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), galactosyltransferase 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 galactosyltransferase, resulting in an increase in the apparent Michaelis constant, Km(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 increasing concentrations of Zn(II). The Zn(II) inhibition of lactose synthase plateaus at [Zn(II)]:[alpha-lactalbumin] approximately 1:1, while for N-acetyllactosamine synthesis there is no plateau at all. The results suggest that Zn(II) binding to alpha-lactalbumin effects lactose synthase. Kinetically, Zn(II) induces a decrease in both the Km(app) and Vm for Mn(II), which results in an apparent increase, followed by a decrease, in lactose synthase activity at Mn(II) concentrations below saturation of the first [Mn(II)] binding site. Increasing Zn(II) also decreases Km(app) and Vm for both glucose and UDP-galactose in the lactose synthase reaction with either both Ca(II)- or apo-alpha-lactalbumin, further suggesting novel interactions between Zn(II)-alpha-lactalbumin 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 slightly effects Km(app) for N-acetylglucosamine and has essentially no effect on Km(app) or Vm for UDP-galactose.
Permyakov
E A
EA
Department of Chemistry, Ohio State University, Columbus 43210.
Reyzer
I L
IL
Berliner
L J
LJ
D000818:Animals; D002413:Cations, Divalent / Q000494:pharmacology; D002417:Cattle; D005260:Female; D007700:Kinetics; D007768:Lactalbumin / Q000737:chemistry / Q000378:metabolism; D007788:Lactose Synthase / Q000378:metabolism*; D008345:Manganese / Q000494:pharmacology; D008892:Milk / Q000201:enzymology; D009237:N-Acetyllactosamine Synthase / Q000378:metabolism*; D015032:Zinc / Q000494:pharmacology*; D006022:beta-N-Acetylglucosaminylglycopeptide beta-1,4-Galactosyltransferase / Q000378:metabolism*
D002413:Cations, Divalent; D008345:Manganese; D007768:Lactalbumin; D007788:Lactose Synthase; D006022:beta-N-Acetylglucosaminylglycopeptide beta-1,4-Galactosyltransferase; D009237:N-Acetyllactosamine Synthase; D015032:Zinc