Two class II D-tagatose-bisphosphate aldolases from enteric bacteria

Escherichia coli, Salmonella enterica, Klebsiella pneumoniaeand Klebsiella oxytocawere found to contain two D-tagatose 1,6-bisphosphate (TagBP)-specific aldolases involved in catabolism of galactitol (genes gatY gatZ) and of N-acetyl-galactosamine and D-galactosamine (genes kbaY kbaZ,also called agaY agaZ). The two aldolases were closely related (> or = 53.8% identical amino acids) and could substitute for each other in vivo. The catalytic subunits GatY or KbaY alone were sufficient to show aldolase activity. Although substantially shorter than other aldolases (285 amino acids, instead of 358 and 349 amino acids), these subunits contained most or all of the residues that have been identified as essential in substrate/product recognition and catalysis for class II aldolases. In contrast to these, both aldolases required subunits GatZ or KbaZ (420 amino acids) for full activity and for good in vivo and in vitro stability. The Z subunits alone did not show any aldolase activity. Close relatives of these new TagBP aldolases were found in several gram-negative and gram-positive bacteria, e.g., Streptomyces coelicolor.