title: Limulus alpha 2-macroglobulin. First evidence in an invertebrate for a protein containing an internal thiol ester bond.
abstract: Intra-chain thiol ester bonds are present in a limited number of proteins. The thiol ester class of proteins includes vertebrate alpha 2-macroglobulin and the complement proteins C3 and C4. We report here the first instance of a thiol ester protein from an invertebrate, the alpha 2-macroglobulin proteinase-inhibitor homologue present in the plasma of the American horseshoe crab Limulus polyphemus. Our evidence is of three kinds: (1) the proteinase-binding activity of Limulus alpha 2-macroglobulin is inactivated by the low-molecular-mass primary amine methylamine; (2) the native protein is subject to autolytic fragmentation during mild thermal denaturation, yielding fragments of approx. 125 kDa and 55 kDa, whereas the methylamine-treated protein is stable under these conditions of thermal treatment; (3) new thiol groups are generated rapidly during reaction of the protein with trypsin. The demonstration of the thiol ester bond in a protein from an ancient invertebrate provides evolutionary evidence for the importance of this bond in the function of plasma forms of the alpha 2-macroglobulin-like proteinase inhibitors.
mesh_terms: D000818:Animals; D055598:Chemical Phenomena; D002621:Chemistry; D004220:Disulfides; D004952:Esters / Q000032:analysis; D006737:Horseshoe Crabs / Q000032:analysis*; D006358:Hot Temperature; D008744:Methylamines; D011489:Protein Denaturation; D011725:Pyridines; D013438:Sulfhydryl Compounds / Q000032:analysis; D000511:alpha-Macroglobulins
chemical_list: D004220:Disulfides; D004952:Esters; D008744:Methylamines; D011725:Pyridines; D013438:Sulfhydryl Compounds; D000511:alpha-Macroglobulins; C007542:4,4'-dipyridyl disulfide; C027451:methylamine