Phosphorylase mechanism. A bound HPO42−group (red) favors the cleavage of the glycosidic bond by donating a proton to the C-4 oxygen of the departing glycosyl group (black). This reaction results in the formation of a carbonium ion and is favored by the transfer of a proton from the protonated phosphate group of the bound pyridoxal phosphate (PLP) group (blue). The carbonium ion and the orthophosphate combine to form glucose 1-phosphate.