Experimental evolution of Ebg enzyme provides clues about the evolution of catalysis and to evolutionary potential

The ebg (evolved beta-galactosidase) operon of Escherichia coli has been used since 1974 as a model system to dynamically study the evolutionary processes which have led to catalytic efficiency and substrate specificity in enzymes. Wild-type ebg beta-galactosidase, encoded by ebgA, is a catalytically feeble enzyme that does not hydrolyze lactose or other beta-galactosidase efficiently enough to permit growth on those substrates. Each of two specific base substitutions at widely separated sites increases catalytic activity sufficiently to permit growth, and the combination of the two mutations further increases catalytic effectiveness and expands the substrate range of the enzyme in a non-additive fashion. Experimental studies suggested that in the 3126 bp coding region those two substitutions were the only mutations capable of increasing activity toward lactose sufficiently to permit growth. Alignment of EbgA with the LacZ beta-galactosidase showed that both mutations were in active site amino acids. Multiple alignment and phylogenetic analysis of EbgA, LacZ, and 12 other related beta-galactosidases showed that EbgA and LacZ diverged from a common ancestor at least 2.2 billion years ago, that they belonged to different subclasses of the family of 14 beta-galactosidases, that the two subclasses differed at 12 of the 15 active site residues, and confirmed that the two previously identified mutations in ebgA are the only ones that can lead to enzyme with sufficient activity on lactose to permit growth. Studies of the catalytic mechanism of Ebg beta-galactosidase have allowed the widely accepted Albery and Knowles model for the evolution of catalysis to be rejected.