Lab Research Focus: Metabolism of Proteoglycans
Research in my laboratory is concerned with the metabolism of proteoglycans, a class of complex carbohydrates which consist of one or more polysaccharide chains linked to a core protein.
Polysaccharide chain initiation. In most mammalian proteoglycans, the polysaccharide chains are composed mainly of repeating disaccharide units containing a hexosamine (glucosamine or galactosamine) and a uronic acid (D-glucuronic acid or its C5-epimer, L-iduronic acid). The first sugar in each chain, however, is a xylose residue, which is linked to the hydroxyl group of a serine residue in the core protein. In the course of proteoglycan biosynthesis, the formation of the polysaccharide chains is initiated by transfer of xylose from uridine diphosphate xylose to specific serine residues in the core protein. A goal of our research is to determine in more detail the structural and catalytic properties of the xylosyltransferase catalyzing this reaction.
Heparin biosynthesis. Since its discovery in 1916 by Jay McLean, then a second-year medical student at Johns Hopkins University, heparin has attracted much interest by virtue of its ability to prevent the coagulation of blood. The structural basis of this property was not elucidated until 1980, when Lindahl and coworkers showed that a specific pentasaccharide sequence in the polysaccharide, with a unique 3-O-sulfated glucosamine residue, binds to antithrombin and thereby accelerates the inactivation of thrombin and other proteases involved in the coagulation process.
The biosynthesis of heparin is a complex process involving many membrane-bound enzymes and includes a unique sugar interconversion, i.e. the C5-epimerization of D-glucuronic acid residues already incorporated into the growing polymer. In collaboration with Lindahl’s laboratory in Uppsala, Sweden, we have purified the C5-epimerase to homogeneity, and the enzyme has subsequently been cloned in Uppsala. Currently, our laboratory is conducting studies of the kinetic properties of the epimerase and its mechanism of action with focus on a particularly intriguing property of the enzyme, i.e. that the Km values increase with increasing enzyme concentration. In an extension of our work on the epimerase, we are also participating in a multi-laboratory effort to produce a heparin-like coagulant by chemical and enzymatic modification of an E. coli K5 capsular polysaccharide, which has the same structure as a polysaccharide intermediate in heparin biosynthesis inasmuch as it is composed of alternating glucuronic acid and N-acetylglucosamine units.
Lennart Rodén (b. 1929), Professor of Biochemistry and Molecular Genetics, received the M.D. degree from the Karolinska Institute in Stockholm, Sweden. Before coming to UAB in 1972, he was Professor of Pediatrics and Biochemistry at the University of Chicago.