The hemocyanin of the squid Sepioteuthis lessoniana:: Structural comparison with other cephalopod hemocyanins

The subunit structure of the hemocyanin from the squid Sepioteuthis lessoniana has been characterized by analytical sedimentation. The protein exists free in the hemolymph as a 60S, multisubunit structure with a molecular mass of approximately 4.0 x 10(6) gm/mol. At extremes of pH this can be dissociated into 10 monomers, of mass 3.8 x 10(5) gm/mol each. At neutral pH, if 5 mM Mg2+ or less is present, the monomers associate to form a dimer of molecular mass 7.7 x 10(5) gm/mol. In the pH range from 10.5 to 7.5, formation of the dimer is facilitated by the binding of one proton per monomer. If the magnesium concentration is raised to 25 mM or more, the dimers reversibly and completely associate into decamers. This process is shown to require the binding of 10 magnesium ions. In its size, subunit structure, and behavior with respect to association-dissociation processes, Sepioteuthis hemocyanin resembles the corresponding proteins of other squid much more than it does those of Octopus or Nautilus. (C) 1997 Elsevier Science Inc.