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Studies on the respiratory protein hemerythrin Bruce, Robert Emerson

Abstract

Hemerythrin from the marine worm Phascolosoma lurco has been isolated and characterized. The protein appears to have the uncommon trimeric structure reported for Phascolosoma agassizii hemerythrin, but is more homogeneous on disc gel electrophoresis, and can be crystallized from ethanol. P. lurco hemerythrin has no free cysteine residue and half the number of tryptophans of octameric hemerythrins. It appears to have a similar secondary structure to Phascolopsis gouldii hemerythrin, with a high degree (~75%) of alpha helix. Proteins which appear to be hemerythrins have been isolated from the sipunculid Phascolosoma noduliferum and the brachiopod Glottidea pyramidata, although they have not been as well characterized as the hemerythrin from P. lurco. Comparative studies using UV-visible and CD spectroscopies of oxy- and metazidohemerythrins from several species of sipunculids have shown that the proteins from all species examined have very similar active sites, despite differences in primary and quaternary structures. New methemerythrin derivatives with selenocyanate and dicyanamide, and the adduct formed between deoxyhemerythrin and nitric oxide, have all been characterized by UV-visible and circular dichroism spectroscopies. Tricyanomethide has been shown to bind "non-specifically" to P. gouldii hemerythrin and this has been related to the accessibility of the active site of the protein. Fluorescence and nuclear magnetic resonance spectroscopies have been shown to be applicable to comparative studies of hemerythrin, with nmr of paramagnetically shifted resonances perhaps providing the best means of investigating the physiologically important colourless deoxy protein. Metal atom replacement studies and protein redox potential determinations were not able to be carried out successfully, and possible reasons for this as well as suggestions for future research are discussed.

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