Biochemistry · MU Pleven

3. Enzymes: General Properties. Enzyme Specificity. Nomenclature and Classification. Isozymes. Mechanism of Enzymatic Action.

Covers enzymes as biological catalysts, starting with their general properties such as catalytic power, specificity and sensitivity to conditions, then moving to simple versus conjugated enzymes and the types of cofactors. It goes on to specificity types and the lock-and-key and induced-fit models, the IUBMB six-class EC nomenclature, and isozymes with LDH and creatine kinase patterns. The note closes with the mechanism of catalysis, factors affecting activity and clinical uses of enzymes, and includes several comparison tables.

Inside: 10 sections · 4 labelled pictures · 9 tables · 1 formula · 20 flashcards · 6 min

Topic 3 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 10 sections, about 437 words (6 min of reading), with 20 flashcards.

In this note

  1. Enzymes. General properties.
  2. Definition of Enzymes
  3. General Properties of Enzymes
  4. Chemical Nature of Enzymes
  5. Enzyme Specificity
  6. Enzyme Nomenclature and Classification
  7. Isoenzymes (Isozymes)
  8. Mechanism of Enzymatic Action
  9. Factors Affecting Enzyme Activity
  10. Clinical Importance of Enzymes

Definition of Enzymes

Enzymes are biological catalysts — mostly proteins (some RNA molecules also act as enzymes, called ribozymes) — that increase the rate of chemical reactions without being consumed in the process.

Chemical Nature of Enzymes

A. Simple Enzymes

  • Composed only of protein.

Example: Pepsin, trypsin.

B. Conjugated (Holo) Enzymes

  • Have protein part (apoenzyme) + non-protein part (cofactor).

Types of Cofactors:

  • Prosthetic group: Tightly bound (e.g. FAD in succinate dehydrogenase).
  • Coenzyme: Loosely bound organic molecules (e.g. NAD⁺, CoA).
  • Metal ions: Fe²⁺, Cu²⁺, Zn²⁺, Mg²⁺ — often stabilize substrate or active site.

Enzyme Specificity

Enzyme specificity refers to the ability of an enzyme to recognize and act only on one substrate or a group of related substrates.

Types of Specificity:

Lock and Key Model (Emil Fischer, 1894)

  • The enzyme’s active site has a fixed shape exactly complementary to the substrate (like a lock and key).
  • Explains specificity, but not flexibility.

Induced Fit Model (Koshland, 1958)

  • The enzyme changes its conformation after substrate binds, optimizing interaction and catalysis.
  • Explains dynamic nature and transition-state stabilization.

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