Enzyme Classification

Enzyme Classification

Enzymes are biological catalysts — proteins (mostly) that accelerate chemical reactions by lowering the activation energy without being consumed. They are highly specific (substrate specificity, reaction specificity, stereospecificity) and regulated.

IUBMB Classification (6 Major Classes)

  • 1. Oxidoreductases: Catalyze oxidation-reduction reactions. Transfer of electrons (H or O). Key examples: Lactate dehydrogenase (LDH), Glucose-6-phosphate dehydrogenase (G6PD), Cytochrome oxidase. Coenzymes: NAD+, NADP+, FAD.
  • 2. Transferases: Transfer functional groups (methyl, phosphate, amino, acyl) from donor to acceptor. Examples: Aminotransferases (ALT, AST — used in LFTs), Hexokinase, Kinases (phosphate transfer). Coenzyme: PLP for aminotransferases.
  • 3. Hydrolases: Cleave bonds using water (hydrolysis). Digestive enzymes — Proteases (pepsin, trypsin, chymotrypsin), Lipases (pancreatic lipase), Amylase, Nucleases. Also ATPases, Phosphatases.
  • 4. Lyases: Cleave bonds WITHOUT water or oxidation; can form double bonds or rings. Examples: Pyruvate decarboxylase, Aldolase (cleaves fructose-1,6-bisphosphate), Carbonic anhydrase (adds CO₂ to water).
  • 5. Isomerases: Interconvert structural isomers/stereoisomers. Examples: Phosphoglucose isomerase (glycolysis), Triose phosphate isomerase, Epimerases, Mutases. No coenzyme required usually.
  • 6. Ligases (Synthetases): Join two molecules using energy from ATP hydrolysis. Examples: DNA Ligase, Pyruvate carboxylase, Acetyl-CoA Carboxylase, Aminoacyl-tRNA Synthetases.

Nomenclature System

Each enzyme is assigned a 4-number EC number: EC [class].[subclass].[sub-subclass].[serial number]. Example: Lactate dehydrogenase = EC 1.1.1.27 (class 1 = oxidoreductase; acts on CH-OH groups; NAD+ as acceptor).

Enzyme Specificity

  • Absolute specificity: Acts on only one substrate (Urease → urea only)
  • Group specificity: Acts on substrates with a particular group (Hexokinase → phosphorylates any hexose)
  • Bond specificity: Acts on a particular type of bond (any peptide bond → protease)
  • Stereospecificity: Distinguishes between optical isomers (L-amino acid oxidase acts only on L-forms)

Cofactors and Coenzymes

Apoenzyme (protein part) + Cofactor = Holoenzyme (active form). Cofactors may be: Metal ions (Mg²⁺ in kinases, Zn²⁺ in carbonic anhydrase, Fe²⁺ in catalase), or Coenzymes (organic, often derived from vitamins — NAD+, FAD, CoA, PLP).

Clinical Applications

  • Serum enzyme levels indicate organ damage (enzyme leak from damaged cells)
  • Enzyme replacement therapy (Gaucher's disease — recombinant glucocerebrosidase)
  • Enzymes as drug targets (Statins inhibit HMG-CoA reductase; ACE inhibitors block angiotensin-converting enzyme)

Quiz - Exam Preparation Strategy

When studying Quiz for your final board exams, it is critical to focus on the core concepts and fundamental formulas. Relying strictly on NCERT textbook solutions and practicing previous year questions (PYQs) is the proven methodology for scoring high marks. Avoid rote memorization and instead focus on the logical application of the theories presented in this chapter.

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

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Board exams tend to favor conceptual application questions and direct formula-based derivations from the NCERT syllabus. Ensure you have solved every single exercise in the official textbook.

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