Possible structure and active site residues of starch, glycogen, and sucrose synthases

A group of enzymes that include muscle glycogen phosphorylase and sugar transferases involved in, for example, the glucosylation of DNA and the synthesis of peptidoglycan are known to possess the same basic three-dimensional fold. Here the possibility is examined that other monosaccharide transferases, those that catalyze synthesis of starch, glycogen, and the disaccharide sucrose, resemble the phosphorylase-type enzymes in structure. In particular, a clear relationship is shown, for the first time, between mammalian glycogen synthases and the phosphorylase structural group of proteins. Domain architecture and secondary structure are discussed, and the possible role of several conserved amino acids at the active site is explored.