Enzymes and Their Functions

Study Notes

Enzymes are biological catalysts that increase reaction rates by lowering activation energy.
Enzymes are proteins, except for ribozymes (RNA molecules).
Enzymes are reusable, not consumed during reactions.
Enzymes are specific for their substrates.

Enzyme specificity refers to their ability to catalyze specific chemical reactions.
Enzyme specificity depends on the properties of their active site.
Important factors for substrate binding include: orientation of the active site, presence of specific binding sites, and the presence of bond breaking/forming groups.
Types of enzyme specificity:
- Relative/low specificity: acts on a particular chemical bond, regardless of surrounding molecular structure.
- Moderate/group specificity: acts on molecules with specific functional groups.
- Absolute specificity: catalyzes a single reaction with a single substrate.
- Optical/stereo specificity: acts on a specific stereoisomer.
- Dual specificity: acts on two substrates through the same type of reaction
Bond specificity relates to an enzyme’s activity on a particular type of bond independent of the surrounding molecule.
Group specificity relates to the enzyme activity on molecules that have particular functional groups surrounding the bond.
Absolute specificity is the enzyme’s activity on a single substrate.
Stereo-chemical/optical specificity describes the enzyme’s activity on specific isomers.
Dual specificity relates to enzymes that work with two substrates through the same reaction.

Substrates bind to a specific active site on the enzyme.
The binding of the substrate to the active site induces a conformational change in the enzyme, creating an enzyme-substrate complex.
Bonds form between the substrate and amino acid residues in the active site.
Additional non-covalent bonds are formed during the transition state.
The substrate is converted to product while bound to the active site.
Products leave the active site and the original enzyme is regenerated.

Enzyme activity is influenced by:
- Environmental conditions (temperature, pH, substrate and enzyme concentration)
- Cofactors and coenzymes.

Temperature affects enzyme activity.
- Enzyme activity increases with increasing temperature until optimum is reached.
- Further increase causes enzyme denaturation and decreased activity.
pH affects enzyme activity.
- Each enzyme has an optimal pH range.
- Changes in pH can alter enzyme structure and hinder its activity.
Substrate and enzyme concentration affects enzyme activity.
Increasing substrate concentration leads to increased enzyme activity until all enzymes are occupied (saturation).

Coenzymes are low molecular weight organic molecules.
- They are heat stable.
Cofactors are non-protein components.
- Some cofactors are metal ions.
- Coenzymes and cofactors are required by some enzymes before they can exert their catalytic activities.
The entire complex of apoenzyme and co-factors are called holoenzymes.
Holoenzyme = apoenzyme + co-factor.

Possible regulations include:

Cell compartmentalization of enzymes.
- Enzymes are contained within cells or organelles to prevent unwanted reactions.
End-product inhibition regulates enzyme activity.
- End products of a metabolic pathway bind to the enzyme and inhibit it.
Covalent modification changes the conformation of the enzyme and adjusts its activity.
Allosteric factors, which are molecules that bind to the enzyme at a site different from the active site to regulate the enzyme's activity

Enzyme presence, absence, or activity can be used diagnostically.
Plasma functional enzymes: Enzymes normally found in the plasma.
- E.g., thrombin.
Plasma non-functional enzymes: Enzymes not normally found in the plasma, but increase in concentration if tissue damage occurs in the body.
- E.g. ALT, acid phosphatase.
Enzyme levels in blood correlate to tissue damage or disease, e.g., myocardial infarction.