CRYSTAL-STRUCTURE OF A FUNCTIONAL UNIT FROM OCTOPUS HEMOCYANIN

Hemocyanins are giant oxygen transport proteins found in many arthropo ds and molluscs. Freely dissolved in the hemolymph, they are multisubu nit proteins that contain many copies of the active site, a copper ato m pair that reversibly binds oxygen. Octopus hemocyanin is composed of ten subunits, each of which contain seven oxygen-binding ''functional units''. The carboxyl-terminal 47 kDa functional unit, Odg, is a prot eolytic isolate that binds oxygen reversibly while exhibiting slight B ohr and magnesium ion effects. Ln this work we present the X-ray struc ture determination and analysis of Odg at 2.3 Angstrom resolution. Odg has two structural domains: a largely alpha-helical copper binding do main, and a five-stranded anti-parallel beta-sandwich with the jelly r oll topology found in many viruses. Six histidine residues ligate the copper atoms, one of which is involved in a thioether bridge. The resu lts show that the hemocyanin from the mollusc and that from the arthro pod have distinct tertiary folds in addition to the long recognized di fferences in their quaternary structures, Nonetheless, a comparison of Octopus and horseshoe crab hemocyanin reveals a similar active site, in a striking example of perhaps both convergent and divergent evoluti on. (C) 1998 Academic Press Limited.