Tertiary structure of myohemerythrin at low resolution.

Abstract

X-ray diffraction studies have produced a low resolution image and also located the iron atoms of a monomeric hemerythrin from muscles of a sipunculan worm. These results reveal the course of the polypeptide chain and some details of the active center.Oxygen transport in certain invertebrate animals is mediated by hemerythrin in erythrocytes of the coelomic fluid. Hemerythrin usually occurs as an octamer of 108,000 molecular weight. It is a non-heme iron protein containing two iron atoms per subunit and it reversibly binds oxygen in the ratio 1 02:2 Fe. Much study has been attended to the structural chemistry of this evolutionary alternative to hemoglobin as an oxygen carrier (1), particularly by Klotz and coworkers, but many details remain obscure. New light can now be shed on hemerythrin structure following the discovery by Klippenstein et al. (2) that the sipunculan worm Themiste (syn. Dendrostomum) pyroides (3) contains a monomeric hemerythrin in its retractor muscles as well as hemerythrin octamers in its erythrocytes-a situation reminiscent of myoglobin and hemoglobin in mammals. Several properties of this myohemerythrin suggest that it bears close structural similarity to the protomers of octameric hemerythrin (2, 4).Myohemerythrin from T. pyroides has been crystallized (4) and the first structural results from studies of these crystals are reported here. These results are mainly at low resolution, but owing to a high helix content and recourse to chemical data, more molecular detail has been gleaned than is ordinarily discernible at low resolution. By way of warning it should be noted that published interpretations of low-resolution density maps have sometimes later been proved incorrect. However, it seems unlikely that such mistakes are repeated here. The quality of this map and the consistency of the model with independent chemical data argue for the basic correctness of the rather detailed molecular interpretation given here.Myohemerythrin is a relatively small protein of 118 aminoacid residues and molecular weight 13,900. This facilitates its crystallographic analysis. In turn, the knowledge of this structure should simplify the analysis of octameric hemerythrins. In any event, further studies on myohemerythrin should be an avenue for gaining a detailed understanding of reversible oxygenation in this fascinating class of proteins. X-ray analysCrystals of metazide myohemerythrin were grown as previously described (4) and then transferred to a stabilizing medium of 80% saturated ammonium sulfate buffered to pH 6.7. These crystals are in space group P2,2121 and have unit cell dimensions of a = 41.58 A; b = 80.03 A; and c = 37.78 X. Derivatives were prepared by soaking native crystals in stabilizing media containing heavy-atom compounds.Many compounds seem to bind, but derivative crystals are often prone to cracking and poor isomorphism. Nevertheless, five derivatives were found to be good enough at least for lowresolution phasing.X-ray diffraction data were measured by w-scans on a fourcircle di...

Structure of the hydrophobic protein crambin determined directly from the anomalous scattering of sulphur

“…Thus, atomic parameters of anomalous scatterers can be refined against |ΔF| values if only the largest differences are included. This was first done for myohaemerythrin to give a quite precise Fe-Fe distance 19 .…”

“…Two of the three aromatic residues are within 4° of the expected transverse conformation at χ 2 , but Tyr 29 has χ 2 = 55°. The disulphide torsion angles, which are also expected to favour ±90°, are −79° (3-40), 106° (4-32) and −86° (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26).…”

“…in using anomalous scattering to locate the iron atoms in myohaemerythrin 19 led us to contemplate an attempt based on the six sulphur atoms in crambin. An estimate of the magnitude of Bijvoet differences to be expected from crambin gave a value about half that found with myohaemerythrin and showed that sulphur scattering would account for 98% of the expected anomalous signal.…”

Location of iron and sulfur atoms in myohemerythrin from anomalous-scattering measurements

“…We have used Bijvoet-difference Patterson maps (Rossmann, 1961), refinement against 'large' observed Bijvoet differences (Hendrickson, Klippenstein & Ward, 1975), Bijvoet-difference Fourier maps (Kraut, 1968) and the 'imaginary' Fourier synthesis (Hendrickson & Sheriff, 1987) in order to investigate the structure of anomalous scatterers in myohemerythrin.…”

Corrigendum

“…3a The presence of a double site suggests the possibility of a cooperative two-electron transfer to oxygen from two close Fe(n), giving two Fe(m), which is followed by the formation of an oxo-bridge by elimination of protons from bound water molecules. Here again we can compare the structure with that of myohaemerythrin 13 …”