Enzymes

Definition: Enzymes are biological catalysts, mostly proteins, that speed up biochemical reactions without being consumed or permanently altered. They work by lowering the Activation Energy of the reaction.

Discovery: The term 'Enzyme' (meaning 'in yeast') was coined by Friedrich Wilhelm Kuhne (1878).

• Protein Nature: Are globular proteins formed by amino acids.
• Specificity: Highly specific; one enzyme catalyzes only one specific reaction.
• Efficiency: Immense catalytic power; a small amount can catalyze a large quantity of substrate.
• Sensitivity: Sensitive to Temp and pH.
• Reversibility: Some enzymatic reactions are reversible.
• Regulation: Activity can be regulated by inhibitors or activators.

enzymes consist of two parts:

• Apoenzyme: The protein part of the enzyme. It is inactive on its own.
• Cofactor: The non-protein part required for activity. Can be:
  • Activator: Detachable inorganic metal ion (e.g., Mg²⁺, Fe²⁺).
  • Coenzyme: Loosely attached organic molecule (e.g., NAD, Vitamins).
  • Prosthetic Group: Tightly/Covalently attached organic unit (e.g., Heme in Hemoglobin).
• Holoenzyme: Active enzyme formed by Apoenzyme + Cofactor.

Active Site: A specific 3D region (cleft/pocket) on the enzyme enzyme binding to substrate. Has two zones: Binding Site (recognizes substrate) and Catalytic Site (performs reaction).

Models:

1. Lock and Key Model (Emil Fischer, 1894): Active site is rigid. Substrate fits exactly like a key in a lock. Explains specificity but not flexibility.
2. Induced Fit Model (Daniel Koshland, 1958): Active site is flexible. Substrate induces a conformational change in the enzyme for a better fit. More accepted model.

• Temperature: Rate increases with temp up to an Optimum Temperature (37°C for humans). Beyond this, enzyme denatures (loses 3D structure) and activity stops.
• pH: Each enzyme has an Optimum pH (e.g., Pepsin=2.0, Trypsin=8.0). pH changes alter the ionization of amino acids at the active site.
• Substrate Concentration: Rate increases with substrate conc. until all active sites are occupied (Saturation). Further increase has no effect (Vmax reached).
• Enzyme Concentration: Rate is directly proportional to enzyme conc., provided substrate is unlimited.

Inhibitors reduce enzyme activity.

• Competitive Inhibitors: Structurally similar to substrate. Compete for the Active Site. Reversible by increasing substrate concentration. (Raises Km, Vmax unchanged). Example: Malonic acid competes with Succinic acid.
• Non-Competitive Inhibitors: Bind to an Allosteric Site (other than active site). Alter enzyme shape so substrate cannot bind/react. Irreversible by substrate. (Lowers Vmax, Km unchanged). Example: Cyanide, Heavy metals.