Enzyme Definitions and Classifications

Enzyme Definitions and Classifications

• Enzymes are biological protein catalysts that increase the rate of biochemical reactions.
• They are proteins (or sometimes ribozymes, which are RNA).
• Enzymes speed up reactions by lowering the activation energy.
• Enzymes are classified into six major classes based on the type of reaction they catalyze.

Oxidoreductases

• These enzymes catalyze oxidation-reduction reactions.
• Oxidation means gaining oxygen or losing hydrogen or electrons.
• Reduction means gaining hydrogen or electrons or losing oxygen.
  • Example: Lactate dehydrogenase.

Transferases

• These enzymes catalyze the transfer of functional groups (other than hydrogen) between two substrates.
• Example: Transaminases (transfer amino groups).

Hydrolases

• These enzymes catalyze hydrolysis reactions, where a bond is broken by the addition of water.
• Example: Digestive enzymes (peptidases, proteinases).

Lyases

• These enzymes catalyze the addition or removal of groups from double bonds.
• Example: Fumarase.

Isomerases

• These enzymes catalyze the interconversion of isomers. This means they rearrange atomic groups without changing the molecule's overall molecular weight.
• Example: Epimerase.

Ligases

• These enzymes catalyze bond formation coupled with ATP hydrolysis.
• Example: Glutamine synthase.

Cofactors and Coenzymes

• Coenzymes are organic molecules that are loosely attached to the enzyme, often transferring a functional group
• Cofactors are inorganic ions (like metal ions) that are involved in enzyme activity.
• Enzymes can be simple proteins, or conjugated proteins (a combination of protein and additional component) called holoenzymes.
• Many coenzymes are vitamins or derived from vitamins.
  • Example: NAD+, NADP+, FAD, FMN (derived from B vitamins).

Isoenzymes

• Isoenzymes are multiple forms of the same enzyme that catalyze the same reaction but have subtle differences in structure.
• These differences can include their location in the body, substrate affinity, or electrophoretic mobility.
• Example: Creatine kinases (CK)

Mechanism of Enzyme Action

• Enzymes lower the activation energy needed for a reaction to occur.
• The active site is the specific region on the enzyme where substrate binding and catalysis occur.
• Two models of active site interactions include Lock and Key and Induced Fit Model.

Regulation of Enzymatic Activity

• Enzyme activity can be regulated at the level of synthesis (long-term) and activity (short-term)
• Enzyme activity regulation can occur via allosteric effectors, covalent modification, and limited proteolysis.

Practical Aspects - Temperature and pH Effects

• Enzyme activity is greatly influenced by temperature.
  • Increasing temperature increases reaction rates until an optimum temperature is reached, then activity decreases due to denaturation.
• Enzyme activity is also influenced by pH.
  • Enzymes have an optimum pH range within which they perform best, and activity drops outside of this range due to denaturation.

Enzyme Inhibitors (Competitive, Noncompetitive, and Uncompetitive)

• Different types of inhibitors affect enzyme activity in various ways. -Competitive inhibitors: compete with substrates for active site; increase Km while keeping Vmax the same. -Noncompetitive inhibitors: do not compete with substrates for active site, decreasing Vmax -Uncompetitive inhibitors: bind only to the enzyme-substrate complex; decrease both Km and Vmax.

Enzyme Definition Summary

• Enzymes are biological catalysts that increase the rate of chemical reactions.
• They are usually proteins, sometimes RNA.
• The active site of an enzyme is where catalysis occurs.
• Enzymes are regulated to optimize their function.
• Specific factors like temperature and pH heavily affect enzyme function.