(i) Substrate concentration : It depends on the type of enzyme reaction. For a competitive enzyme reaction, the increase in substrate concentration can have an inverse effect on the activity, but for a normal enzyme reaction, as long as the & substrate concentration isn’t saturated, in general it will increase activity. At lower concentrations, the active sites on most of the enzyme molecules are not filled because there is not much substrate. Higher concentrations cause more-collisions between the molecules. With more molecules and collisions, enzymes are more likely to encounter molecules of reactant. The maximum velocity of a reaction is reached when the active sites are almost continuously filled. Increased substrate concentration after this point will not increase the rate. Reaction rate, therefore, increases as substrate concentration is increased but it levels off.
(ii) Enzyme Concentration : If there is insufficient enzyme present, the reaction will not proceed as fast as it otherwise would because all of the active sites are occupied with the reaction. Additional active sites could speed up the reaction. As the amount of enzyme is increased, the rate of reaction increases. This is because when more enzyme molecules are present,more substrate molecules can be acted upon at the same time. This means that the total substrate molecules are broken down quickly. If there are more enzyme molecules than are needed, adding additional enzyme will not increase the rate. Reaction rate therefore increases as enzyme concentration increases but then its levels off.
(iii) Temperature : Higher temperature generally causes more collisions among the molecules and therefore, increases the rate of a reaction. More collisions increase the likelihood that substrate will collide with the active site of the enzy me, thus increasing the rate of an enzyme-catalyzed reaction.
Above a certain temperature, activity begins to decline because the enzyme begins to denature. The rate of chemical reactions, therefore, increases with temperature but then decreases as enzymes denature.
Optimum pH : Every enzyme has an optimum pH when it is most effective. A rise or fall in pH reduces enzyme activity by changing the degree of ionization of its side chains. A change in pH may also start reverse reaction. Fumarase catalyses fiimarate → malate at 6.2 pH and reverse at 7.5 pH. Most of the intracellular enzymes function near neutral pH with the exception of several digestive enzymes which work either in acidic range of pH or alkaline, e.g., 2.0 pH for pepsin, 8.5 for trypsin.