Enzymes

### Introduction to Enzymes

Enzymes are globular proteins that function as highly efficient biological catalysts. This means they accelerate the rate of biochemical reactions within living organisms without being chemically altered or consumed in the process. Their role is fundamental to life, facilitating thousands of metabolic reactions, from digestion to DNA replication. A key characteristic of enzymes is their specificity; each enzyme typically catalyses only one specific reaction or acts on a specific type of molecule, known as a substrate.

### Mechanism of Enzyme Action

For a chemical reaction to occur, reactant molecules must overcome an energy barrier called the activation energy. Enzymes work by providing an alternative reaction pathway with a lower activation energy. This allows reactions to proceed millions of times faster than they would otherwise, at the mild temperatures and pressures found within cells.

The interaction between an enzyme and its substrate occurs at a specific region on the enzyme called the active site. This site is a three-dimensional pocket or groove with a shape and chemical environment complementary to the substrate.

Two models describe this interaction:

The catalytic cycle can be summarized as follows:

Enzyme + Substrate ⇌ Enzyme-Substrate Complex → Enzyme + Products

First, the substrate binds to the active site, forming a temporary enzyme-substrate complex. Within this complex, the enzyme facilitates the conversion of the substrate into products. Finally, the products detach from the active site, leaving the enzyme unchanged and ready to bind with another substrate molecule.

### Factors Affecting the Rate of Enzyme Activity

The rate at which an enzyme works is influenced by several environmental factors. Graphing these effects is a common requirement in examinations.

1. Temperature

As temperature increases, the kinetic energy of both enzyme and substrate molecules increases. This leads to more frequent and energetic collisions, increasing the rate of reaction. This continues until the enzyme reaches its optimum temperature, the point of maximum activity (around 37°C for human enzymes). Beyond this point, the rate rapidly declines. The high temperature provides enough energy to break the weak hydrogen bonds that maintain the enzyme's specific 3D tertiary structure. This irreversible change in the shape of the active site is called denaturation. The denatured enzyme is no longer functional.

2. pH

Every enzyme has an optimum pH at which its activity is highest. For example, pepsin in the acidic environment of the stomach works best at pH 2, whereas trypsin in the alkaline small intestine has an optimum pH of around 8. Extreme changes in pH away from the optimum can cause denaturation. The H+ or OH- ions interfere with the ionic bonds and hydrogen bonds holding the protein's structure together. This alters the shape of the active site, preventing the substrate from binding and rendering the enzyme inactive.

3. Substrate Concentration

With a fixed enzyme concentration, increasing the substrate concentration initially increases the rate of reaction. This is because there are more substrate molecules to collide with the enzyme's active sites. However, the rate eventually plateaus and a maximum velocity (Vmax) is reached. At this point, the active sites of all enzyme molecules are occupied, or saturated, with substrate. The reaction cannot proceed any faster, and the enzyme concentration becomes the limiting factor.

4. Enzyme Concentration

Assuming there is an excess of substrate, the rate of reaction is directly proportional to the enzyme concentration. Doubling the enzyme concentration will double the number of available active sites, thus doubling the reaction rate. This linear relationship holds true as long as the substrate is not a limiting factor.