EFFECTS OF ZN(II) ON GALACTOSYLTRANSFERASE ACTIVITY

The enzyme beta-4-galactosyltransferase (GT) catalyzes the transfer of a galactosyl group from UDP-galactose to N-acetylglucosamine (GlcNAc) on glycoproteins. In the presence of alpha-lactalbumin (alpha-LA), ga lactosyltransferase catalyzes the transfer of galactose to glucose to yield lactose. It is known that, in the absence of alpha-lactalbumin, Zn(II) competes with Mn(II) for the same binding site(s) in galactosyl transferase, resulting in an increase in the apparent Michaelis consta nt, K-m(app), for Mn(II)-activation of N-acetyllactosamine synthesis. In the presence of alpha-lactalbumin (i.e., lactose synthase), the Mn( II)-activation is biphasic and the initial phase is inhibited by incre asing concentrations of Zn(II). The Zn(II) inhibition of lactose synth ase plateaus at [Zn(II)]:[alpha-lactalbumin] approximate to 1:1, while for N-acetyllactosamine synthesis there is no plateau at all. The res ults suggest that Zn(II) binding to alpha-lactalbumin effects lactose synthase. Kinetically, Zn(II) induces a decrease in both the K-m(app) and V-m for Mn(II), which results in an apparent increase, followed by a decrease, in lactose synthase activity at Mn(II) concentrations bel ow saturation of the first [Mn(II)] binding site. Increasing Zn(II) al so decreases K-m(app) and V-m for both glucose and UDP-galactose in th e lactose synthase reaction with either both Ca(II)- or apo-alpha-lact albumin, further suggesting novel interactions between Zn(II)-alpha-la ctalbumin and the lactose synthase complex, presumably mediated via a Zn(II)-induced conformational change upon binding to alpha-lactalbumin . On the other hand, in N-acetyllactosamine synthesis, Zn(II) only sli ghtly effects K-m(app) for N-acetylglucosamine and has essentially no effect on K-m(app) or V-m for UDP-galactose.