Enzymes

Enzymes are biological catalysts, which are substances that speed up the rate of biochemical reactions without being used up in the process. The vast majority of enzymes are proteins. They are highly specific, meaning each enzyme typically catalyses only one or a small number of specific reactions. Enzymes function by lowering the activation energy required for a reaction to occur.

The active site is a specific, three-dimensional region on the enzyme molecule where the substrate binds. It is formed by the folding of the protein chain and has a unique shape and chemical properties complementary to its specific substrate. The substrate is the molecule upon which the enzyme acts. When the substrate binds to the active site, an enzyme-substrate complex is formed temporarily, facilitating the conversion of the substrate into products.

Proposed by Emil Fischer in 1894, this model suggests that the active site of an enzyme has a rigid, fixed shape that is perfectly complementary to the shape of its specific substrate, much like a key fits into a specific lock. The substrate fits precisely into the active site, forming the enzyme-substrate complex, and then the reaction proceeds.

Proposed by Daniel Koshland in 1958, this model refines the Lock and Key model. It suggests that the active site is not entirely rigid but is somewhat flexible. When the substrate binds to the active site, it induces a slight conformational change in the enzyme, causing the active site to mould itself more tightly around the substrate. This 'induced fit' optimises the enzyme-substrate interaction, bringing the catalytic groups of the enzyme into the correct orientation to act on the substrate.

Enzyme activity is highly sensitive to temperature. At low temperatures, enzyme activity is low because molecules have less kinetic energy, leading to fewer collisions between enzyme and substrate. As temperature increases, kinetic energy increases, leading to more frequent and energetic collisions, thus increasing the reaction rate. Each enzyme has an optimum temperature at which it exhibits maximum activity (typically around 37°C for human enzymes). Beyond the optimum temperature, the enzyme's three-dimensional structure begins to break down (denaturation), particularly the active site, causing a rapid decrease in activity. Denaturation is usually irreversible.

Enzyme activity is also highly sensitive to pH. Each enzyme has an optimum pH at which it functions most efficiently. For example, pepsin (in the stomach) has an optimum pH of around 2, while amylase (in the mouth) has an optimum pH of around 7. Extreme pH values (too acidic or too alkaline) can alter the charges on the amino acids within the enzyme, disrupting the enzyme's tertiary structure and changing the shape of the active site. This leads to denaturation and a loss of enzyme activity.

At low substrate concentrations, increasing the substrate concentration generally increases the rate of reaction because there are more substrate molecules available to bind with the active sites of the enzymes. However, once all the active sites are saturated with substrate molecules, the reaction rate reaches its maximum (Vmax) and plateaus. At this point, the enzyme concentration becomes the limiting factor, as all available enzyme active sites are continuously occupied.

Assuming there is an excess of substrate available, increasing the enzyme concentration will directly increase the rate of reaction. This is because more enzyme molecules mean more active sites are available to bind with substrate molecules, leading to a faster conversion of substrate into product. Conversely, decreasing enzyme concentration will decrease the reaction rate.

Enzyme inhibition is the process by which the activity of an enzyme is reduced or stopped by a molecule called an inhibitor. Inhibition can be reversible or irreversible.

In competitive inhibition, the inhibitor molecule has a similar shape to the substrate and competes with the substrate for binding to the enzyme's active site. If the inhibitor binds, it prevents the substrate from binding, thus reducing the reaction rate. This type of inhibition can often be overcome by increasing the substrate concentration, which increases the likelihood of the substrate binding to the active site over the inhibitor.

In non-competitive inhibition, the inhibitor binds to a site on the enzyme other than the active site (an allosteric site). This binding causes a conformational change in the enzyme, altering the shape of the active site and reducing its ability to bind to the substrate or to catalyse the reaction effectively. Non-competitive inhibition cannot be overcome by increasing the substrate concentration, as the inhibitor's effect is independent of the active site's availability.