Biomolecules - Enzymes: Types, properties, enzyme action

Enzymes are biological catalysts that speed up biochemical reactions without being consumed. Almost all enzymes are proteins, though some nucleic acids behave like enzymes (e.g., Ribozymes).

Chemical reactions require an initial input of energy called Activation Energy. Enzymes work by lowering this energy barrier ($E_a$), allowing the reaction to proceed faster at physiological temperatures.

The catalytic cycle involves: 1. Substrate ($S$) binding to the Active Site. 2. Formation of the $ES$ complex. 3. Transformation of substrate into product ($P$). 4. Release of product and regeneration of the free Enzyme ($E$).

Properties of enzymes include high specificity (Lock and Key or Induced Fit models), sensitivity to temperature and $pH$, and high turnover numbers (molecules of substrate converted per unit time).

Factors affecting enzyme activity: Enzyme activity is highest at optimum temperature and $pH$. Activity increases with substrate concentration $[S]$ until it reaches maximum velocity ($V_{max}$).

The $Michaelis$ constant ($K_m$) is the substrate concentration at which the reaction velocity is half of $V_{max}$ ($\frac{1}{2} V_{max}$). A low $K_m$ indicates high affinity for the substrate.

Enzyme Inhibition: Competitive inhibitors resemble the substrate and compete for the active site (e.g., Malonate inhibits Succinate dehydrogenase). Non-competitive inhibitors bind to a site other than the active site.

Classification of Enzymes: 1. Oxidoreductases (Redox reactions), 2. Transferases (Group transfer), 3. Hydrolases (Hydrolysis using $H_2O$), 4. Lyases (Removal of groups without water), 5. Isomerases (Interconversion of isomers), 6. Ligases (Joining two compounds using $ATP$).

Cofactors: Non-protein constituents bound to the enzyme to make it catalytically active. The protein part is called the Apoenzyme. The whole active unit is the Holoenzyme ($Holoenzyme = Apoenzyme + Cofactor$).