Enzyme Catalysis and Classification Quiz

Enzyme Catalysis

• Enzymes are biological catalysts.
• Enzymes are mostly proteins, with the exception of some RNA molecules that catalyze self-cleavage.
• Enzymes can increase the rate of a reaction by a factor of 10^9 to 10^20.
• Some enzymes catalyze the reaction of only one compound.
• Some enzymes are stereospecific.
• Some enzymes catalyze reactions of specific types of compounds or bonds.

Classification of Enzymes

• Enzymes are commonly named after the reaction or reactions they catalyze.
• Enzymes are classified into six major groups:
  • Oxidoreductases: oxidation-reduction reactions.
  • Transferases: group transfer reactions.
  • Hydrolases: hydrolysis reactions.
  • Lyases: addition of groups to a double bond, or removal of groups to create a double bond.
  • Isomerases: isomerization reactions.
  • Ligases: the joining of two molecules.

Terms in Enzyme Chemistry

• Apoenzyme: the protein part of an enzyme.
• Cofactor: a nonprotein portion of an enzyme that is necessary for catalytic function, like metallic ions.
• Coenzyme: a nonprotein organic molecule that acts as a cofactor, often a B vitamin.
• Substrate: the compound or compounds whose reaction an enzyme catalyzes.
• Active site: the specific portion of the enzyme to which a substrate binds during reaction.

Terms in Enzyme Chemistry (Continued)

• Activation: any process that initiates or increases the activity of an enzyme.
• Inhibition: any process that makes an active enzyme less active or inactive.
• Competitive inhibitor: any substance that binds to the active site of an enzyme thereby preventing binding of substrate.
• Noncompetitive inhibitor: any substance that binds to a portion of the enzyme other than the active site and thereby inhibits the activity of the enzyme.

Enzyme Activity

• Enzyme activity is a measure of how much a reaction rate is increased.
• How much an enzyme activity is influenced by:
  • Enzyme concentration
  • Substrate concentration
  • Temperature
  • pH

Lock-and-key Model

• The enzyme is a rigid three-dimensional body.
• The enzyme surface contains the active site.

Induced-fit Model

• The active site becomes modified to accommodate the substrate.

Mechanism of Action

• Both the lock-and-key model and the induced-fit model emphasize the shape of the active site.
• The most important factor is the chemistry of the active site.
• Five amino acids participate in the active sites of more than 65% of enzymes.

Catalytic Power

• Enzymes provide an alternative pathway for reaction with a lower activation energy.
• This results in a faster rate.

Enzyme Regulation

• Feedback control: an enzyme-regulation process where the product of a series of enzyme-catalyzed reactions inhibits an earlier reaction in the sequence.

Enzyme Regulation (Continued)

• Proenzyme (zymogen): an inactive form of an enzyme that must have part of its polypeptide chain hydrolyzed and removed before it becomes active.

Enzyme Regulation (Continued)

• Trypsin, a digestive enzyme, is an example of a proenzyme.
• Trypsinogen is initially inactive.
• It becomes active after a six-amino acid fragment is hydrolyzed and removed from its N-terminal end.
• This removal changes the primary and tertiary structure, allowing the molecule to become active.

Enzyme Regulation (Continued)

• Allosterism: enzyme regulation based on an event occurring at a place other than the active site that creates a change in the active site.
• Allosteric enzyme: An enzyme regulated by this mechanism.
• Negative modulation: inhibition of an allosteric enzyme.
• Positive modulation: stimulation of an allosteric enzyme.
• Regulator: a substance that binds to an allosteric enzyme.

Enzyme Regulation (Continued)

• Protein modification: the process of affecting enzyme activity by covalently modifying it.
• Phosphorylation/dephosphorylation: a common example.
• Pyruvate kinase (PK) is active.
• Phosphorylation inactivates the enzyme to pyruvate kinase phosphate (PKP).

Enzyme Regulation (Continued)

• Isoenzyme: an enzyme that occurs in multiple forms, each catalyzing the same reaction.
• Lactate dehydrogenase (LDH) is an example.
• LDH catalyzes the oxidation of lactate to pyruvate.
• The enzyme is a tetramer of H and M chains.
• H4 is predominantly in heart muscle.
• M4 is predominantly in the liver and skeletal muscle.
• H3M, H2M2, and HM3 also exist.