Unit 4: Enzymes and Vitamins

1. Nomenclature, Classification, and Properties of Enzymes

Enzymes are biological catalysts that speed up chemical reactions without being consumed in the process. They are primarily globular proteins with high specificity.

Nomenclature and Classification

Enzymes are generally named by adding the suffix "-ase" to the substrate they act upon (e.g., Lactase acts on Lactose). The IUBMB classifies enzymes into six major classes:

1. Oxidoreductases: Catalyze oxidation-reduction reactions.
2. Transferases: Transfer functional groups (e.g., methyl or phosphate groups).
3. Hydrolases: Catalyze the cleavage of bonds by addition of water.
4. Lyases: Catalyze the removal of groups to form double bonds.
5. Isomerases: Catalyze structural shifts within a molecule (isomers).
6. Ligases: Catalyze the joining of two molecules using ATP energy.

General Properties

• Catalytic Power: They increase reaction rates by 10^6 to 10^12 times.
• Specificity: Each enzyme is highly specific to its substrate and the reaction it catalyzes.
• Regulation: Their activity can be controlled by inhibitors, activators, and pH/temperature changes.

2. Mechanism of Enzyme Action

Enzymes work by lowering the activation energy required for a reaction to proceed. This is achieved through the formation of an Enzyme-Substrate (ES) complex at the Active Site.

Models of Enzyme-Substrate Binding

• Lock and Key Model: Proposed by Emil Fischer; suggests the active site has a rigid shape that exactly fits the substrate.
• Induced Fit Model: Proposed by Koshland; suggests the active site is flexible and changes shape to fit the substrate upon binding.

3. Enzyme Kinetics: Km, Vmax, and L-B Plot

Enzyme kinetics studies the rate of enzyme-catalyzed reactions.

Key Concepts

• Vmax: The maximum velocity of a reaction when the enzyme is saturated with substrate.
• Km (Michaelis Constant): The substrate concentration at which the reaction velocity is half of Vmax. It indicates the affinity of an enzyme for its substrate (Low Km = High affinity).

Lineweaver-Burk (L-B) Plot

Also known as the double-reciprocal plot, it is used to determine Km and Vmax more accurately by plotting 1/v against 1/[S].

4. Factors Affecting Enzyme Activity

The rate of enzyme-catalyzed reactions is influenced by several environmental factors:

• Temperature: Most enzymes have an optimum temperature. High temperatures cause denaturation.
• pH: Changes in pH can alter the charge of amino acids at the active site, affecting binding.
• Substrate Concentration: Increasing [S] increases velocity until Vmax is reached.
• Enzyme Concentration: Rate is directly proportional to enzyme concentration, provided substrate is in excess.

5. Vitamins: Classification and Functions

Vitamins are organic compounds required in small amounts for normal growth and metabolic processes.

Classification of Vitamins

6. Exam Focus Enhancements