Paper 2 · Enzymes
How do extreme pH values affect the structure of the enzyme pepsin, reducing its catalytic activity?
AThey break the peptide bonds linking amino acids together, splitting the polypeptide chain into several smaller separate fragments that can no longer fold properly.
BThey alter the charges on R-groups of amino acids, disrupting the ionic and hydrogen bonds that hold the tertiary structure together and changing the active site's shape.
CThey raise the kinetic energy of substrate molecules so much that the molecules can no longer collide with or enter the enzyme's active site region.
DThey form a thick layer of water molecules across the active site, physically blocking the substrate from binding to the enzyme's surface completely.
Explanation: Extreme pH values add or remove hydrogen ions from acidic and basic R-groups, changing their charges; this disrupts the ionic and hydrogen bonds that hold the tertiary structure in place, distorting the shape of the active site so the substrate no longer fits (denaturation). Peptide bonds themselves are not broken by pH alone, and pH does not act by changing substrate kinetic energy or coating the active site with water.
Derived from ZIMSEC Biology Paper 2, November 2004, Q7
