Enzymes and Catalysis

Key Terms and Definitions

This section introduces essential vocabulary for understanding enzyme function and regulation in biological systems.

• Substrate: The specific reactant that an enzyme acts upon during a chemical reaction.

• Enzyme: A biological catalyst, typically a protein, that accelerates chemical reactions without being consumed.

• Catalyst: Any substance that increases the rate of a chemical reaction by lowering the activation energy, without itself undergoing permanent change.

• Active Site: The region on an enzyme where substrate molecules bind and undergo a chemical reaction.

• Activation Energy: The minimum energy required to initiate a chemical reaction.

• Transition State: A high-energy, unstable state during a reaction in which old bonds are breaking and new bonds are forming.

• Coenzyme: An organic non-protein molecule that assists enzymes in catalyzing reactions.

• Cofactor: A non-protein chemical compound (often a metal ion) required for enzyme activity.

• Competitive Inhibition: A type of enzyme inhibition where an inhibitor competes with the substrate for binding to the active site.

• Non-competitive Inhibition: Inhibition where the inhibitor binds to a site other than the active site, altering enzyme function.

• Allosteric Regulation: Regulation of enzyme activity through binding of molecules at sites other than the active site (allosteric sites).

• Allosteric Site: A specific site on an enzyme, distinct from the active site, where regulatory molecules bind.

Enzyme Function and Reaction Mechanisms

Enzymes are crucial for facilitating biochemical reactions by lowering activation energy and increasing reaction rates.

• Spontaneous Reactions and Enzymes: Even spontaneous reactions may require enzymes because the activation energy barrier can be too high for the reaction to proceed at a biologically relevant rate.

• Limits to Catalysis: Factors such as substrate concentration, enzyme concentration, temperature, pH, and presence of inhibitors can limit the rate of catalysis.

• Transition State: Enzymes stabilize the transition state, reducing the activation energy required for the reaction.

• Induced Fit Model: The enzyme changes shape upon substrate binding, enhancing the fit and facilitating catalysis.

• Primary Structure and Affinity: The amino acid sequence of an enzyme determines its three-dimensional structure and thus its substrate affinity.

• Enzyme-Substrate Interactions: The number and type of chemical interactions between an enzyme and its substrate contribute to binding strength and specificity.

Enzyme Catalysis and Reaction Types

Enzymes can catalyze a wide variety of chemical reactions, each with specific mechanisms and requirements.

• Types of Catalyzed Reactions: Enzymes catalyze reactions such as hydrolysis, oxidation-reduction, transfer of functional groups, and isomerization.

• Coenzymes, Cofactors, and Prosthetic Groups:

  • Coenzyme: Organic molecule, often derived from vitamins, that assists in enzyme activity (e.g., NAD+).

  • Cofactor: Inorganic ion (e.g., Mg2+, Zn2+) required for enzyme function.

  • Prosthetic Group: Tightly bound cofactor or coenzyme essential for enzyme activity.

Enzyme Inhibition and Regulation

Cells regulate enzyme activity through various mechanisms to maintain homeostasis and respond to environmental changes.

• Competitive vs. Non-competitive Inhibition:

  • Competitive Inhibition: Inhibitor resembles the substrate and competes for the active site, reducing enzyme activity.

  • Non-competitive Inhibition: Inhibitor binds to an allosteric site, changing enzyme conformation and reducing activity regardless of substrate concentration.

• Allosteric Regulation: Regulatory molecules bind to allosteric sites, causing conformational changes that increase or decrease enzyme activity.

• Regulation of Enzyme Activity: Cells regulate enzymes to control metabolic pathways, prevent wasteful reactions, and respond to signals.

Table: Comparison of Competitive and Non-competitive Inhibition

Key Equations

• Michaelis-Menten Equation:

• Activation Energy:

Examples and Applications

• Example: Hexokinase is an enzyme that catalyzes the phosphorylation of glucose in glycolysis, requiring Mg2+ as a cofactor.

• Application: Enzyme inhibitors are used as drugs to treat diseases by modulating enzyme activity (e.g., statins inhibit HMG-CoA reductase in cholesterol synthesis).

Additional info: Academic context and definitions have been expanded for clarity and completeness.