Structural Basis of the Bifunctionality of Marinobacter salinexigens ZYF650^T Glucosylglycerol Phosphorylase in Glucosylglycerol Catabolism

2-O-α-Glucosylglycerol (GG) is a natural heteroside synthesized by many cyanobacteria and a few heterotrophic bacteria under salt stress conditions. Bacteria produce GG in response to stimuli and degrade it once the stimulus diminishes. Heterotrophic bacteria utilize GG phosphorylase (GGP), a member of the GH13_18 family, via a two-step process consisting of phosphorolysis and hydrolysis for GG catabolism. However, the precise mechanism by which GGP degrades GG remains elusive. We determined the 3D structure of a recently identified GGP (MsGGP) of the deep-sea bacterium Marinobacter salinexigens ZYF650^T, in complex with glucose and glycerol, α-D-glucose-1-phosphate (αGlc1-P), and orthophosphate (Pi) at resolutions of 2.5, 2.7 and 2.7 Å, respectively. Notably, the first αGlc1-P complex structure in the GH13_18 family, the complex of MsGGP and αGlc1-P, validates that GGP catalyzes GG decomposition through consecutive phosphorolysis and hydrolysis. Additionally, the structure reveals the mechanism of high stereoselectivity on αGlc1-P. Glu231 and Asp190 were identified as the catalytic residues. Interestingly, these structures closely resemble each other, indicating minimal conformational changes upon binding end-product glucose and glycerol, or the intermediate αGlc1-P. The structures also indicate that the substrates may follow a specific trajectory and a precise order towards the active center in close proximity and in a geometrically favorable orientation for catalysis in a double displacement mechanism.