The noncatalytic triad of α-amylases: A novel structural motif involved in conformational stability

Chloride-activated α-amylases contain a noncatalytic triad, independent of the glycosidic active site, perfectly mimicking the catalytic triad of serine-proteases and of other active serine hydrolytic enzymes. Mutagenesis of Glu, His, and Ser residues in various α-amylases shows that this pattern is a structural determinant of the enzyme conformation that cannot be altered without losing the intrinsic stability of the protein. 1H-15N NMR spectra of a bacterial α-amylase reveal proton signals that are identical with the NMR signature of catalytic triads and especially a deshielded proton involving a protonated histidine and displaying properties similar to that of a low barrier hydrogen bond. It is proposed that the H-bond between His and Glu of the noncatalytic triad is an unusually strong interaction, responsible for the observed NMR signal and for the weak stability of the triad mutants. Furthermore, a stringent template-based search of the Protein Data Bank demonstrated that this motif is not restricted to α-amylases, but is also found in 80 structures from 33 different proteins, amongst which SH2 domain-containing proteins are the best representatives. Proteins 2008. © 2007 Wiley-Liss, Inc.