The synthesis of lactose by mammary gland is a particular example. Lactose synthetase catalyses the synthesis of lactose and consists of a catalytic subunit. The catalytic subunit (refer Figure 6.1) by its own cannot synthesise lactose; it helps or catalyses the attachment of galactose to a protein–carbohydrate (glycoprotein).

Figure 6.1 Structure of Catalytic and Modifier Subunit of Lactose Synthetase

Modifier subunit (refer Figure 6.2) converts the specificity of the catalytic subunit in order to link galactose to glucose to form lactose. This subunit is controlled by hormones. During pregnancy, catalytic subunit is formed in abundance in the mammary gland, but little modifier subunit is formed. At the time of birth or parturition, modifier subunit is synthesised in large amount. The modifier subunit that binds to catalytic subunit and synthesise lactose in large amount during the period of parturition.

Figure 6.2 Modifier Subunit