Biochemistry: Enzyme Structure and Function

Active Site

• The specific region on an enzyme where the substrate binds, forming an enzyme-substrate complex.
• Both binding and catalytic reactions occur at the active site.

Substrate

• The substance acted on by an enzyme.

Product

• The substance derived from a transformed substrate (enzyme + substrate).

Allosteric Site

• A cavity on the enzyme, separate from the active site, that can bind regulatory molecules.
• This site influences the enzyme's structure and function.

Apoenzyme

• The enzyme portion that cannot function alone, requiring a coenzyme.

Holoenzyme

• The complete enzyme complex composed of an apoenzyme and a coenzyme.

Metalloenzyme

• An enzyme that contains an inorganic activator as part of its molecular structure.

Primary Structure

• The specific amino acid sequence of an enzyme.

Enzyme Variants

• Heteroenzyme: Enzymes with the same enzymatic activity but specific to different biological species (e.g., Lactate dehydrogenase (LDH) of rabbits and humans).
• Plasma-Specific Enzymes: Enzymes secreted by the liver and released into the plasma. They contribute to plasma function (e.g., blood clotting, fibrinolytic system).
• Bilocular: Enzymes released by mitochondria and cell sap.

Isoenzymes

• Multiple forms of an enzyme that catalyze the same reaction but have slightly different structures and properties.
• CK-MB: A sub-unit of creatine kinase (CK) important for heart and skeletal muscle function. More specific and sensitive than CK-MM. Used in diagnosing acute myocardial infarction, particularly within the first 12 hours.

Characteristics of Isoenzymes

• Electrophoresis: Used to classify isoenzyme sub-units based on their charge and mobility.
• Response to Activation and Inactivation: Isoenzymes respond differently to activation and inactivation processes (e.g., formaldehyde activation of ACP-RBC and inactivation of ACP-Prostate).
• Response to Inhibitors: Plasma enzymes are susceptible to inhibitors. Various substances like fluoride, heparin, or oxalic acid can inhibit ACP activity.

Nomenclature and Classifications of Enzymes

Empirical Names

• Not specific, based on organ, observation, experiences, or the type of chemical reaction.
  • Arbitrary: Not specific (e.g., Ptyalin, Trypsin, Pepsin, Rennin).
  • Suffix -ase: Added to the name of the substrate or group acted upon by the enzyme to describe the catalyzed reaction (e.g., Maltase).

Classifications of Enzymes

• Oxidoreductase: Catalyze oxidation (addition of H ion) and reduction reactions (removal of H ion) between two substances (e.g., Oxidases, Dehydrogenases).
• Transferase: Catalyze the transfer of an intact group of atoms (amine or phosphate group) from one substrate to another (e.g., Aminotransferase, Kinase, Phosphorylase, Methyltransferase).
• Hydrolases: Catalyze hydrolysis of various bonds, requiring water for the reaction to proceed (e.g., Phosphatase, Phosphodiesterase, Protease).
• Lyases: Catalyze the removal of groups from substrates without hydrolysis. This leads to double bonds (e.g., Decarboxylase).

Factors Affecting Enzyme Binding

• Energy: Activation energy required for a reaction.
• Molecular Compatibility: The similarity between the enzyme and substrate.
• Space Availability: The number of enzymes and substrates available for reaction.
• Specificity: The enzyme's ability to catalyze a specific substrate.

Specificity of Enzyme Action

• Lock and Key Theory: Proposed by Emil Fischer. The active site of the enzyme is always complementary to the substrate, ensuring recognition between the two.
• Induced Fit Theory: Proposed by Daniel Koshland. The enzyme modifies its shape upon substrate binding, ensuring complementarity between the two only after binding.

Explore the fundamental concepts related to enzyme structure, including active sites, substrates, and types of enzymes. Understand how different sites and structures, such as allosteric and holoenzymes, influence enzymatic activity. This quiz will deepen your knowledge of biochemistry and enzyme functionality.