The catalytic domain of endoglucanase A from Clostridium cellulolyticum: effects of arginine 79 and histidine 122 mutations on catalysis.

Sequence analysis of the endoglucanase EGCCA of Clostridium cellulolyticum indicates the existence of two domains: a catalytic domain extending from residue 1 to residue 376 and a reiterated domain running from residue 390 to 450. A small deletion in the C terminal end of the catalytic domain inactivated the protein. From the analysis of the sequences of 26 endoglucanases belonging to family A, we focused on seven amino acids which were totally conserved in all the catalytic domains compared. The roles of two of these, Arg-79 and His-122, were studied and defined on the basis of the mutants obtained by introducing various substitutions. Our findings suggest that Arg-79 is involved in the structural organization of the protein; the His-122 residue seems to be more essential for catalysis. The role of His-123, which is conserved only in subfamily A4, was also investigated.