Two histidines are essential for the activity of the beta-galactosidase from Lactobacillus delbrückii subsp. bulgaricus.

Twelve tyrosines, ten glutamates, and five histidines were individually substituted for phenylalanine, glutamine, and asparagine, respectively, in the beta-galactosidase from Lactobacillus delbrückii subsp. bulgaricus. Only Y509, E464, H351, and H534 appear to be essential for the activity of the enzyme. The residues Y509 and E464 are homologous to Y503 and E461, respectively, of the Escherichia coli lacZ beta-galactosidase which have been shown to be necessary for its activity. Surprisingly, a number of amino acids that are highly conserved in the sequence alignments of nine beta-galactosidases and four beta-glucuronidases are not essential for enzyme activity.