Chapter 7: Enzyme Mechanisms – Study Notes for Biochemistry I

Overview of Enzymes

Definition and Biological Role

Enzymes are biological catalysts that accelerate chemical reactions in living systems. Most enzymes are proteins, though some RNA molecules (ribozymes) also possess catalytic activity.

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

• Advantages over inorganic catalysts:

  • Specificity: Enzymes exhibit high specificity for their substrates, ensuring precise control over biochemical pathways.

  • Regulation: Enzyme activity can be regulated, allowing cells to control metabolic flux.

Models of Enzyme-Substrate Binding

Induced Fit and Conformational Selection

Enzyme-substrate binding can be described by two main models, both of which explain how enzymes achieve specificity and catalytic efficiency.

• Lock-and-Key Model: The active site of the enzyme has a fixed shape that matches the substrate.

• Induced Fit Model: The enzyme changes conformation upon substrate binding, optimizing interactions for catalysis.

• Conformational Selection Model: The enzyme exists in multiple conformations; substrate binding stabilizes the most favorable one.

Example: Hexokinase undergoes a significant conformational change when glucose binds, demonstrating the induced fit mechanism.

Critical Aspects of Enzymes

Protein Structure and Active Site

The active site of an enzyme is a specialized region where substrate binding and catalysis occur. The protein structure supports and positions the active site for optimal function.

• Binding Site: Orients and binds substrates.

• Catalytic Site: Reduces chemical activation energy.

Substrate Binding and Conformational Changes

Mechanistic Implications

Substrate binding often induces conformational changes in the enzyme, which can block water from the active site and promote specific reactions, such as phosphorylation in hexokinase.

Regulation of Enzyme Activity

Modes of Regulation

Enzyme activity is tightly regulated to maintain cellular homeostasis.

• Bioavailability: Regulation of enzyme presence (gene expression).

• Catalytic Efficiency: Post-translational modifications (e.g., phosphorylation) alter enzyme activity.

• Effector Molecules: Allosteric regulation by small molecules (e.g., AMP).

Cofactors and Coenzymes

Definitions and Roles

Many enzymes require non-protein molecules for activity.

• Cofactors: Small molecules, often inorganic ions (e.g., Fe2+, Cu2+, Mg2+), that assist in catalysis.

• Coenzymes: Organic cofactors, often derived from vitamins (e.g., NAD+, FAD).

• Prosthetic Groups: Coenzymes that are permanently associated with the enzyme.

Common Metal-Ion Cofactors

Cofactor	Representative Enzymes	Role in Catalysis
Fe2+	Cytochrome oxidase	Oxidation-reduction
Mg2+	Hexokinase	Helps bind ATP
Mn2+	Ribonucleotide reductase	Oxidation-reduction
Cu2+	Nitrite reductase	Oxidation-reduction
Zn2+	Alcohol dehydrogenase	Helps bind the substrate
Ni2+	Urease	Required in the catalytic site
K+	Pyruvate kinase	Increases enzyme activity
Se	Glutathione peroxidase	Oxidation-reduction
Mo	Xanthine oxidase	Oxidation-reduction

Redox Coenzymes

• NAD+/NADH: Functions as a mobile two-electron carrier in redox reactions.

• Lipoamide: Covalently attached coenzyme involved in redox reactions; the lipoyl group attaches to a lysine residue.

Enzymes as Catalysts and Nomenclature

Catalytic Properties

Enzymes accelerate reactions by lowering the activation energy ($E_a$) without altering the equilibrium constant ($K_{eq}$) or the overall free energy change ($riangle G$).

• Key equation:

  • $\triangle G_{activation}$ is decreased by enzyme action.

  • $\triangle G_{reaction}$ remains unchanged.

Enzyme Nomenclature

Number	Enzyme Class	Type of Reaction	Generic Examples
1	Oxidoreductase	Oxidation-reduction, transfer of H or O atoms	Oxidases, dehydrogenases
2	Transferase	Transfer of functional groups (e.g., methyl, acyl, amino, phosphate)	Transaminases, kinases
3	Hydrolase	Formation of two products by hydrolyzing a bond	Peptidases, lipases
4	Lyase	Cleavage of C–C, C–O, C–N, or other bonds by means other than hydrolysis or oxidation	Decarboxylases, aldolases
5	Isomerase	Interconversion of isomers	Isomerases, mutases
6	Ligase	Formation of C–C, C–S, C–O, and C–N bonds by condensation reactions coupled with ATP cleavage	Synthetases, carboxylases

• Most enzyme names end in -ase.

