To elucidate the quaternary structure of the extracellular haemoglobin (Hb) of the marine polychaete Arenicola marina (lugworm) it was subjected to multi-angle laser-light scattering (MALLS) and to electrospray-ionisation mass spectrometry (ESI-MS). It was also subjected to SDS/PAGE analysis for comparative purposes. MALLS analysis gave a molecular mass of 3648 % 24 kDa and a gyration radius of 11.3 rt 1.7 nm. Maximum entropy analysis of the multiply charged electrospray spectra of the native, dehaemed, reduced and carbamidomethylated Hb forms, provided its complete polypeptide chain and subunit composition. We found, in the reduced condition, eight globin chains of molecular masses 15952. Keywords: Arenicola ; haemoglobin ; quaternary structure ; multi-angle laser-light scattering; electrospray mass spectrometry.The giant extracellular haemoglobins (Hb) and chlorocruorins found in annelids [1] and vestimentiferans [2, 31 are characterised by an acidic isoelectric point and by an hexagonal symmetry in electron micrographs consisting of two superimposed hexagonal arrays of twelve spherical subunits, the hexagonal bilayer (HBL). These HBL Hb also contain a low haem and iron contents (about 67% of those observed in other Hbs) and consist of two types of chains, globin chains (~1 6 -1 8 kDa), accounting for approximately 70% of the total mass, and haem-deficient linker chains (~2 4 -2 8 kDa) necessary for the assemblage into the HBL structure [1,. The intertidal marine polychaete Arenicola marina (L.) was one of the first 60s (~3 6 0 0 kDa)
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