The affinity reagent N10-(bromoacetyl)-5,8-dideazafolate has previously been shown to inactivate glycinamide ribonucleotide transformylase (EC 22.214.171.124) from Escherichia coli in an active-site-directed manner with a 1:1 stoichiometry [Inglese et al. (1990) Biochemistry 29, 1436–1443]. After a series of mild proteolytic digestions, the dideazafolate label was localized to an active-site peptide attached by an ester linkage to the highly conserved residue Asp 144. Subsequent site-specific mutagenesis of Asp 144 to Asn 144 resulted in a catalytically inactive enzyme that retained the ability to bind substrates and inhibitors. The Asn 144 mutant could be further labeled with the affinity reagent in an active-site-directed stoichiometric fashion; however, the site of modification in this case was His 119. These results imply that Asp 144 may function as a general base within the catalytic center of the transformylase and is in close proximity to His 119 in the folded protein.
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