ISSN:
1573-4943
Keywords:
α-Lactalbumin
;
zinc binding
;
lactose synthase
;
galactosyltransferase
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract The enzyme β-4-galactosyltransferase (GT) catalyzes the transfer of a galactosyl group from UDP-galactose to N-acetylglucosamine (GlcNAc) on glycoproteins. In the presence of α-lactalbumin (α-LA), galactosyltransferase catalyzes the transfer of galactose to glucose to yield lactose. It is known that, in the absence of α-lactalbumin, Zn(II) competes with Mn(II) for the same binding site(s) in galactosyltransferase, resulting in an increase in the apparent Michaelis constant,K m (app), for Mn(II)-activation of N-acetyllactosamine synthesis. In the presence of α-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)]:[α-lactalbumin] ≈ 1:1, while for N-acetyllactosamine synthesis there is no plateau at all. The results suggest that Zn(II) binding to α-lactalbumin effects lactose synthase. Kinetically, Zn(II) induces a decrease in both theK m (app) andV m 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 decreasesK m (app) andV m for both glucose and UDP-galactose in the lactose synthase reaction with either both Ca(II)- or apo-α-lactalbumin, further suggesting novel interactions between Zn(II)-α-lactalbumin and the lactose synthase complex, presumably mediated via a Zn(II)-induced conformational change upon binding to α-lactalbumin. On the other hand, in N-acetyllactosamine synthesis, Zn(II) only slightly effectsK m (app) for N-acetylglucosamine and has essentially no effect onK m (app) orV m for UDP-galactose.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01025128
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