Mechanistic snapshots of lipid-linked sugar transfer.

Enzymes undergo dynamic conformational changes during catalysis, yet conventional high-resolution structural methods typically capture only the most stable states. Here, we address this gap using rapid UV photolysis of a chemically caged substrate with cryogenic time-resolved electron microscopy (cryo-TREM). We elucidate the catalytic mechanism of GtrB, a membrane-bound glycosyltransferase that transfers glucose from UDP-glucose to the lipid carrier undecaprenyl phosphate. We visualized how GtrB, which has an active site ~15 Å from the membrane, transitions during the catalytic cycle to move each substrate in proximity for catalysis. From a single dataset, we resolved distinct conformational states: the initial substrate-bound state, a catalytically poised intermediate, and the product-bound state. Through molecular dynamics simulations and biochemical analyses, we identify coordinated movements within the active site that drive catalysis. These findings provide a molecular framework for understanding how glycosyltransferases function and highlight a broadly applicable strategy for capturing dynamic enzymatic states in native-like environments.