Question

Describe how the protein structure is effected when pH is increased or decreased thus effecting catalysis

Answer 1

Changes in pH change the attractions between the groups in the side chains of the protein by affecting the interaction between acidic and basic ends of side chains.

Changing the pH disrupts the hydrogen bonds, and this changes the shape of the protein.

Salt bridges are ionic bonds between positively and negatively charged side chains of amino acids. An example is the attraction between a -COO^- ion of lysine and an -NH⁺³ ion of aspartic acid.

• Increasing the pH by adding a base converts the -NH⁺³ ion to a neutral -NH² group.
• Decreasing the pH by adding an acid converts the –COO^- ion to a neutral -COOH group.

In each case the ionic attraction disappears, and the protein shape unfolds.

If the pH strays too far from optimum, the presence of H+ or OH– ions will disrupt the bonds in the tertiary structure. It also affects the electrostatic attraction between the enzyme and substrate. Each amino acid has a residual or ‘R’ group. The charge of the R group is often opposite to the charge of the specific substrate. The two opposite charges attract, and this helps to bring the substrate to the enzyme. If there are other ions present, then the electrostatic attraction can occur between the ions of the solution and the R groups, and again this decreases the rate of reaction. The disruption of the bonds causes the enzyme to denature. If the pH is too extreme then the changes will be irreversible, however often the changes are reversible.