The kinetic mechansim of bovine milk galactosyltransferase. The role of alpha-lactalbumin.

Initial rate parameters obtained with bovine galactosyltransferase at saturating Mn2+ concentrations, and a variety of acceptors including N-acetylglucosamine, glucose, ovalbumin, and di-N-acetylglucosamine are inconsistent with an ordered addition of UDP-galactose and acceptor substrates to the enzyme-Mn2+ complex. Inhibition patterns with N-acetylglucosamine or UDP-glucose as inhibitors of the galactosylation of ovalbumin indicated that either UDP-galactose or N-acetylglucosamine can bind to an enzyme-Mn2+ complex by a random equilibrium mechanism. Initial rate studies also indicate that alpha-lactalbumin may bind to either an enzyme-Mn2+-acceptor complex or an enzyme-Mn2+-UDP-galactose complex, suggesting that lactose synthesis also proceeds by a random equilibrium addition of substrates and alpha-lactalbumin. From the initial rate data assuming the random equilibrium mechanism, the dissociation constants for UDP-galactose, acceptor substrates, and alpha-lactalbumin from the appropriate complexes have been calculated. These values are in good agreement with those obtained independently by nonkinetic methods, providing additional support for the proposed random equilibrium mechanism. From similar studies with a cross-linked complex of alpha-lactalbumin and transferase, dissociation constants for UDP-galactose and acceptor substrates from the enzyme-Mn2+-alpha-lactalbumin complex were calculated. Comparison of each of the dissociation constants in the substrate addition phase shows that the binding of acceptor substrates and alpha-lactalbumin to enzyme-Mn2+ complexes is highly synergistic; the affinity of alpha-lactalbumin for the enzyme-Mn2+ acceptor complex is about 2 orders of magnitude greater than for the enzyme-Mn2+ complex. Similarly, the affinity of the acceptor for the enzyme-Mn2+-alpha-lactalbumin complex is about 2 orders of magnitude greater than the enzyme-Mn2+ complex. Synergism is also observed between alpha-lactalbumin and UDP-galactose binding but the synergism is much less than that observed with acceptor substrates and alpha-lactalbumin. Thus, the large decrease in the Michaelis constant for glucose in the presence of alpha-lactalbumin, which is observed for lactose synthesis by the galactosyltranferase, is primarily the result of the high degree of synergism in the binding of alpha-lactalbumin and glucose to enzyme-Mn2+ complexes. This synergism also accounts for the activation of N-acetyllactosamine synthesis by alpha-lactalbumin at low concentrations (less than 2 mM) of N-acetylglucosamine. An abortive enzyme-Mn2+-UDP-acceptor complex in the product release phase of the reaction appears to account for the inhibition of either lactose, or N-acetyllactosamine synthesis at a high concentration of either N-acetylglucosamine or glucose. This abortive complex is further stabilized by alpha-lactalbumin, thus the resulting substrate inhibition is observed at much lower acceptor concentrations in the presence of alpha-lactalbumin.