Pharma gyan Pandit

Enzyme Activity Factors:

An enzyme catalyzed reaction may be written as:

E is the enzyme

S is the substrate (reactant)

ES is the enzyme-substrate complex

P is the product

The Equation which describes rate of reaction (r) as a function of substrate concentration (S) is the Michaelis-Menton Equation.

Vmax is the function of the concentration of the enzyme [E] and the Turnover Number of the enzyme.

Km is the Michaelis-Menton constant and is determined experimentally, just as the rate constant is for a rate law equation.

Remember, the Turnover Number refers to the efficiency of the enzyme and is expressed as the number of molecules of substrate converted to product per second.

The Turnover Number of enzymes can range from 10 to 100,000 molecules per second, demonstrating the effective catalytic nature of some enzymes.

Substrate Concentration

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 (98.6^oF) 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.