Structure of the binuclear iron complex in metazidohaemerythrin from Themiste dyscritum at 2.2. Å resolution

Abstract

Haemerythrin and myohaemerythrin are non-haem iron proteins utilized by several marine invertebrates in transporting and storing oxygen. Haemerythrin is oligomeric [usually octameric, but also trimeric^1,2, and tetrameric and dimeric (C. A. Appleby, personal communication)] whereas myohaemerythrin is a monomeric analogue of the haemerythrin subunit. Physico-chemical investigations^3–5 of these molecules have shown that (1) each subunit binds two iron atoms by means of amino acid side chains, (2) one molecule of O₂ is bound to each binuclear iron centre, and (3) the iron atoms are in the +3 oxidation state in oxy and met forms of the proteins and +2 in deoxy. Models have been proposed for the complexes in haemerythrin and myohaemerythrin on the basis of crystallographic studies of the proteins. A μ-oxo bridged model (Fig. 1a) has been proposed from the results of a crystallographic analysis of metazidomyo-haemerythrin from Themiste zostericola⁶. Based on a similar analysis of methydroxohaemerythrin from Themiste dyscritum, previously referred to as metaquohaemerythrin⁷, we proposed a confacial bi-octahedron model with no atom identified as a μ-oxo bridge (Fig. 1b)⁸. Previously⁹, we stated that “at least one of the models is incorrect”. In fact, neither completely accounts for the electron density from the present 2.2 Å resolution difference map of the complex and surrounding electron density of metazidohaemerythrin from T. dyscritum.