Structure and function of enzymes of the Leloir pathway for galactose metabolism.

In most organisms, the conversion of -D-galactose to the more metabolically useful glucose 1-phosphate is accomplished by the action of four enzymes that constitute the Leloir pathway (Scheme 1). In the first step of this pathway, -D-galactose is epimerized to -D-galactose by galactose mutarotase. The next step involves the ATP-dependent phosphorylation of -D-galactose by galactokinase to yield galactose 1-phosphate. As indicated in Scheme 1, the third enzyme in the pathway, galactose-1-phosphate uridylyltransferase, catalyzes the transfer of a UMP group from UDP-glucose to galactose 1-phosphate, thereby generating glucose 1-phosphate and UDP-galactose. To complete the pathway, UDP-galactose is converted to UDP-glucose by UDP-galactose 4-epimerase. In humans, defects in the genes encoding for galactokinase, uridylyltransferase, or epimerase can give rise to the diseased state referred to collectively as galactosemia (1, 2). Although galactosemia is rare, it is potentially lethal with clinical manifestations including intellectual retardation, liver dysfunction, and cataract formation, among others. Indeed, the enzymes of the Leloir pathway have attracted significant research attention for well over 30–40 years, in part because of their important metabolic role in normal galactose metabolism. As of this year, the three-dimensional structures of all of the enzymes of the Leloir pathway have now been defined. It is thus timely to present in this minireview recent advances in our understanding of the structure and function of these enzymes. For a discussion of the literature prior to 1996, see Ref. 3.