Structure and Function of Enzymes of the Leloir Pathway for Galactose Metabolism

Abstract

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. Galactose MutarotaseGalactose mutarotase activity was first reported in Escherichia coli in 1965 (4), and the gene encoding it was defined in 1994 (5). Since 1986, genes encoding for proteins with mutarotase activities have been identified in other organisms including Lactococcus lactis (6). With respect to the catalytic mechanism of galactose mutarotase, it was first suggested by Hucho and Wallenfels (7) that the reaction proceeds through the abstraction of the proton from the 1-hydroxyl group of the sugar by an active site base and donation of a proton to the C-5 ring oxygen by an active site acid, thereby leading to ring opening. Subsequent rotation of 180 o about the C-1-C-2 bond followed by abstraction of the proton on the C-5 oxygen and donation of a proton back to the C-1 oxygen generates product. A kinetic analysis of the enzyme from E. coli was recently reported (8).In 2002, the first structure of a galactose mutarotase (from L. lactis) was determined by Thoden et al. (9,10). A ribbon representation of the dimeric enzyme is displayed in Fig. 1. Each subunit contains 339 amino acid residues and adopts a distinctive ␤-sandwich motif. Despite the lack of amino acid sequence homology, the overall topology of the ␤-sandwich is similar to that first observed in domain 5 of ␤-galactosidase from E. coli (11). This ␤-sheet architecture has since been seen in the central domain of copper amine oxidase (12), the C-te...

Xyr1 receives the lactose induction signal and regulates lactose metabolism in Hypocrea jecorina

“…In H. jecorina this is mainly encoded by the bga1 gene [17]. The a-anomer of galactose is then specifically metabolized by galactokinase (EC 2.7.1.6, encoded by gal1, [15]) to give D D-galactose-1-phosphate [18] which then enters the Leloir pathway [19]. The first step of an alternative pathway to the Leloir pathway [15,20,21] was described to be catalyzed by D D-xylose reductase (encoded by xyl1) [20].…”

Altered architecture of substrate binding region defines the unique specificity of UDP‐GalNAc 4‐epimerases

“…2 These enzymes are important in myriad biological pathways such as lipopolysaccharide (LPS) biosynthesis 3,4 and galactose metabolism. 5 LPS is implicated in several facets of host-pathogen interactions such as resistance to serum-mediated killing, phagocytosis and killing by cationic peptides [6][7][8][9][10] whereas a malfunctioning UDP-hexose 4-epimerase can lead to epimerase-deficiency galactosemia in humans. 11,12 General details of the epimerization reaction have been thoroughly investigated by Wilson and Hogness, 13 Bauer et al, 2 Thoden et al, 14 Liu et al, 15 Wohlers et al 11 among others.…”

Characterization of two different types of UDP-glucose/-galactose4-epimerase involved in galactosylation in fission yeast

“…The metabolism of galactose in most organisms occurs via the Leloir pathway (Frey, 1996;Holden et al, 2003), comprising galactokinase (EC 2.7.1.6), galactose-1-phosphate uridylyltransferase (EC 2.7.7.12) and UDP-glucose/-galactose 4-epimerase (EC 5.1.3.2) (see Fig. 7b).…”