Untitled Quiz

Enzymes

• Enzymes are biological catalysts.
• They are organic, thermo-labile catalysts.
• They speed up chemical reactions without being used up in the reaction.

Chemical Nature of Enzymes

• Most enzymes are proteins, except ribozymes which are RNA.
• Protein enzymes are classified into two types:
  • Simple protein enzymes: Composed only of protein.
  • Complex (conjugated) protein enzymes: Composed of a protein part (apoenzyme) and a non-protein part (cofactor).

Cofactors

• Cofactors are non-protein molecules that help enzymes function.
• Cofactors can be either coenzymes or prosthetic groups.
  • Coenzymes: Organic, thermo-labile, loosely attached to the enzyme.
    • Often derived from vitamins B.
    • Examples include FAD and NAD.
  • Prosthetic groups: Inorganic, thermo-stable, firmly attached to the enzyme.
    • Often metal ions.
    • Examples include Ca and Zn.
  • The whole enzyme with its cofactor is called a holoenzyme.

Enzyme Vocabulary

• Substrate: Reactant that binds to the enzyme.
• Enzyme-substrate complex: Temporary association between enzyme and substrate.
• Product: End result of the reaction.
• Active site: Enzyme's catalytic site, where substrate binds.

Active Site

• A specific region of the enzyme.
• It binds to the substrate.
• Formed by amino acid sequences in a polypeptide chain.

Mechanism of Enzyme Action

• Substrate (S) binds to the enzyme's active site.
• This forms an enzyme-substrate complex (ES).
• The complex is stabilized and broken down, releasing products (P) and the unchanged enzyme (E).

Enzyme Action

• Enzymes speed up reactions by lowering the activation energy.
• Activation energy is the energy needed to start a reaction.
• Enzymes decrease the energy barrier between reactants and products.

Factors Affecting Enzyme Rate

• Enzyme concentration: Reaction rate increases with higher enzyme concentration, up to a limit.
• Substrate concentration: Reaction rates increase with rising substrate concentration, up to a maximal rate.
• Temperature: Rate increases with rising temp. until optimal temperature, after which rate decreases due to denaturation of the enzyme
  • Optimal temperature 37-40°C for many enzymes
• pH: Each enzyme has an optimal pH at which its activity is maximum.
• Concentration of coenzymes: Increased coenzyme concentration increases reaction rate in conjugated enzymes requiring coenzymes.
• Concentration of ion activators: metal ion activators increase the reaction rate
• Time: Rate decreases with time due to substrate depletion and product build-up and pH change.
• Enzyme inhibitors: presence decreases or stops enzyme activity.

Michaelis Constant (Km)

• The substrate concentration that yields half the maximum velocity (Vmax) of an enzyme-catalyzed reaction.

Enzyme Affinity

• Enzymes with low Km have high affinity for the substrate; they reach maximum velocity at low substrate concentrations.
• High Km enzymes have low affinity, reacting at maximum velocity only at high substrate concentration.

Effects of Temperature

• Reaction rate increases with temperature increase to an optimum, then decreases due to enzyme denaturation.

Effects of pH

• Each enzyme has an optimum pH where its activity is highest.
• Deviation from the optimum pH results in decreased activity and the enzyme may denature.

Concentration of Coenzymes

• In conjugated enzymes, increasing the coenzyme concentration increases the reaction rate.

Concentration of Ion Activators

• Metal ions activate some enzymes, increasing the reaction rate.

Effect of Time

• Reaction rate decreases over time, due to substrate depletion, product buildup and pH change.

Enzyme Inhibitors

• Inhibitors decrease or block enzyme activity.
• Competitive inhibitors bind to the active site, competing with the substrate.
• Non-competitive inhibitors bind to an allosteric site, changing the enzyme's shape and thus preventing the substrate from binding.

Biomedical Importance of Enzymes

• Enzymes play crucial roles in countless biological processes.