Identification and Modification of the Uridine-binding Site of the UDP-GalNAc (GlcNAc) Pyrophosphorylase

Abstract

UDP-GalNAc pyrophosphorylase (UDP-GalNAcPP; AGX1) catalyzes the synthesis of UDP-GalNAc from UTP and GalNAc-1-P. The 475-amino acid protein (57 kDa protein) also synthesizes UDP-GlcNAc at about 25% the rate of UDP-GalNAc. The cDNA for this enzyme, termed AGX1, was cloned in Escherichia coli, and expressed as an active enzyme that cross-reacted with antiserum against the original pig liver UDP-HexNAcPP. In the present study, we incubated recombinant AGX1 with N(3)-UDP-[(32)P]GlcNAc and N(3)-UDP-[(32)P]GalNAc probes to label the nucleotide-binding site. Proteolytic digestions of the labeled enzyme and analysis of the resulting peptides indicated that both photoprobes cross-linked to one 24-amino acid peptide located between residues Val(216) and Glu(240). Four amino acids in this peptide were found to be highly conserved among closely related enzymes, and each of these was individually modified to alanine. Mutation of Gly(222) to Ala in the peptide almost completely eliminated UDP-GlcNAc and UDP-GalNAc synthesis, while mutation of Gly(224) to Ala, almost completely eliminated UDP-GalNAc synthesis, but UDP-GlcNAc was only diminished by 50%. Both of these mutations also resulted in almost complete loss of the ability of the mutated proteins to cross-link N(3)-UDP-[(32)P]GlcNAc or N(3)-UDP-[(32)P]GalNAc. On the other hand, mutations of either Pro(220) or Tyr(227) to Ala did not greatly affect enzymatic activity, although there was some reduction in the ability of these proteins to cross-link the photoaffinity probes. We also mutated Gly(111) to Ala since this amino acid was reported to be necessary for catalysis (Mio, T., Yabe, T., Arisawa, M., and Yamada-Okabe, H. (1998) J. Biol. Chem. 273, 14392-14397). The Gly(111) to Ala mutant lost all enzymatic activity, but interestingly enough, this mutant protein still cross-linked the radioactive N(3)-UDP-GlcNAc although not nearly as well as the wild type. On the other hand, mutation of Arg(115) to Ala had no affect on enzymatic activity although it also reduced the amount of cross-linking of N(3)-UDP-[(32)P]GlcNAc. These studies help to define essential amino acids at or near the nucleotide-binding site and the catalytic site, as well as peptides involved in binding and catalysis.