lactose metabolism begins with hydrolysis of the disaccharide into its component monosaccharide, a reaction catalyzed by the enzyme B-galactosidase.the gene for beta-galactosidase(lacZ) is part of the lactose operon, which includes two other genes coding for enzymes that function in the use of lactose.the entire transcription unit is under the command of one main operator, codes for an allosteric repressor protein that can switch off the lactose operon by binding to the operator.