• The substrate is usually included in the name.

Enzyme databases such as ExplorEnz, BRENDA, and SwissProt provide nomenclature, classification, and literature references.

Enzyme Structure and Function

Mechanisms of Rate Enhancement

Enzymes increase reaction rates by:

• Lowering activation energy ($E_a$): In all cases, enzymes lower $\triangle G_{activation}$.

• Orienting reactants: Active site residues attract and bind substrates, optimizing their orientation for reaction.

• Providing alternative pathways: Active site residues may participate directly in chemical transformations.

How Active Sites Contribute to Catalytic Properties

Physical and Chemical Properties

• Sequestered microenvironment: The active site provides an optimal orientation for the substrate and excludes excess solvent.

• Binding interactions: These stabilize the transition state and lower the activation energy.

• Catalytic functional groups: Specific amino acid side chains participate in catalysis.

Active Site Microenvironment

Example: Aldolase arranges substrates to promote reaction, combining two 3-carbon compounds into a 6-carbon product. The active site environment can alter the pKa of amino acid residues, enabling catalysis.

Stabilization of the Transition State

Enzymes stabilize the transition state, lowering the energy barrier for the reaction.

• Transition state analogs are stable molecules that mimic the transition state and bind tightly to the active site, often acting as inhibitors.

Common Classes of Catalytic Mechanisms

Catalytic Functional Groups

• General acid-base catalysis: Involves amino acid side chains acting as proton donors or acceptors (other than water).

• Covalent catalysis: The enzyme forms a transient covalent bond with the substrate via a nucleophilic side chain.

• Metal ion catalysis: Metal ions stabilize charges or participate directly in electron transfer (redox reactions).

Examples of Catalytic Mechanisms

• Acid-base catalysis: Pancreatic ribonuclease uses two His residues as general acid/base; water is required.

• Covalent catalysis: Glyceraldehyde-3-phosphate dehydrogenase forms a covalent thiohemiacetal intermediate with Cys.

• Metal ion catalysis: Carbonic anhydrase uses Zn2+ to support nucleophilic attack on CO2:

$\mathrm{CO_2 + H_2O \xrightarrow{Zn^{2+}} HCO_3^- + H^+}$

Types of Enzyme-Mediated Reactions

Coenzyme-Dependent Redox Reactions

• Involve electron transfer using coenzymes such as NAD+ and FAD.

• Example: Lactate dehydrogenase uses NAD+ to oxidize lactate to pyruvate.

Metabolic Transformation Reactions

• Include polymerization, building, and breaking down of molecules.

• Example: Post-translational modifications (PTMs) such as phosphorylation.

Reversible Covalent Modifications

• Enzymes can be regulated by reversible addition or removal of chemical groups (e.g., phosphorylation/dephosphorylation).

• Example: Insulin signaling involves phosphorylation of key enzymes.

Practice Questions and Applications

• Enolase in glycolysis requires Mg2+ and is an example of metal ion catalysis.

• Folic acid is a coenzyme; tetrahydrofolate is a coenzyme involved in nucleotide synthesis.

• Transition state analogs are often planar and mimic the geometry of the transition state to inhibit enzymes.

Summary Table: Enzyme Mechanism Types

Mechanism	Description	Example
Acid-base catalysis	Amino acid side chains act as proton donors/acceptors	Pancreatic ribonuclease
Covalent catalysis	Enzyme forms covalent bond with substrate	Glyceraldehyde-3-phosphate dehydrogenase
Metal ion catalysis	Metal ions stabilize charges or participate in redox	Carbonic anhydrase (Zn2+)

Additional info:

• Some slides referenced post-translational modifications (PTMs) as a regulatory mechanism, especially phosphorylation.

• Enzyme databases are essential for standardized nomenclature and research.