| Summary: | Abstract Human blood group A and B glycosyltransferases (GTA, GTB) are retaining glycosyltransferases, requiring a catalytic mechanism that conserves the anomeric configuration of the hexopyranose moiety of the donor substrate (UDP‐GalNAc, UDP‐Gal). Previous studies have shown that GTA and GTB cycle through structurally distinct states during catalysis. Here, we link binding and release of substrates, substrate‐analogs, and products to transitions between open, semi‐closed, and closed states of the enzymes. Methyl TROSY based titration experiments in combination with zz‐exchange experiments uncover dramatic changes of binding kinetics associated with allosteric interactions between donor‐type and acceptor‐type ligands. Taken together, this highlights how allosteric control of on‐ and off‐rates correlates with conformational changes, driving catalysis to completion.
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