, pharmacology, and biotechnology.

Key Concepts in Enzyme Kinetics

1. Michaelis-Menten Kinetics

Definition: A model that describes the rate of enzyme-catalyzed reactions based on substrate concentration.
Key Parameters:
- Km (Michaelis constant): Represents the substrate concentration at which the reaction velocity is half of Vmax. It is inversely related to the affinity of the enzyme for its substrate; lower Km indicates higher affinity.
- Vmax (maximum velocity): The maximum rate of reaction achieved by the system at saturation. It is directly proportional to enzyme concentration.

2. Saturation and Inhibition

Enzymatic reactions typically follow a hyperbolic curve, indicating that increasing substrate concentration leads to increased reaction velocity until saturation is reached.
In contrast, reactions exhibiting a sigmoid curve suggest cooperative kinetics (e.g., hemoglobin).

3. Types of Inhibitors

Competitive Inhibitors:
- Resemble the substrate and bind to the active site.
- Can be overcome by increasing substrate concentration.
- Effects:
  - Increases Km (lower affinity)
  - Vmax remains unchanged.
Noncompetitive Inhibitors:
- Bind to an allosteric site, not the active site.
- Cannot be overcome by increasing substrate concentration.
- Effects:
  - Km remains unchanged.
  - Vmax decreases.

4. Lineweaver-Burk Plot

A double-reciprocal plot was used to determine Km and Vmax.
Interpretation:
- The closer to 0 on the Y-axis, the higher the Vmax.
- The closer to 0 on the X-axis, the higher the Km.
- Competitive inhibitors cross each other, while noncompetitive inhibitors do not.

5. Summary of Inhibitor Characteristics

Type of Inhibitor	Resembles Substrate	Overcome by Increased [S]	Binds Active Site	Effect on Vmax	Effect on Km
Competitive (Reversible)	Yes	Yes	Yes	Unchanged	↑
Competitive (Irreversible)	Yes	No	Yes	Unchanged	↑
Noncompetitive	No	No	No	↓	Unchanged

Summary of Inhibitor Characteristics

Points to Remember

Km is a measure of enzyme affinity; lower values indicate higher affinity.
Vmax is dependent on enzyme concentration.
Competitive inhibitors affect Km but not Vmax, while noncompetitive inhibitors decrease Vmax without changing Km.
The Lineweaver-Burk plot is useful for visualizing enzyme kinetics and determining kinetic parameters.

Bibliography

Berg, J. M., Tymoczko, J. L., & Stryer, L. (2015). Biochemistry (8th ed.). W.H. Freeman.
Voet, D., & Voet, J. G. (2011). Biochemistry (4th ed.). Wiley.
Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry (7th ed.). W.H. Freeman.
Michaelis, L., & Menten, M. L. (1913). “Die Kinetik der Invertinwirkung”. Biochemische Zeitschrift, 49, 333-369.

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