Interpreting and explaining results: effect of pH

The graph shows the rate of reaction at different pH’s for an enzyme-catalysed reaction.

A

• at the optimum pH, the shape of the active site enables bonds to form successfully with the substrate

• greatest frequency of enzyme-substrate complex formation and highest rate of reaction.

B

• in low pH there is a high concentration of H⁺ ions (acid conditions)

• more amino groups will have a positive charge so will affect hydrogen and ionic bonding in the protein

• this will change the 3D shape of the active site

• as the pH becomes more acidic fewer bonds can form between the active site and the substrate molecules

• fewer enzyme-substrate complexes form and the rate decreases.

C

• at pH values above the optimum, not enough H⁺ ions are present

• increasing number of carboxylic acid groups have a negative charge

• hydrogen and ionic bonding are affected and the 3D shape of the active site changes making it less able to form bonds with the substrate

• as the pH becomes more basic, fewer bonds can form between the active site and the substrate molecules

• fewer enzyme-substrate complexes form and the rate decreases.

Small changes in pH are reversible but large changes in pH are irreversible – new bonds form that permanently change the 3D shape of the polypeptide chain.

Because enzymes are affected by pH, when you investigate the effect of pH on an enzyme you need to use a buffer – this is a solution of chemicals that can maintain a constant pH.