The above diagram shows a two-dimensional representation of an enzyme binding with a substrate molecule and them reacting together to form an enzyme-substrate complex, then an enzyme-product complex and finally the product leaving the active site. The enzyme is not changed in any way by the reaction and so is reusable. The same enzyme can also catalyse the reverse reaction.
However, it has been discovered that competitors for an active site (similar in shape to the substrate) could fit even though they are larger than the substrate. This means that the substrate and active site are a little flexible. This is known as the induced fit hypothesis this hypothesis suggests, the substrate does not simply bind with the active site. It has to bring about changes to the shape of the active site to activate the enzyme and make the reaction possible. So small molecules may enter the active site, but they cannot induce the changes in shape to make the enzyme behave like a catalyst. The hypothesis suggests that when the enzyme's active site comes into contact with the right substrate, the active site slightly changes or moulds itself around the substrate for an effective fit (the substrate induces the active site to change shape). The reaction will take place and the product, being a different shape to the substrate, moves away from the active site. The active site then returns to its original shape. This shape adjustment triggers catalysis and helps to explain why enzymes only catalyse specific reactions. Below is a two-dimensional diagram to show the induced fit hypothesis.
Reactions proceed because the products have less energy than the substrates. However, most substrates require an input of energy to get the reaction going. The energy required to initiate the reaction is called the activation energy. When the substrate reacts, they need to form a complex called the transition state before the reaction actually occurs. This transition state has a higher energy level than either the substrates or the product. Outside the body, high temperatures often supply the energy required for a reaction. This would be hazardous inside the body though. Fortunately enzymes provide an alternative way with a different transition state and lower activation energy. Below is a graph showing that the activation energy of a reaction is smaller in the presence of an enzyme.
The active site is held together by hydrogen bonds and ionic bonds. If the bonds break, the shape of the active site changes, and therefore the enzyme can no longer act as a catalyst, it is denatured. There are two factors that can affect an enzyme’s shape, and therefore how much it functions, are temperature and pH. The temperature at which an enzyme works best is known as its optimum temperature. Below this, an increase in temperature provides more kinetic energy to the molecules involved. The numbers of collisions between enzyme and substrate will increase so the rate of reaction will too. Above the optimum temperature, and the enzymes are denatured, because the kinetic energy causes the enzyme to vibrate, if the enzymes vibrate too much then the bonds holding the structure together will be broken and the active site loses its shape and will no longer work. Most enzymes have an optimum temperature between 40 and 50°C.
The pH at which an enzyme works best is called it optimum pH. pH is a measure of hydrogen ions (H+), or the number of hydrogen ions in a certain volume. The concentration of hydrogen ions affects the bonds and ionic bonds. If the pH changes much from the optimum, the chemical nature of the amino acids can change. This may result in a change in the bonds and so the tertiary structure may break down. The active site will be disrupted and the enzyme will be denatured.
Some enzymes are exceptions to these temperatures and pHs. Some enzymes can live cold environments, since there are more cold environments than hot ones, for example, oceans, which have average temperatures from 1-3°C. These organisms are called psychrophiles. Also some organisms like the warmer climate, they are called thermophiles, they can grow and reproduce in temperatures higher than 45°C, for example in hot springs. Similarly some enzymes like acidic conditions, with a pH lower than 5, they are called Acidophiles. Some enzymes also prefer alkaline environments with a pH above 9 they are called alkophiles.
Enzyme and substrate concentration can also affect enzyme activity. At low enzyme concentrations there is great competition for the active sites and the rate of reaction is low. As the enzyme concentration increases, there are more active sites and the reaction can proceed at a faster rate. Eventually, increasing the enzyme concentration beyond a certain point has no effect because the substrate concentration becomes the limiting factor. We call this the point of optimal concentration. At a low substrate concentration there are many active sites that are not occupied. This means that the reaction rate is low. When more substrate molecules are added, more enzyme-substrate complexes can be formed. As there are more active sites, and the rate of reaction increases. Eventually, increasing the substrate concentration yet further will have no effect. The active sites will be saturated so no more enzyme-substrate complexes can be formed. We call this the point of optimal concentration.
Enzymes are extremely valuable biological catalysts to living organisms as they control biological processes and allow them to take place in the conditions, which occur inside living organisms.