At low substrate concentrations, each enzyme molecule is reacting with fewer substrate molecules than is suggested by its turnover number. As the substrate concentration is increased, each enzyme is able to locate and react with more substrate molecules and the observed enzyme activity increases. However, once the turnover number is reached, the addition of more substrate does not further increase the rate.
Enzyme Concentration
Under normal biological conditions the substrate is present in a large excess (there is much more substrate than enzyme). As long as this condition is maintained, the addition of more enzyme results in a proportional increase in rate.
pH
Enzyme activity is influenced by pH with each enzyme having an optimum pH. The optimum pH is the pH at which the activity of a particular enzyme is at a maximum. The optimum pH's for enzymes found in the body are matched to the pH of the biological systems in which they operate. For example, pepsin, a digestive enzyme, has an optimum pH of 1.5, the pH of the stomach. At pH's far from the optimum pH, enzyme activity can be reduced to nearly zero. If the reduction in activity is reversible the enzyme has been denatured. If it is not reversible, the enzyme has been digested.
Temperature
Just as each enzyme has an optimum pH, each has an optimum temperature as well. Most human enzymes have an optimum temperature about that of body temperature (98.6oF) and are denatured or digested at extreme temperatures.
Presence of Cofactors
Some enzymes are capable of catalytic activity by themselves. Others require the presence of an additional substance called a cofactor to induce this behavior. If the cofactor is an organic compound, it is called a coenzyme. If it is a metal ion, it is called a metal ion activator. If a required cofactor is not present, the catalytic activity of the enzyme is dramatically reduced.
Presence of Inhibitors
Inhibitors are substances that reduce the rate of enzyme activity, usually by binding with the enzyme and interfering with the formation of the enzyme-substrate complex. While some heavy metals act as metal ion activators, they can also act as enzyme inhibitors.
