Biological Reactions are Regulated by Enzymes

Understanding Enzymes

Enzymes are specialised proteins acting as biological catalysts to speed up the rate of reactions in cells.
Each enzyme has a particular active site that fits with its specific substrate akin to a lock and key model, leading to its specificity.
Enzymes function best at an optimum pH and temperature, deviations from which can result in a decrease in activity or denaturation.

Enzyme-Substrate Interactions

An enzyme and its substrate form a temporary enzyme-substrate complex to lower the activation energy for the reaction.
The changed shape of the active site after substrate binding is described by the induced fit model, leading to a transition state where the reaction occurs.
Once the reaction is complete, the product is released and the enzyme is free to catalyse another reaction, thus demonstrating their reusability.

Roles of Enzymes in Metabolic Pathways

Enzymes are integral in anabolic (building up) and catabolic (breaking down) metabolic pathways.
They assist in the synthesis of molecules such as proteins, DNA, RNA and polysaccharides in anabolic pathways.
In catabolic pathways, enzymes facilitate the breakdown of complex molecules to smaller ones releasing energy, such as in respiration and digestion.

Regulatory Mechanisms

The activity of enzymes can be regulated by factors such as the availability of substrates or enzyme inhibitors.
In competitive inhibition, molecules similar to the substrate bind to the active site of an enzyme preventing substrate binding.
In non-competitive inhibition, the inhibitor binds to an allosteric site (sites other than the active site) causing a structural change in the enzyme rendering it non-functional.
Feedback inhibition occurs when the product of a reaction inhibits the enzyme that produced it, helping to maintain homeostasis.

Clinical Significance of Enzymes

Malfunction or deficiency of enzymes can lead to pathological conditions. For instance, phenylketonuria results from a deficiency of the enzyme that metabolises phenylalanine.
Enzymes are used for therapeutic purposes, such as digestive enzymes in cystic fibrosis and DNA polymerase in PCR for DNA replication.
Understanding enzyme kinetics and regulation is vital for the design of effective therapeutic drugs.