Carbohydrates and Glycoconjugates (5D) Practice Test

A biochemical assay measures the initial rate of an enzyme that cleaves a specific glycosidic bond in a glycolipid. The enzyme is tested at pH 5.0 and pH 8.0 with identical substrate concentration. The rate is much higher at pH 5.0. The active site is proposed to use general acid catalysis to protonate the glycosidic oxygen during bond cleavage.

Which change is most likely to reduce activity at pH 5.0 by disrupting the proposed mechanism?

Reference: General acid catalysis requires a protonated residue to donate H$^+$ during the reaction.

A physiology study compares red blood cells (RBCs) placed in two buffered solutions at 37°C: Solution 1 contains 300 mOsm of NaCl; Solution 2 contains 300 mOsm of glucose. After 30 minutes, RBCs in Solution 1 maintain volume, while RBCs in Solution 2 swell and lyse. Assume RBC membranes are highly permeable to glucose via GLUT transporters but have low permeability to Na$^+$ and Cl$^-$.

Which statement best explains the different outcomes?

Reference: A solution can be iso-osmotic yet not isotonic if its solutes can cross the membrane.