A kinetic and equilibrium study of ligand binding to the monomeric and dimeric haem-containing globins of two chitons

The radular muscles of the amphineuran molluscs Amaurochiton glaucus and Sipharochiton pelliserpentis contain both a dimeric and a monomeric form of myoglobin. The dimeric form of the protein is composed of two polypeptide chains covalently linked to each other via one or more disulphide bonds. The dimeric protein shows co-operative O2-binding curves. Kinetic investigations indicate that CO binding is co-operative in the dimeric protein, subsequent to full photolysis, but mono-exponential following 10% photolysis. O2 recombination following part photolysis is mono-exponential in the dimeric form, whereas O2 dissociation kinetics indicates the presence of chain heterogeneity. The monomeric form of the protein exhibits mono-exponential time courses in all the experimental situations explored. Although the rate constants associated with the reactions of individual dimer and monomer molecular species are very different, the two species of chiton investigated show remarkably similar properties when compared with each other. All the reactions studied are pH-independent in the range pH 6-8. Amino acid analysis indicates that the monomeric units that combine to form the dimeric species are not identical with the naturally occurring monomeric form. A comparison is made between the chiton myoglobins and other similar O2-binding proteins.