High resolution crystal structures of T4 phage β-glucosyltransferase: induced fit and effect of substrate and metal binding

β-Glucosyltransferase (BGT) is a DNA-modifying enzyme encoded by bacteriophage T4 that transfers glucose from uridine diphosphoglucose to 5-hydroxymethyl cytosine bases of phage T4 DNA. We report six X-ray structures of the substrate-free and the UDP-bound enzyme. Four also contain metal ions which activate the enzyme, including Mg2+ in forms 1 and 2 and Mn2+ or Ca2+. The substrate-free BGT structure differs by a domain movement from one previously determined in another space group. Further domain movements are seen in the complex with UDP and the four UDP-metal complexes. Mg2+, Mn2+ and Ca2+ bind near the β-phosphate of the nucleotide, but they occupy slightly different positions and have different ligands depending on the metal and the crystal form. Whilst the metal site observed in these complexes with the product UDP is not compatible with a role in activating glucose transfer, it approximates the position of the positive charge in the oxocarbonium ion thought to form on the glucose moiety of the substrate during catalysis.