A unique pair of zinc binding sites in the human .alpha.2-macroglobulin tetramer. A chlorine-35 and chlorine-37 NMR study

Abstract

35Cl NMR has been used to demonstrate that human alpha 2-macroglobulin tetramer possesses a unique pair of zinc binding sites. Zinc bound at these sites does not affect the 35Cl NMR line width of free Cl-. Additional lower affinity zinc sites exist that bind chloride weakly and cause broadening of the free chloride resonance through fast exchange with bound chloride. Using both 35Cl and 37Cl relaxation measurements it has been shown that chloride bound at these sites has an internal correlation time of 5.1 ns and a quadrupolar interaction, chi, of 4.2 MHz with zinc. Manganese binds to apo-alpha 2-macroglobulin analogously to zinc. alpha 2-macroglobulin that has been reacted with methylamine still possesses two classes of zinc sites per tetramer, but their relative affinities differ more than for unreacted alpha 2-macroglobulin. These data are discussed with respect to possible models for the subunit arrangement in the tetramer.