Carbohydrates and Glycoconjugates (5D) Practice Test

A researcher studies an enzyme that transfers a monosaccharide from an activated nucleotide-sugar donor to a hydroxyl group on a protein, forming a new glycosidic bond. In vitro, the reaction rate increases when the concentration of UDP-glucose is raised, but plateaus at high UDP-glucose. The researcher repeats the assay with a mutant enzyme that binds UDP-glucose more weakly; at the same enzyme concentration, the mutant requires higher UDP-glucose to reach half-maximal velocity. Which kinetic conclusion is most consistent with these observations?

A research group studies a lectin that binds specific carbohydrate motifs on cell surfaces. Binding is quantified by fluorescence anisotropy using a fluorescently labeled oligosaccharide ligand. When the ligand’s terminal sialic acid is enzymatically removed (neuraminidase treatment), the dissociation constant $K_d$ increases from 20 nM to 2.0 µM under identical ionic strength and temperature.

Which finding is most consistent with the effect of removing the terminal sialic acid?

Reference: More negative $\Delta G$ corresponds to stronger binding; $\Delta G = RT\ln K_d$ (at constant standard state).