On the donor substrate dependence of group‐transfer reactions by hydrolytic enzymes: Insight from kinetic analysis of sucrose phosphorylase‐catalyzed transglycosylation

Chemical group‐transfer reactions by hydrolytic enzymes have considerable importance in biocatalytic synthesis and are exploited broadly in commercial‐scale chemical production. Mechanistically, these reactions have in common the involvement of a covalent enzyme intermediate which is formed upon enzyme reaction with the donor substrate and is subsequently intercepted by a suitable acceptor. Here, we studied the glycosylation of glycerol from sucrose by sucrose phosphorylase (SucP) to clarify a peculiar, yet generally important characteristic of this reaction: partitioning between glycosylation of glycerol and hydrolysis depends on the type and the concentration of the donor substrate used (here: sucrose, α‐d‐glucose 1‐phosphate (G1P)). We develop a kinetic framework to analyze the effect and provide evidence that, when G1P is used as donor substrate, hydrolysis occurs not only from the β‐glucosyl‐enzyme intermediate (E‐Glc), but additionally from a noncovalent complex of E‐Glc and substrate which unlike E‐Glc is unreactive to glycerol. Depending on the relative rates of hydrolysis of free and substrate‐bound E‐Glc, inhibition (Leuconostoc mesenteroides SucP) or apparent activation (Bifidobacterium adolescentis SucP) is observed at high donor substrate concentration. At a G1P concentration that excludes the substrate‐bound E‐Glc, the transfer/hydrolysis ratio changes to a value consistent with reaction exclusively through E‐Glc, independent of the donor substrate used. Collectively, these results give explanation for a kinetic behavior of SucP not previously accounted for, provide essential basis for design and optimization of the synthetic reaction, and establish a theoretical framework for the analysis of kinetically analogous group‐transfer reactions by hydrolytic enzymes.