Lactase hydrolyzes the milk sugar lactose but leaves the similarly sized disaccharide sucrose untouched. Which structural feature best explains this selectivity?

Core Concept

A catalytic pocket arranges specific amino acid side chains in a precise three dimensional geometry. Only substrates with matching contours and chemical groups position properly for bond breaking, often after slight induced fit adjustments of the complex. Lactose presents its galactose glucose linkage in the productive orientation, while the sucrose linkage cannot bind usefully, so the enzyme excludes it despite similar molecular size.