Phascolopsis Gouldii Research Articles

EPR spectroscopy of semi-methemerythrin

EPR spectra of semi-met forms of octameric hemerythrin from Themiste zostericola, prepared by one electron reduction of methemerythrin or by one electron oxidation of deoxyhemerythrin, have been visualized at liquid helium temperatures. The spectrum of that prepared by one electron reduction has principal g-values of 1.96 ± 0.01, 1.88 ± 0.01, and 1.67± 0.02 while that obtained by one electron oxidation has g = 1.95 ± 0.01, 1.72 ±0.01, and 1.68 ± 0.02. The amplitude of either spectrum decreases with time on incubation at room temperature according to a first order rate with t 1 2 = 5–8 min , apparently because of an intramolecular disproportionation. Similar EPR spectra have been obtained with semi-metmyohemerythrin of T. zostericola and with the octameric semi-met form of Phascolopsis gouldii. However, these forms disproportionate to a much lesser degree. The azide adduct of the octameric semi-met form of T. zostericola has g-values of 1.94 ± 0.01, 1.85 ± 0.01, and 1.57 ± 0.02. Its EPR spectrum differs somewhat from those of the azide adducts of the octamer of P. gouldii and the monomer of T. zostericola although all are resistant to disproportionation. Methemerythrin and deoxyhemerythrin have no EPR spectra even at liquid helium temperature.

Role of mixed oxidation states in the oxidation of hemerythrin species by ferricyanide ion.

The oxidation of deoxy- and oxyhemerythrin to methemerythrin by Fe(CN)6(3-) has been examined. The first step produces (semi-met)O, a half-oxidized form of hemerythrin which has different spectral and kinetic properties from (semi-met)R which is produced by one-electron reduction of methemerythrin. Further oxidation of (semi-met)O to methemerythrin usually occurs only indirectly via disproportionation of (semi-met)O to met and deoxy forms within the octameric framework. Oxidation of (semi-met)R by Fe(CN)6(3-) is a direct second-order reaction. (Semi-met)O is reduced rapidly by dithionite to deoxy, whereas that of (semi-met)R is a slow biphasic process. The oxidation of oxyhemerythrin occurs via the deoxy species, little, if any, reactivity being attributable to the oxy form. The oxidation of the azide adduct of (semi-met)R is biphasic, in which in step one N3(-) is removed during the oxidation to methemerythrin, which in the second step recombines with N3(-). Rate parameters for all these processes at pH 8.2 and 6.3 for protein from Phascolopsis gouldii and Themiste zostericola have been obtained. The implications of these findings to hemerythrin chemistry are discussed.

Sequence homology between the tyrosine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase from Escherichia coli and hemerythrin from Sipunculida.

The first enzyme of the common aromatic biosynthetic pathway in Escherichia coli, the 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase, contains iron as an integral part of the polypeptide chain, and the enzyme shows an absorption maximum around 350 nm (McCandliss, R.J., and Herrmann, K.M. (1978) Proc. Natl. Acad. Sci. U. S. A. 75, 4810-4813). These two properties are also found in hemerythrin, the oxygen carrier of certain marine invertebrates. The amino acid sequence of residues 10 to 18 of the enzyme from E. coli, His-Ile-Thr-Asp-Glu-Gln-Val-Leu-Met, is highly homologous to the sequence of residues 54 to 62 of hemerythrin from Phascolopsis gouldii, His-Phe-Leu-Asn-Glu-Gln-Val-Leu-Met. His54 and Glu58 of hemerythrin have previously been identified through x-ray and protein sequence analysis as iron ligands. We suggest that residues 10 to 18 of the E. coli enzyme represent part of the iron binding fold in this protein, and that His10 and Glu14 are iron ligands.

The reduction of methemerythrin by e-aq and CO2- from pulse radiolysis studies.

The rate constants for reduction of methemerythrin from Phascolopsis gouldii and Themiste pyroides by hydrated electrons are 2.0 and 3.9 x 10(9) M(-1)s(-1), respectively, at pH 8.2, I = 0.03 M, and 25 degrees C. There is only a small increase in rate when the pH is lowered to 6.3 and a very small decrease when the ionic strength is raised to 0.1 M. Adding azide ion (to form the met-azide adduct) has little effect on the reactivity towards e-aq. For the monomer form, metmyohemerythrin from T. pyroides, the reaction rate constant is 4.5 x 10(9) M(-1)s(-1). Methemerythrin from T. pyroides reacts with CO2- with a rate constant 6.8 x 10(7) M(-1)s(-1). The reactivity sequence e-aq greater than CO2- greater than SO2- (from dithionite reduction) towards methemerythrin is the same as that observed with reduction of heme proteins but the rate constants are some 10 to 100 times smaller for the former. Only 10 to 20% of the e-aq or CO2- radicals generated effect reduction of the iron centers in methemerythrin.

Reduction of methemerythrin by deoxymyoglobin: a protein-protein redox reaction not involving electron-transfer proteins.

The stoichiometry and kinetics of reaction of methemerythrin with the deoxy forms of myoglobin and hemoglobin have been examined at I = 0.2 M and 25 degrees C. One mole of methemerythrin (on the basis of the monomer unit containing two irons) reacts with 2 mol of deoxymyoglobin and with 0.5 mol of deoxyhemoglobin. All reactions are second order. Rate constants for reaction with deoxymyoglobin are 0.25 M-1s-1 (Phascolopsis gouldii) and 5.6 M-1s-1 (Themiste pyroides) at pH 6.3. There is little effect of raising the ionic strength to 1.35 M and only a small decrease in rate when the pH is adjusted to 8.2. The rate constant for reaction of deoxyhemoglobin with P. gouldii methemerythrin is approximately 0.1 M-1s-1 at pH 6.3. Metmyohemerythrin from T. pyroides reacts slightly slower than the octamer form (k = 2.0 M-1s-1 at pH 6.3 and 7.0). Oxymyoglobin is converted to metmyoglobin by methemerythrin. The electron-transfer path is discussed and a self-exchange rate constant for hemerythrin assessed as 10(-3) M-1s-1 on the basis of Marcus's theory.

New evidence for glutamic acid as an iron ligand in hemerythrin

The amino acid sequence of the respiratory protein, hemerythrin, from Phascolopsis gouldii was reinvestigated in the region of residue 58. Sequenator analyses were performed on peptide 50–113, obtained by trypsin digestion of the intact protein, and on peptide 50–62, obtained by cyanogen bromide cleavage of peptide 50–113. In both peptides residue 58 was unambiguously identified as glutamic acid and residue 59 as glutamine. This corrects a previous mistake in the assignment of residue 58 and makes its proposed role as an iron ligand more plausible.

Structure of the iron complex in methemerythrin.

The coordination of the ligands about the iron atoms in methemerythrin from Themiste dyscritum has been deduced from a 2.8 A resolution electron density map. The complex can be described in terms of two trigonal antiprisms about the pair of iron atoms in each subunit, the antiprisms having one face in common. Ligands at eight of the nine coordination positions are protein side chains, the ninth presumably being water. Comparison of the electron density map for T. dyscritum methemerythrin with the sequence of Phascolopsis gouldii hemerythrin suggests six aromatic side chain ligands (five histidine and one tyrosine) and two nonaromatic side chain ligands. The latter provide atoms at two of the three vertices of the face shared by the two antiprisms, and these along with the presumed water at the third vertex form bridges between the iron atoms of each pair.