Enzyme Classification and Regulation: GOB Chemistry Study Notes

Enzyme Classification & Regulation

Introduction to Enzymes

Enzymes are biological catalysts, typically proteins (or RNA), that accelerate chemical reactions in living organisms. Their names usually end in "-ase" and are often derived from the substrate they act upon.

• Enzyme: A protein or RNA molecule that speeds up a biochemical reaction without being consumed.

• 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, essential for certain enzyme functions.

• Apoenzyme: The protein portion of an enzyme, lacking its cofactor/coenzyme.

• Holoenzyme: The complete, catalytically active enzyme (apoenzyme plus cofactor/coenzyme).

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

How Enzymes Work as Catalysts

Enzymes increase reaction rates by lowering the activation energy required for a reaction to proceed. They achieve this through several effects:

• Proximity effect: Bringing reactants close together.

• Orientation effect: Positioning reactants in the optimal orientation for reaction.

• Energy effect: Inducing strain in substrate bonds to lower activation energy.

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

Components of a Biological Reaction

• Enzyme-substrate complex (ES): Temporary association between enzyme and substrate.

• Product: The result of the enzymatic reaction.

Enzyme Classes

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 the rearrangement of atoms within a molecule.

• Lyases: Catalyze the addition or removal of groups to form double bonds.

• Ligases: Catalyze the joining of two molecules, often coupled with ATP hydrolysis.

Oxidoreductases

• General Reaction: Catalyze oxidation or reduction of substrates.

• Dehydrogenases: Remove hydrogen atoms to form double bonds; require coenzymes like FAD or NAD+.

Transferases

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

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

Hydrolases

• Lipases: Hydrolyze ester bonds in lipids.

• Proteases: Hydrolyze peptide bonds in proteins.

• Nucleases: Hydrolyze phosphate ester bonds in DNA/RNA.

• Amylases: Hydrolyze glycosidic bonds in starch.

Isomerases

• General Reaction: Rearrangement of atoms to form isomers.

Lyases

• Decarboxylases: Remove CO2.

• Dehydrases: Remove H2O.

• Deaminases: Remove NH3.

• Synthases: Add small molecules to double bonds.

Ligases

• Synthetases: Join two molecules using ATP energy.

• Carboxylases: Add CO2 to form carboxylate groups.

Enzyme Specificity

Specific vs. Nonspecific Enzymes

Enzyme specificity refers to the ability of an enzyme to select its substrate(s).

• 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).

Binding Specificity and Stereochemistry

The active site of an enzyme must be chemically compatible with the substrate, including its stereochemistry.

Models of Substrate Binding

Lock-and-Key Model

The lock-and-key model suggests that the enzyme's active site is a perfect fit for the substrate, like a key fitting into a lock.

Induced Fit Model

The induced fit model proposes that substrate binding causes a conformational change in the enzyme, enhancing the fit and facilitating catalysis. This model more accurately describes enzyme-substrate interactions.

Mechanism of Enzyme Action

Atomic-Level Mechanism (Example: Hydrolase)

Enzymes often use a combination of amino acid residues to facilitate catalysis. For example, a hydrolase may use Asp, Ser, and His residues to cleave peptide bonds.

• The substrate binds to a hydrophobic pocket in the enzyme.

• Serine is activated to attack the substrate's amide bond.

• Histidine donates a hydrogen, breaking the bond.

• Water replaces the leaving group, completing hydrolysis.

• The enzyme is reset for another reaction.

Factors Affecting Enzyme Activity

Substrate Concentration

• Increasing substrate concentration increases reaction rate until the enzyme is saturated, at which point the rate levels off.

Enzyme Concentration

• Reaction rate increases linearly with enzyme concentration when substrate is in excess.

Temperature

• Enzyme activity increases with temperature up to an optimum, then decreases as the enzyme denatures.

pH

• Each enzyme has an optimal pH; activity decreases outside this range due to denaturation or altered active site chemistry.

Summary Table: Enzyme Classes and Subclasses

Class	General Reaction	Key Subclasses
Oxidoreductases	Oxidation-reduction	Dehydrogenases
Transferases	Group transfer	Kinases, Transaminases
Hydrolases	Hydrolysis	Lipases, Proteases, Nucleases, Amylases
Isomerases	Isomerization	---
Lyases	Add/remove groups to/from double bonds	Decarboxylases, Dehydrases, Deaminases, Synthases
Ligases	Bond formation (ATP hydrolysis)	Synthetases, Carboxylases

Key Equations

• Activation Energy Lowering: $\Delta G^\ddagger_{\text{with enzyme}} < \Delta G^\ddagger_{\text{without enzyme}}$

• General Enzyme Reaction: $\text{Enzyme} + \text{Substrate} \rightarrow \text{Enzyme-Substrate Complex} \rightarrow \text{Enzyme} + \text{Product}$

Conclusion

Enzymes are essential biological catalysts, classified by the reactions they catalyze. Their activity is influenced by substrate and enzyme concentration, temperature, and pH. Understanding enzyme specificity and mechanisms is fundamental to biochemistry and GOB Chemistry. Additional info: These notes expand on lecture points and textbook readings to provide a comprehensive, exam-ready summary of enzyme classification and regulation.