Reversion reactions of beta-galactosidase (Escherichia coli).
The reversion reactions of beta-galactosidase (Escherichia coli) produced beta-galactosyl-galactoses and beta-galactosyl-glucoses. About 10 beta-galactosyl-galactose and 10 beta-galactosyl-glucose gas-liquid chromatographic peaks were detected and it is thus very likely that every possible isomer of beta-galactosyl-galactose and beta-galactosyl-glucose was formed by the reversion reactions (taking into account both anomers for each isomer). The presence of lactose and allolactose among the beta-galactosyl-glucoses was confirmed with standards. An important finding relating to the role of allolactose as an inducer of the lac operon was that allolactose (beta-D-galactosyl-(1----6)-D-glucose) was the only disaccharide formed initially, and at equilibrium it was present in the largest amount (50%). Obviously the enzyme is specific in its ability to form allolactose, and allolactose is the most stable beta-galactosyl-glucose, both important inducer properties. The equilibrium constant (concentration of disaccharides divided by the concentration of reactants at equilibrium) of the reaction was about 9.5 mM-1. This is the first report of an equilibrium constant for the beta-galactosidase reaction. Of mechanistic significance is the fact that only three compounds were able to replace D-galactose as a reversion reactant. Two of these (L-arabinose and D-fucose) had alterations at carbon 6. The 6 position, therefore, is not essential for reactivity. The third compound was D-galactal. Any other sugars tested (even with very minor changes relative to D-galactose) did not react. Of special consequence is the 2 position. The results strongly suggest that there has to be either an equatorial hydroxyl at the 2 position of a sugar or a special reactivity (as with D-galactal) in order for the enzyme to catalyze the beta-galactosidase reaction.
Huber
R E
RE
Hurlburt
K L
KL
D001665:Binding Sites; D002849:Chromatography, Gas; D004187:Disaccharides / Q000096:biosynthesis; D004926:Escherichia coli / Q000201:enzymology*; D005690:Galactose / Q000378:metabolism; D005696:Galactosidases / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D007536:Isomerism; D007700:Kinetics; D009005:Monosaccharides / Q000378:metabolism; D001616:beta-Galactosidase / Q000378:metabolism*
D004187:Disaccharides; D009005:Monosaccharides; D005696:Galactosidases; D001616:beta-Galactosidase; D005947:Glucose; D005690:Galactose