title: Hydrophobic stabilization of chiton hemocyanins: effects of ureas, Hofmeister salts and pH on their dissociation.
abstract: The subunit dissociation of the hemocyanins from five members of the Polyplacophora families, Acanthochitonidae, Callistoplacidae, Chitonidae, Ischnochitonidae and Mopalidae, represented by the chitons Cryptochiton stelleri, Nutallina fluxa, Acanthopleura granulata, Stenoplax conspicua and Mopalia mucosa, respectively, have been investigated by light-scattering molecular-weight and ultracentrifugation methods, using the hydrophobic reagents of the urea series and the Hofmeister salt series as probes of the contact areas of the hemocyanin subunits. The polyplacophoran hemocyanins are decamers with molecular weights of (4.2-4.6) X 10(6). The effectiveness of dissociation by the ureas follows the order of increasing hydrophobicity of the reagent, i.e., urea, methyl-, ethyl-, propyl- and butylurea, as expected of hydrophobically stabilized subunit systems. The urea dissociation is found to be a two-step reaction: the dissociation of parent decamers to dimers followed by dissociation of the dimers to monomers. Analysis of the observed decrease in molecular weight requires the interaction with urea of about 27 to 35 apparent amino-acid groups (Napp) at the contact areas of the dimers, and a much larger number of apparent binding groups ranging from about 100 to 120 per monomer at the contact areas of the monomers. Fitting of the pH dissociation profiles of A. granulata, C. stelleri, M. muscosa and S. conspicua requires the participation of a much smaller number of amino-acid residues in the interaction with the probe-solvent components. The ionization or protonation of one acidic and one basic group per dimer, and five to eight acidic and basic groups per monomer is found to be adequate for the description of the two-step pH dissociation reaction.
mesh_terms: D000818:Animals; D002704:Chlorates / Q000494:pharmacology; D002942:Circular Dichroism; D019791:Guanidine; D006146:Guanidines / Q000494:pharmacology; D006433:Hemocyanins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008027:Light; D046911:Macromolecular Substances; D008970:Molecular Weight; D008974:Mollusca / Q000378:metabolism*; D012492:Salts / Q000494:pharmacology*; D012542:Scattering, Radiation; D012974:Sodium Iodide / Q000494:pharmacology; D013861:Thiocyanates / Q000494:pharmacology; D014461:Ultracentrifugation; D014508:Urea / Q000494:pharmacology*
chemical_list: D002704:Chlorates; D006146:Guanidines; D046911:Macromolecular Substances; D012492:Salts; D013861:Thiocyanates; C024553:sodium thiocyanate; D014508:Urea; D006433:Hemocyanins; D012974:Sodium Iodide; D019791:Guanidine; C032706:sodium chlorate