Enzymes:
- Are globular proteins which act as catalysts. They speed up chemical reactions by lowering the activation energy, and remain unchanged at the end of the reaction.
- They provide an alternate reaction pathway, which requires less energy to start.
- Part of the molecule is a specifically shaped active site, into which a substrate fits to form an enzyme-substrate complex.
- The lock and key hypothesis suggested an exact match between the shapes of the substrate and active site. How does it occur?
- Substrate diffuses into the active site
- Substrate binds to the active site
- Bonds in the substrate are broken as a result
- Products form and unbind from the active site and diffuse out of the active site
- The induced fit hypothesis describes the active site moulding around the substrate once it is in place.
- Limitations of enzymes:
- An increase in temperature (and thus an increase in the kinetic energy of the molecules) increases the chances of a collision between enzyme and substrate molecules. The rate of reaction increases.
- Beyond the optimum temperature, the increased vibrations of the atoms in the protein molecule break the bonds maintaining the tertiary structure. The active site of the enzyme is irreversibly denatured.
- pH changes around the enzymes optimum pH alter the charge distribution in the active site, reducing the compatibility of enzyme and substrate. Tertiary structure bonds are again affected and extreme changes will denature the enzyme.
- An increase in either substrate or enzyme concentration will increase the rate of reaction until the other acts as a limiting factor.