Explain how a change in pH from the physiological pH 7.4 to pH 3.0 can affect the tertiary structure of the enzyme amylase and consequently its catalytic activity. In your answer discuss (i) the nature of ionic interactions in the protein, (ii) the impact on the active‑site geometry, and (iii) the overall effect on the reaction rate.
Answer
At pH 3.0 many ionizable side chains become protonated, disrupting salt bridges and hydrogen bonds; the active‑site conformation is altered, leading to loss of substrate binding and a marked decrease in catalytic rate.
Step-by-step solution
- Identify ionizable groups in proteins (‑COO⁻ of Asp/Glu and –NH₃⁺ of Lys/Arg).
- At pH 3.0 the carboxylate groups are largely protonated (‑COOH), removing negative charges that participate in salt bridges with positively charged residues.
- Loss of these salt bridges weakens the tertiary structure, causing partial unfolding of the protein.
- The active site of amylase depends on a precise three‑dimensional arrangement of residues; disruption of tertiary contacts distorts this geometry.
- Distorted active‑site geometry reduces substrate binding affinity and proper positioning of catalytic residues.
- Consequently the enzyme’s catalytic rate drops sharply, often to near‑zero, reflecting loss of functional activity.
Common mistake: Assuming denaturation only involves breaking of covalent bonds, ignoring the role of ionic (salt‑bridge) interactions.
Concept tested: Effect of pH on protein tertiary structure
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