Enzymes: Structure, Function, and Classification

What are Enzymes?

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. Most enzymes are proteins, though some are RNA molecules. Enzyme names typically end in "-ase" and are often named after the substrate they act upon.

• Substrate: The reactant in an enzyme-catalyzed reaction.

• Turnover number: The number of substrate molecules converted to product per unit time.

• Cofactor: A nonprotein molecule (often a metal ion) required for enzyme activity.

• Coenzyme: An organic cofactor, often derived from vitamins, required for catalysis.

• Apoenzyme: The protein portion of an enzyme, without its cofactor/coenzyme.

• Holoenzyme: The catalytically active enzyme, including its cofactor/coenzyme.

• Active site: The region of the enzyme where substrate binding and catalysis occur.

How Enzymes Work as Catalysts

Enzymes speed up reactions by lowering the activation energy required for the reaction to proceed. They do not alter the equilibrium or the overall energy change of the reaction.

• Proximity effect: Enzymes bring reactants together.

• Orientation effect: Enzymes position reactants optimally for reaction.

• Energy effect: Enzymes induce strain in substrate bonds, lowering activation energy.

• Catalytic effect: Enzymes provide functional groups (acidic, basic, etc.) necessary for catalysis.

Enzyme Classes and Their Functions

The Six Major Classes of Enzymes

Enzymes are classified based on the type of reaction they catalyze:

• Oxidoreductases: Catalyze oxidation-reduction reactions.

• Transferases: Transfer functional groups between molecules.

• Hydrolases: Catalyze hydrolysis reactions (breaking bonds with water).

• Isomerases: Catalyze rearrangement of atoms within a molecule.

• Lyases: Add or remove groups to form double bonds or break double bonds.

• Ligases: Join two molecules together using energy from ATP hydrolysis.

Oxidoreductases

These enzymes catalyze oxidation and reduction reactions, often requiring coenzymes such as NAD+ or FAD.

• Dehydrogenases: Remove hydrogen atoms to form double bonds.

Transferases

Transferases move functional groups between molecules. Two important subclasses are:

• Kinases: Transfer phosphoryl groups (–PO32–).

• Transaminases: Transfer amino groups (–NH3+).

Hydrolases

Hydrolases catalyze the cleavage of bonds by adding water. Four key subclasses:

• Lipases: Hydrolyze ester bonds in lipids.

• Proteases: Hydrolyze peptide bonds in proteins.

• Nucleases: Hydrolyze phosphate ester bonds in nucleic acids.

• Amylases: Hydrolyze glycosidic bonds in starch.

Isomerases

Isomerases catalyze the rearrangement of atoms within a molecule to form isomers.

Lyases

Lyases catalyze the addition or removal of groups to/from double bonds.

• Decarboxylases: Remove CO2.

• Dehydrases: Remove H2O.

• Deaminases: Remove NH3.

• Synthases: Add small molecules to double bonds.

Ligases

Ligases join two molecules together, often using ATP hydrolysis for energy.

• Synthetases: Bond two substrate molecules.

• Carboxylases: Add CO2 to form carboxylate groups.

Enzyme Specificity and Substrate Binding

Specific vs. Nonspecific Enzymes

Enzyme specificity refers to the ability of an enzyme to select its substrate:

• Highly specific enzymes: Catalyze one type of reaction for a single substrate (e.g., urease hydrolyzes urea).

• Nonspecific enzymes: Catalyze a single reaction for a group of similar substrates (e.g., aminopeptidase releases N-terminal amino acids from polypeptides).

Induced Fit Model vs. Lock-and-Key Model

The induced fit model describes substrate binding more accurately than the lock-and-key model. In this model, substrate binding triggers a conformational change in the enzyme, optimizing the fit and facilitating catalysis.

• Lock-and-key model: Substrate fits into the enzyme without changing its shape.

• Induced fit model: Enzyme changes shape upon substrate binding.

Mechanism of Enzyme Action

Atomic-Level Steps in Enzyme Catalysis

Enzyme catalysis involves several steps, including substrate recognition, bond cleavage, and product release. The enzyme is recycled and ready for another reaction after the process.

• Enzyme recognizes substrate via chemical compatibility.

• Active site residues (e.g., Ser, His, Asp) participate in catalysis.

• Substrate is cleaved, products are released, and enzyme returns to its original state.

Factors Affecting Enzyme Activity

Substrate Concentration

As substrate concentration increases, the reaction rate increases until it reaches a maximum (saturation point), where all enzyme active sites are occupied.

Enzyme Concentration

With excess substrate, increasing enzyme concentration increases the reaction rate linearly.

Temperature

Enzyme activity increases with temperature up to an optimum, after which activity decreases due to denaturation.

pH

Each enzyme has an optimal pH range. Deviations from this range can reduce activity or denature the enzyme.

Summary Table: Enzyme Classes and Subclasses

Key Equations

• General enzyme-catalyzed reaction:

• Michaelis-Menten equation (for enzyme kinetics):

Additional info: These notes expand on lecture content to provide a comprehensive overview of enzyme classification, mechanism, and regulation, suitable for GOB Chemistry students.