Identification of catalytic amino acids in the human GTP fucose pyrophosphorylase active site

GTP-l-fucose pyrophosphorylase(GFPP) catalyzes the reversible formation of the nucleotide-sugar GDP-beta-l-fucose from guanosine triphosphate and beta-l-fucose-1-phosphate. The enzyme functions primarily in the mammalian liver and kidney to salvage free fucose during the breakdown of glycoproteins and glycolipids. GFPP shares little primary sequence identity with other nucleotide-sugar metabolizing enzymes, and the three-dimensional structure of the protein is unknown. The enzyme does contain several sequences that could be nucleotide binding sites, but none of them are an exact match to consensus sequences. Using a combination of site-directed mutagenesis and UV photoaffinity cross-linking, we have identified five amino acid residues that are critical for catalysis. Some of these amino acids are found within the poorly conserved nucleotide binding consensus structures, while others represent new motifs. Two active site lysines can be cross-linked to photoaffinity probes. The site of cross-linking depends on the probe used. The identification of these critical residues highlights how distinct GFPP is from other nucleotide-sugar pyrophosphorylases.