RESUMO
a-L-Fucosidases are widely occurring enzymes that remove fucose residues from N- and O-fucosylated glycoproteins. Comparison of amino acid sequences of fucosidases reveals that although the nucleophile is conserved among all a-L-fucosidases, the position of the acid/base residue is quite variable. Although several site-directed mutation studies have previously been performed on bacterial fucosidases, the only eukaryotic fucosidase so studied was the human fucosidase. Recent alignments indicate that human and Arthropoda a-L-fucosidases share at least 50% identity and the acid/base residue seems to be conserved among them suggesting a common acid/base residue in Metazoa. Here we describe the cloning and expression in Pichia pastoris of a very active a-L-fucosidase from the spider Nephilingis cruentata (NcFuc) with a Km value for pNPFuc of 0.4 mM. NcFuc hydrolyzed fucoidan, 2'fucosyllactose and also lacto-N-difucohexaose II. Mutants modified at the conserved residues D214N, E209A, E59A were expressed and characterized. The 500-fold lower kcat of D214N than the wild type was consistent with a role in catalysis, as was the 8000-fold lower kcat value of E59A. This was supported by the 57-fold increase in the kcat of E59A upon addition of azide. A complex pH/rate profile was seen for the wild-type and mutant forms of NcFuc, similar to those measured previously for the Sulfolobus fucosidase. The non-conservative catalytic structure and distinct active site organization reinforce the necessity of structural studies of new fucosidases.
RESUMO
l-fucose is a constituent of glycoconjugates in different organisms. Fucosidases catalyze the removal of fucose residues, and have been correlated to different physiological and pathological processes, such as fertilization, cancer, fucosidosis, and digestion in molluscs and ticks. An alpha-L-fucosidase sequence was identified from the transcriptome and proteome from the midgut diverticula of the synanthropic spider Nephilingis cruentata. In this article, we describe the isolation of this alpha-L-fucosidase and the characterization of its activity using substrates and inhibitors demonstrating different specificities among fucosidases. The enzyme had a K-m of 32 and 400 mu M for 4-methylumbelliferyl alpha-L-fucopyranoside and 4-nitrophenyl alpha-L-fucopyranoside, respectively; and was unable to hydrolyze fucoidan. Nephilingis cruentata alpha-L-fucosidase was inhibited competitively by fucose and fuconojyrimycin. The fucosidase had two distinct pH optima even in the isolated form, due to oligomerization dependent on pH, as previously described to other fucosidases. Alignment and molecular homology modeling of the protein sequence with other fucosidases indicated that the active sites and catalytic residues were different, including residues involved in acid/base catalysis. Phylogenetic analysis showed, for the first time, gene-duplication events for fucosidases in Arachnida species. All these data reveal that studies on fucosidases in organisms distinct from bacteria, fungi, and humans are important.