Mucopolysaccharidosis type I (MPS I; McKusick 25280) results from a deficiency in alpha-L-iduronidase activity. Using a bioinformatics approach, we have previously predicted the putative acid/base catalyst and nucleophile residues in the active site of this human lysosomal glycosidase to be Glu182 and Glu299, respectively. To obtain experimental evidence supporting these predictions, wild-type alpha-L-iduronidase and site-directed mutants E182A and E299A were individually expressed in Chinese hamster ovary-K1 cell lines. We have compared the synthesis, processing, and catalytic properties of the two mutant proteins with wild-type human alpha-L-iduronidase. Both E182A and E299A transfected cells produced catalytically inactive human alpha-L-iduronidase protein at levels comparable to the wild-type control. The E182A protein was synthesized, processed, targeted to the lysosome, and secreted in a similar fashion to wild-type alpha-L-iduronidase. The E299A mutant protein was also synthesized and secreted similarly to the wild-type enzyme, but there were alterations in its rate of traffic and proteolytic processing. These data indicate that the enzymatic inactivity of the E182A and E299A mutants is not due to problems of synthesis/folding, but to the removal of key catalytic residues. In addition, we have identified a MPS I patient with an E182K mutant allele. The E182K mutant protein was expressed in CHO-K1 cells and also found to be enzymatically inactive. Together, these results support the predicted role of E182 and E299 in the catalytic mechanism of alpha-L-iduronidase and we propose that the mutation of either of these residues would contribute to a very severe clinical phenotype in a MPS I patient.