Galactose-1-phosphate uridyl transferase, class I His-active site <p>Galactose-1-phosphate uridyl transferase <db_xref db="EC" dbkey="2.7.7.12"/> (galT) catalyses the transfer of an uridyldiphosphate group on galactose (or glucose) 1-phosphate. During the reaction, the uridyl moiety links to a histidine residue. In the <taxon tax_id="562">Escherichia coli</taxon> enzyme, it has been shown [<cite idref="PUB00004352"/>] that two histidine residues separated by a single proline residue are essential for enzyme activity. The first H binds zinc and the second H is an active site residue. </p><p>On the basis of sequence similarities, two apparently unrelated families seem to exist. Class-I enzymes are found in eukaryotes as well as some bacteria such as E. coli or <taxon tax_id="1916">Streptomyces lividans</taxon>, while class-II enzymes have been found so far only in bacteria such as <taxon tax_id="1423">Bacillus subtilis</taxon> or <taxon tax_id="1587">Lactobacillus helveticus</taxon> [<cite idref="PUB00002146"/>].</p>