What do Enzymes do?
Enzymes are biological catalysts. This means that they are highly specific to a reaction and control the rate at which it occurs at in the cell. This involves lowering the activation energy. So that the enzyme and substrate can collide successfully with lower kinetic energy (so reactions will work at fairly low temperatures), and enzyme substrate complexes will form so a reaction will occur.
How do Enzymes Work?
For a reaction to work molecules must collide successfully with enough energy, known as the activation energy. The greater the kinetic energy molecules have, the greater the chances of a successful collision and therefore the greater the rate of reaction will be. Enzymes make it easier for the reaction to occur, by the formation of enzyme – substrate complexes reducing the activation energy of the reaction.
Enzymes are substrate specific. This means that it has a highly specific 3D shape. The Active site of an enzyme is both shaped and charged to allow a specific substrate to bind with the enzyme forming an enzyme – substrate complex. They have complimentary shapes. The reaction can then proceed and a product is formed. The enzyme then releases the products from its active site, and another substrate molecule can bind with the enzyme.
This is often described as the lock and key mechanism. As the enzyme acts as the lock and the substrate acts as the key.
A newer theory is the Induced Fit Hypothesis. The enzymes active site is similarly shaped to fit the shape of the substrate, however when the substrate and the enzyme are close together and bind, the active site of the enzyme changes shape and becomes fully complimentary to the substrates shape.
Enzymes are Sensitive…
The rate at which an enzyme-controlled reaction will proceed at is effected by outside conditions.
Enzymes work at an optimum temperature. At higher temperatures the molecules have greater kinetic energy so more collisions occur so the rate of reaction is higher. However at very high temperatures enzymes are denatured. This is when the tertiary structure of the enzyme (as it is a protein) is lost by hydrogen bonds being broken. So the enzyme looses its specific shape and therefore its active site. This means that enzyme – substrate complexes cannot form and the reaction cannot occur.
The concentration of either the enzyme or substrate can limit the rate of reaction. The greater the concentration the greater the chances of a successful collision between an enzyme and a substrate, and the formation of an enzyme – substrate complex so a reaction occurs at a higher rate.
Enzymes work at an optimum pH. In very acidic or alkaline conditions, the number of hydrogen bonds and ionic bonds in the tertiary structure of the protein is affected. So the enzyme changes shape, loosing its specifically shaped active site and is denatured, so the reaction does not proceed.
Some enzymes rely on the presence of cofactors. These are often a metal ion, which bonds with either the enzyme or the substrate to reduce the activation energy. It may also be an organic molecule called a coenzyme that temporarily bonds to the enzyme active site during the reaction, it can also act as the link between two reactions. Also an enzyme may have a coenzyme permanently bonded to it, this is known as a prosthetic group.
Enzyme controlled reactions can be prevented by inhibition.
Competitive inhibitors prevent enzyme-substrate complexes forming. The inhibitor competes with the substrate to fit in the active site of the enzyme. So less enzyme-substrate complexes form and the rate of reaction is decreased. This can be overcome by increasing the amount / concentration of substrate.
Non-competitive inhibitors act by binding to the enzyme at an area away from its active site, however the specific shape of the active site is changed. Therefore it is no longer specific to the substrate, so enzyme – substrate complexes do not form and the rate of reaction is decreased.
Uses of Enzymes
Enzymes are used in living organisms to control anabolic and catabolic reactions.
E.g. Starch + Amalyase = 2Maltose
Enzymes are used in food technology to avoid spoilage in processed foods.
Enzymes that are acid tolerant are added to animal feed grains to make them more digestible.
Diagnostic enzymes are used as biosensors, for example Clinistix to detect glucose in urine. Or, breathalyser to detect ethanol (alcohol). Ethanol and glucose act as substrates for enzymes on the biosensors.
