title: Structural arrangement of the proteinase binding sites in human alpha 2-macroglobulin.
abstract: Using singlet-singlet energy transfer measurements with labeled-chymotrypsin-alpha 2-macroglobulin complexes, we find that the two proteinase binding sites of alpha 2-macroglobulin are separated from each other by 44 A. The free thiol groups generated upon reaction of alpha 2-macroglobulin with trypsin or chymotrypsin react with thiopropyl Sepharose, indicating that they are located at the surface of the complexes. Singlet-singlet energy transfer experiments from labeled proteinases to labeled thiols of alpha 2-macroglobulin show that the thiol groups are in close contact with the proteinase molecules whether the latter are covalently or noncovalently bound to alpha 2-macroglobulin. In addition, they are remote from the association interface between the Mr = 360,000 halves of alpha 2-macroglobulin. Using the same approach we demonstrate that the active sites of chymotrypsin molecules are separated by a distance of at least 20 A from the thiols group of each alpha 2-macroglobulin subunit.
mesh_terms: D001665:Binding Sites; D002918:Chymotrypsin / Q000378:metabolism*; D004735:Energy Transfer; D006801:Humans; D007700:Kinetics; D010196:Pancreatic Elastase / Q000037:antagonists & inhibitors; D011480:Protease Inhibitors; D011485:Protein Binding; D013438:Sulfhydryl Compounds / Q000032:analysis; D014357:Trypsin / Q000378:metabolism*; D000511:alpha-Macroglobulins / Q000378:metabolism*
chemical_list: D011480:Protease Inhibitors; D013438:Sulfhydryl Compounds; D000511:alpha-Macroglobulins; D002918:Chymotrypsin; D010196:Pancreatic Elastase; D014357:Trypsin