1HXQ

Nucleotidyltransferases catalyse the covalent modification of a variety of biological molecules. These reactions are crucial for the synthesis of coenzymes, cyclic nucleotides, polynucleotides, and nucleotide sugars. These reactions involve substitutions at the R -phosphorus of a nucleotidyl donor substrate and result in displacement of a phosphoryl ester or pyrophosphate. Substrates for such reactions may include nucleoside di- or triphosphates, as well as nucleotide sugars, such as UDP-Glc. Galactose-1-phosphate uridylyltransferase (hexose-1-phosphate uridylyltransferase) catalyses the exchange of the UMP moiety between the hexose 1-phosphates of Glc and Gal and their corresponding UDP-sugar. The enzyme is distinct among nucleotidyl transferases that use phosphates as acceptor groups in that it is the only one that does not utilise nucleoside di- or triphosphates as the nucleotidyl donor substrate. The reaction is part of the Leloir pathway of galactose metabolism required for the normal equilibration of UDP-hexoses among most organisms. Deficiencies in uridylyltransferase activity culminate in the metabolic disease galactosemia, which occurs as an autosomal recessive trait.