Polymerization of turtle α-macroglobulin through newly exposed sulfhydryls reveals the location of ex-thiolester bonds

Green turtle α-macroglobulin, which has previously been shown to contain thiolester bonds, formed linear polymers after being treated with proteinases. Biochemical analyses showed that the polymerization proceeded through disulfide-bond formation between monomers. The only sulfhydryl groups available for such polymerization after proteinase treatment were these created as the product of thiolester hydrolysis. Electron micrographs of polymers revealed H-shaped monomeric units aligned lengthwise in linear polymers. The average length per monomeric unit in the polymer estimated from the discrete distribution of polymer lengths was approximately 80% of the average length of free monomers, indicating that monomers overlapped each other within a region of about 4 nm. From such observations we concluded that the newly produced sulfhydryl groups were located on the four arms of the H-shaped molecule. The location of sulfhydryls can be taken as the site of the exposure of thiolesters which were originally sequestered in the hydrophobic interior of the molecule. Since the structure of turtle α-macroglobulin is very similar to that of human serum α₂-macroglobulin the results predict a similar location of sulfhydryls in human α₂-macroglobulin after proteinase treatment. The observed polymerization property is unique to sea turtle α-macroglobulin and has not been observed with human α₂-macroglobulin or other homologous proteins.