Evolutionary link between glycogen phosphorylase and a DNA modifying enzyme.

We report here an unexpected similarity in three‐dimensional structure between glucosyltransferases involved in very different biochemical pathways, with interesting evolutionary and functional implications. One is the DNA modifying enzyme beta‐glucosyltransferase from bacteriophage T4, alias UDP‐glucose:5‐hydroxymethyl‐cytosine beta‐glucosyltransferase. The other is the metabolic enzyme glycogen phosphorylase, alias 1.4‐alpha‐D‐glucan:orthophosphate alpha‐glucosyltransferase. Structural alignment revealed that the entire structure of beta‐glucosyltransferase is topographically equivalent to the catalytic core of the much larger glycogen phosphorylase. The match includes two domains in similar relative orientation and connecting helices, with a positional root‐mean‐square deviation of only 3.4 A for 256 C alpha atoms. An interdomain rotation seen in the R‐ to T‐state transition of glycogen phosphorylase is similar to that observed in beta‐glucosyltransferase on substrate binding. Although not a single functional residue is identical, there are striking similarities in the spatial arrangement and in the chemical nature of the substrates. The functional analogies are (beta‐glucosyltransferase‐glycogen phosphorylase): ribose ring of UDP‐pyridoxal ring of pyridoxal phosphate co‐enzyme; phosphates of UDP‐phosphate of co‐enzyme and reactive orthophosphate; glucose unit transferred to DNA‐terminal glucose unit extracted from glycogen. We anticipate the discovery of additional structurally conserved members of the emerging glucosyltransferase superfamily derived from a common ancient evolutionary ancestor of the two enzymes.