Enzymes

Enzymes A catalyst is a substance that speeds up a reaction, without itself being used up in the reaction. Enzymes are catalysts made of protein. They are referred to as biological catalysts. They are made in living cells and they speed up reactions without being used up themselves. Enzymes are folded into globular shapes. They are described as globular proteins. The three dimensional shape of an enzyme means that it will fit neatly and react only with a substance of a shape that matches the enzyme. Similar to a lock and key mechanism. Anything that changes the shape of an enzyme will reduce the efficiency of the enzyme to speed up a reaction. The parameters that cause enzymes to change shape would include pH and temperature.

Enzyme Action The substance in which an enzyme interacts with is called the substrate. The substances that an enzyme form(s) is called the product. The active site of an enzyme is the region that binds the substrate and converts it to the product. It is usually a relatively small part of the whole enzyme molecule. The active site is normally a crevice or surface on the enzyme molecule. The enzyme substrate complex is formed through binding of a substrate to the active site. Enzymes work because they have the correct shape to fit the substate. Enzyme reactions are reversible. It can cause a reaction to proceed in either direction. This means that an enzyme can be either anabolic or catabolic. Anabolic means forming more complex compounds. Catabolic means breaking down larger compounds. When a substrate binds to an enzyme, an enzyme-substrate complex is formed. This is called the "Lock and Key Model".

The Role of EnzymesCatabolic Enzymes break down larger compounds. For example, Amylase is an enzyme that converts starch into maltose. Starch is broken down into simpler parts. Amylase is produced by the salivary glands in the mouth and by the pancreas. Anabolic Enzymes form more complex compounds. For example, DNA polymerase is an enzyme that forms and repairs DNA. It converts simpler molecules into a more complex form. DNA polymerase is found in a vast majority of organisms.

Enzyme InhibitorsInhibitors are chemicals that attach to an enzyme and destroy its shape. The enzyme is said to be denatured (unfolded). The enzyme activity is decreased through the action of inhibitors. The binding of an inhibitor stops the substrate from interacting with the enzymes active site and/or hindering the enzyme from catalising its reaction. When an inhibition binds to an enzyme, an Enzyme-Inhibitor complex is formed. Many drugs act as enzyme inhibitors. Therefore these inhibitors are of benefit to humans. For example, Enzyme inhibitors are often used in cancer therapeutics. Folic acid is a substrate of dihydrofolate reductase. This is an enzyme involved in making nucleotides. It is potently inhibited by methotrexate. Methotrexate blocks the action of dihydrofolate reductase and thereby halts the production of nucleotides. This block of nucleotide biosynthesis is more toxic to rapidly growing cells. Therefore methotrexate is used as a chemotherapeutic agent blocking the rapidly dividing cancer cells. Another form of enzyme inhibition is seen in the use of antibiotics. Antibiotics are used to inhibit the survival of pathogens. For example, bacteria is surrounded by a thick cell wall. The components that make up the cell wall are peptidoglycans. Many antibiotics, such as penicillin, inhibit the enzymes that produce peptidoglycan.

Factors affecting enzyme activityEnzymes work best under ideal conditions. Any change in these conditions will slow down the rate of the reaction. Two factors that can affect the enzyme activity include Temperature and pH. TemperatureAt very low temperatures, ice is formed. At these low temperatures, the cell contents become solid and enzymes cannot work. As the temperature increases from zero degrees celsuis, the rate of molecular movement increases. This causes the substrate molecules and enzymes to 'bump' into each other more often. The rate of the reaction increases as a result. Human enzymes work best at a temperature of 37 degrees celsius. This is the optimum body temperature for humans. Plant enzymes work best at temperatures between 20-30 degrees celsius. Above a certain temperature, the enzyme loses its three dimensional shape. The rate of the reaction begins to fail. WHen the shape of the enzyme is fully lost, above 50 degrees celsius, the enzyme is denatured.pHEach enzyme has an optimum pH at which the rate of the reaction that it catalyses is at its maximum. Small deviations from this pH can lead to a decrease in the activity. Large deviations in this pH can lead to enzyme denaturation. The pH scale runs from 0 to 14. Between values 0 to 7, the pH is acidic, pH 7 is neutral, between 7-14 pH is basic (alkaline). Enzymes work over a very narrow pH. For most enzymes, this is between pH 6-8. The optimum pH for most enzymes is at pH 7. This can vary depending on where the enzyme carries out its action. For example, Pepsin is the enzyme in the stomach. Its optimum pH is at a pH of 2, as the stomach is an acidic environment. Both graphs on the right describe the relationship between the rate of enzyme activity with pH or temperature. The curve is called a 'Bell-Shaped' Curve.

Substrate and Enzyme ConcentrationAnother factor that affects the enzyme activity is the enzyme and substrate concentration. At low substrate concentrations ([S]), a doubling of [S] leads to a doubling of V0 (Initial Velocity). Whereas at a higher [S], the enzyme becomes saturated and there is no further increase in the V0, known as the Vmax. A graph of V0 against [S] will give a hyperbolic curve.

Enzymes