The pH level affects the attraction between the enzyme and the substrate. The pH level is measured by how many hydrogen ions (H+) are present: the more H+ ions the lower the pH level. As with temperature, enzymes have an optimum pH level. For example, in the body:

• intracellular enzymes work best at pH 7
• digestive enzymes work best in acidic conditions (pH 1 to 6)
• enzymes found in oral cavities work best in alkaline conditions (pH 8 to 14)

If a pH level is too high or too low for an enzyme it will become denatured. When this happens the ionic bonds will break causing the enzyme to change shape. The charge on its amino acids can also change which changes the attraction between the enzyme and the substrate, preventing the formation of the ES complex.

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Substrate Concentration

The concentration of substrate present will also affect the rate of reaction.

• If all other conditions are kept constant then the concentration of enzyme is proportional to the rate of reaction.
• The substrate concentration is also proportional to the rate of reaction however only until there are more substrates present than there are enzymes.