Biological Reactions and Enzymes

Biological Reactions

Metabolism refers to all the chemical reactions that take place within an organism to maintain life.
Anabolic reactions are those that involve the building up of complex molecules from simpler ones, often requiring energy. These include protein synthesis and DNA replication.
Catabolic reactions are those that involve the breaking down of complex molecules into simpler ones, often releasing energy. These include digestion and cellular respiration.

Energy is needed to start a reaction and is known as the activation energy. This is the energy needed to break the bonds in the reactants.
The energy released or absorbed in a chemical reaction is determined by the differences in the bond energies between the reactants and the products.
Exothermic reactions release energy, usually as heat. Cellular respiration is an example of an exothermic reaction.
Endothermic reactions absorb energy from the surroundings. These reactions lead to a decrease in temperature. Photosynthesis is an example of an endothermic reaction.

Enzymes are biological catalysts which increase the speed of a reaction without being consumed in the process.
An enzyme’s function is determined by its three-dimensional structure, which allows it to recognise and bind to specific substrates.
Substrates are the reactants in a reaction catalysed by an enzyme. Upon binding, an enzyme-substrate complex is formed.
The active site is the part of an enzyme where substrate binds and is transformed into product. The shape and chemical environment of the active site is key to its function.

The lock-and-key model describes enzymes and substrates as having exactly complementary shapes.
In the induced fit model, binding of the substrate induces a change in the shape of the enzyme to enhance the fit, and therefore the catalytic activity.
After the reaction, the products are released and the enzyme is available to accept another substrate molecule.

Temperature: Increasing temperature increases enzyme activity up to an optimal point, above which the enzyme starts to denature, losing its structure and thus its function.
pH: Each enzyme has an optimal pH at which it operates most efficiently. Deviations from this can affect the enzyme’s shape and activity.
Substrate concentration: Increasing the concentration of substrate increases the rate of reaction to a point, but once all active sites are occupied (saturation point), adding more substrate doesn’t increase the rate.
Inhibition: Certain substances can reduce enzyme activity. Competitive inhibitors compete with substrate for the active site, while non-competitive inhibitors bind to another part of the enzyme, altering its shape and function.