title: Domain structure of human alpha 2-macroglobulin. Characterization of a receptor-binding domain obtained by digestion with papain.
abstract: Digestion of methylamine-treated alpha 2-macroglobulin (alpha 2M X MA) with catalytic amounts of papain at pH 4.5 has been investigated. Cleavage of Lys(1313)-Glu resulted in two major products, which could be separated by gel chromatography: a large disulfide bridged fragment set nearly the size of intact alpha 2M X MA, and an 18 kDa fragment, constituting the carboxy-terminal domain of alpha 2M. This domain contained the receptor recognition site, exposed as a result of cleavage of the internal beta-cysteinyl-gamma-glutamyl thiol esters in alpha 2M. Compared with alpha 2M-trypsin complex the apparent affinity for binding to rat hepatocyte receptors was 0.1 and 2% at 4 and 37 degrees C, respectively. The receptor-binding domain presumably forms a compact globular beta-barrel-type structure, stable at pH 2.5-9.0. Chemical modification experiments suggest that receptor binding is contributed by a determinant formed by the precise folding of the polypeptide chain.
mesh_terms: D000818:Animals; D001665:Binding Sites; D055598:Chemical Phenomena; D002621:Chemistry; D002850:Chromatography, Gel; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000378:metabolism; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D010206:Papain; D010446:Peptide Fragments / Q000302:isolation & purification; D051381:Rats; D011971:Receptors, Immunologic / Q000378:metabolism*; D014357:Trypsin; D000511:alpha-Macroglobulins / Q000378:metabolism*
chemical_list: D026503:Low Density Lipoprotein Receptor-Related Protein-1; D010446:Peptide Fragments; D011971:Receptors, Immunologic; D000511:alpha-Macroglobulins; D014357:Trypsin; D010206:Papain