Structure of the iron complex in methemerythrin.

Abstract

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.

Comparison of hemerythrins from four species of sipunculids by optical absorption, circular dichroism, fluorescence emission, and resonance Raman spectroscopy

“…The crystallographic models of hemerythrin from P. gouldii and T. dyscritum have very similar peptide backbone structures and conformations but substantially different active-site structures Klotz et al, 1976;Stenkamp et al, 1976a,b). In the model for methemerythrin from T. dyscritum, the active site is viewed as a pair of iron octahedra sharing a face (Stenkamp et al, 1976a). One iron is coordinated to three His, a bridging Glu, and a bridging Asp.…”

Resonance Raman spectroscopy in biochemistry and biology

“…Another comparative study (Dunn et al 1977) based on absorption, fluorescence, CD and RR is at variance with preliminary X-ray data in that it suggests that there are no major site differences for P. gouldi and I. dyscritum haemocyanins. The X-ray data (Stenkamp, Sieker &Jensen, 1976 andKlotz, Klippenstein &Hendrikson, 1976) suggests substantially different sites for these proteins. Mixed isotopes 16 0-18 0 can be used to probe the symmetry of the oxygen binding sites (Kurtz, Shriver & Klotz, 1976).…”

Hydroxide binding equilibrium in hemerythrin

“…The major significance of this value now 1 is in its assistance in interpreting previous work. When the crystal structure was done on !• dyscritum hemerythrin (Stenkamp, et al, 1976) the crystals were believed to have been aquomethemerythrin because the pH of the mother liquor was 6.5 (Loehr, personal communication) but later the assignment was changed to hydroxyrnethemerythrin on the basis of the absorption spectrum of the mother liquor which, having two maxima in the 300-400 nm region, appeared more like hydroxymet than aquomet. The existence of hydroxymet in a solution of pH 6.5 seemed to imply that the pKa was much lower for the Themiste dyscritum protein than that from Phascolopsis gouldii, but Themiste hemerythrin was observed to undergo a color change and drastic solubility change at pH values closer to 8.…”

“…An X-ray diffraction crystal structure has been done on aquomethemerythrin from Themiste dyscritum (Stenkamp, • et al, 1976). The first model at low ( 2.RA) resolution had the iron atoms as the centers of octahedra which shared a 5 common face.…”