Mannosidases play a key role in the processing of
glycoproteins and thus are of considerable
pharmaceutical interest and indeed have emerged as targets for the development of anti-
cancer therapies. Access to useful quantities of the mammalian
enzymes has not yet been achieved; therefore, jack bean
mannosidase, a readily available
enzyme, has become the model system. However, the relevance of this
enzyme has not been demonstrated, nor is anything known about the active site structure of this, or any other,
mannosidase. Hydrolysis by this
enzyme occurs with net retention of
sugar anomeric configuration; thus, a double displacement mechanism involving a mannosyl-
enzyme intermediate is presumably involved. Two new mechanism-based inhibitors, 5-fluoro-alpha-D-mannosyl
fluoride and
5-fluoro-beta-L-gulosyl fluoride, which function by the steady state trapping of such an intermediate, have been synthesized and tested. Both show high affinity for jack bean
alpha-mannosidase (Ki' = 71 and 86 microM, respectively), and the latter has been used to label the active site nucleophile. The labeled
peptide present in a peptic digest of this trapped glycosyl-
enzyme intermediate was identified by neutral loss scans on an electrospray ionization triple quadrupole mass spectrometer. Comparative liquid chromatographic/mass spectrometric analysis of peptic digests of labeled and unlabeled
enzyme samples confirmed the unique presence of this
peptide of m/z 1180.5 in the labeled sample. The label was cleaved from the
peptide by treatment with
ammonia, and the resultant unlabeled
peptide was purified and sequenced by Edman degradation. The
peptide identified contained only one candidate for the catalytic nucleophile, an
aspartic acid. This residue was contained within the sequence
Gly-Trp-Gln-Ile-
Asp-Pro-
Phe-Gly-
His-Ser, which showed excellent sequence similarity with regions in mammalian lysosomal and Golgi
alpha-mannosidase sequences. These mammalian alpha-
mannosidases belong to family 38 (or class II alpha-
mannosidases) in which the Asp in the above sequence is totally conserved. This finding therefore assigns jack bean
alpha-mannosidase to family 38, validating it as a model for other pharmaceutically interesting
enzymes and thereby identifying the catalytic nucleophile within this family.