Enzymes – Definition, Classification, Coenzymes, Cofactors, and Isoenzymes

I. Introduction to Enzymes

1. Definition:
   • Enzymes are biological catalysts that speed up biochemical reactions without being consumed in the process.
   • They are mostly proteins (with a few RNA molecules acting as ribozymes) and are essential for sustaining life by facilitating metabolic reactions.
2. Key Features of Enzymes:
   • Highly specific for their substrates.
   • Operate under mild physiological conditions (temperature, pH).
   • Can be regulated to meet the metabolic needs of the cell.
   • Often require cofactors or coenzymes for activity.

II. Classification of Enzymes

Enzymes are classified by the International Union of Biochemistry and Molecular Biology (IUBMB) into six main classes based on the type of reaction they catalyze:

1. Oxidoreductases:
   • Catalyze oxidation-reduction reactions.
   • Example: Lactate dehydrogenase, which converts lactate to pyruvate.
2. Transferases:
   • Transfer functional groups from one molecule to another.
   • Example: Alanine transaminase (ALT), which transfers an amino group from alanine to α-ketoglutarate.
3. Hydrolases:
   • Catalyze the hydrolysis of chemical bonds.
   • Example: Amylase, which breaks down starch into maltose.
4. Lyases:
   • Catalyze the addition or removal of groups to form double bonds.
   • Example: Fumarase, which catalyzes the conversion of fumarate to malate.
5. Isomerases:
   • Catalyze the rearrangement of molecular structure within a single molecule.
   • Example: Phosphoglucose isomerase, which converts glucose-6-phosphate to fructose-6-phosphate.
6. Ligases (Synthetases):
   • Catalyze the joining of two molecules with the input of energy, often from ATP.
   • Example: DNA ligase, which joins DNA strands during replication and repair.

III. Coenzymes and Cofactors

1. Coenzymes:
   • Organic, non-protein molecules that bind to enzymes to facilitate catalysis.
   • Often derived from vitamins.
   • Function as carriers of chemical groups or electrons.
   • Example:
     • NAD⁺/NADH (Nicotinamide adenine dinucleotide): Coenzyme in redox reactions.
     • FAD/FADH₂ (Flavin adenine dinucleotide): Involved in electron transport.
     • Coenzyme A: Transfers acyl groups in metabolic reactions.
2. Cofactors:
   • Inorganic ions or molecules that are essential for enzyme activity.
   • Examples:
     • Metal ions: Mg²⁺, Zn²⁺, Fe²⁺, Mn²⁺.
     • Prosthetic groups: Heme in cytochrome enzymes.
3. Differences Between Coenzymes and Cofactors:
   • Coenzymes are organic, while cofactors can be inorganic.
   • Coenzymes often serve as transient carriers, while cofactors can be tightly bound as part of the enzyme structure.

IV. Isoenzymes (Isozymes)

1. Definition:
   • Isoenzymes are different molecular forms of the same enzyme that catalyze the same reaction but differ in structure, kinetic properties, and regulatory mechanisms.
2. Characteristics:
   • Encoded by different genes.
   • May vary in amino acid sequence, tissue distribution, and optimum conditions for activity.
   • Allow fine-tuned regulation of metabolic pathways in different tissues or developmental stages.
3. Examples of Isoenzymes:
   • Lactate Dehydrogenase (LDH):
     • Composed of H (heart) and M (muscle) subunits, forming combinations like LDH-1 (H4) in the heart and LDH-5 (M4) in muscles.
   • Alkaline Phosphatase (ALP):
     • Different isoenzymes found in liver, bone, and intestines.

V. Mechanism of Enzyme Action

1. Key Steps:
   • Substrate Binding: The substrate binds to the enzyme’s active site.
   • Formation of Enzyme-Substrate Complex: A temporary complex is formed.
   • Catalysis: Enzyme facilitates the conversion of substrate into product.
   • Product Release: Product is released, and the enzyme is free for another reaction cycle.
2. Models of Enzyme Action:
   • Lock and Key Model: The enzyme’s active site is perfectly shaped for the substrate.
   • Induced Fit Model: The enzyme undergoes a conformational change upon substrate binding.

VI. Factors Affecting Enzyme Activity

1. Temperature:
   • Optimal temperature for activity; extreme heat denatures enzymes.
2. pH:
   • Each enzyme has an optimum pH (e.g., pepsin in the stomach works best at pH 2).
3. Substrate Concentration:
   • Follows Michaelis-Menten kinetics: Increased substrate concentration increases activity until the enzyme is saturated.
4. Inhibitors:
   • Competitive inhibitors bind the active site.
   • Non-competitive inhibitors bind elsewhere, altering enzyme activity.

VII. Clinical Applications of Enzymes

1. Diagnostics:
   • Enzyme levels (e.g., ALT, AST) are used to assess liver function.
2. Therapeutics:
   • Enzyme replacement therapy for diseases like Gaucher’s disease.
3. Industrial Uses:
   • Enzymes in food processing, detergents, and pharmaceuticals.