Enzymes: Biological Catalysts

Enzymes: Biological Catalysts

• Enzymes accelerate biochemical reactions in living organisms by lowering the energy of activation without being altered or consumed.
• All enzymes are proteins, except for ribozymes, which are RNA-based.

Chemical Nature of Enzymes

• There are two types of protein enzymes: simple and complex (conjugated) proteins.
• Simple protein enzymes are formed of protein only.
• Complex (conjugated) protein enzymes are formed of a protein part (apoenzyme) and a non-protein part (cofactor).
• The non-protein part can be a coenzyme (organic, thermo-labile, and loosely attached) or a prosthetic group (inorganic, thermo-stable, and tightly attached).

Enzymatic Reaction Steps

• The five steps of enzymatic reaction are:
  • Substrate approaches active site
  • Enzyme-substrate complex forms
  • Substrate is transformed into products
  • Products are released
  • Enzyme is recycled

Enzyme-Substrate Binding

• The "lock and key" model proposes that the active site of an enzyme is complementary in shape to the substrate.
• The "induced fit" theory suggests that the catalytic site of an enzyme is not fully formed until the substrate binds, causing changes in the shape of the active site.

Enzyme Classification

• Enzymes are grouped into 6+1 classes:
  • Oxidoreductases (catalyze oxidations and reductions)
  • Transferases (catalyze transfer of moieties)
  • Hydrolases (catalyze hydrolytic cleavage of covalent bonds)
  • Lyases (catalyze cleavage of covalent bonds by atom elimination)
  • Isomerases (catalyze geometric or structural changes within a molecule)
  • Ligases (catalyze the joining together of two molecules)
  • Translocases (catalyze the movement of ions or molecules across membranes)

Enzyme Nomenclature

• Enzyme names are formed from the Greek words "en" (meaning inside) and "zyme" (meaning yeast).
• There are three methods for naming enzymes:
  • Old trivial names (e.g., pepsin and trypsin)
  • Substrate name with the suffix "-ase" added (e.g., lactase and sucrase)
  • Two words, one for the substrate and the other for the type of reaction (e.g., succinate dehydrogenase and pyruvate decarboxylase)
• The International Union of Biochemistry and Molecular Biology (IUBMB) developed an unambiguous system of enzyme nomenclature using a unique name and code number (EC number) for each enzyme.

Enzyme Commission (EC) Number

• Each enzyme has a numerical code (EC number) consisting of four digits separated by dots:
  • The first digit denotes the class (reaction type) of the enzyme
  • The second digit denotes the functional group upon which the enzyme acts
  • The third digit denotes the substrate/acceptor
  • The fourth digit denotes the serial number of the enzyme (individual enzyme number)

The Catalytic Activity of Enzymes

• Assays of the catalytic activity of enzymes are used in research and clinical laboratories to detect and quantify enzyme concentration.
• The analysis of certain enzymes in blood plasma aids in the diagnosis of several disease processes.

Principal Serum Enzymes Used in Clinical Diagnosis

• Quantitative analysis of the activity of released enzymes or other proteins in plasma or serum provides information concerning diagnosis, prognosis, and response to treatment.