Hemerythrins

Tertiary structure of monomeric myohemcrythrin (reproduced with permission). 

The smooth curves represent deconvolution of the 822-cm feature into two components. The difference between observed and fitted curves is shown below the spectrum near 822 cm . The vertical lines, a, b, c, and d, show the calculated peak positions for models I and 11 of Fe- Oj (845 cm- ), Fe- 0 0 (825 cm ), Fe- O O (818 cm ), and Fe- Oz (797 cm ), respectively. [Reprinted with permission from /. An Chem. Stic, 98,5034 (1976). 1976 American Chemical 

Armstrong ei al. prepared mode) compounds of the active site of Hr. According to X-ray analysis, the complexes of the type [(HBpz3)Fe0(02CR)2Fe(NBpz3 ] (R=H, CHj, QHj, and HBpz3 = tri-I-pyrazolylborate ion) take a structure similar to that shown in Fig. V-14. The acetate complex (R=CH3) exhibits the p (FeOFe) al 751 cm in IR and i s(FeOFe) at 528 cm in RR spectra. 

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Gofactors. Frequendy proteins exist in their native state in association with other nonprotein molecules or cofactors, which are cmcial to their function. These may be simple metal ions, such as Fe " in hemerythrin or Ca " in calmodulin a heme group, as for the globins nucleotides, as for dehydrogenases, etc. 

FIGURE 6.46 The oligomeric states of hemerythrin in various marine worms, (a) The hemerythrin in Thermkte zostericola crystallized as a monomer (b) the octameric hemerythrin crystallized from Phascolopsis gouldii (c) the trimeric hemerythrin crystallized from Sipiwnosoma collected in mangrove swamps in Fiji. 

Rubrerythrin (Rr) was first isolated in 1988 from cellular extracts of D. vulgaris Hildenborough (38), and later also found in D. desulfuri-cans (39). Rr is constituted by two identical subunits of 22 kDa and it was shown that each monomer contains one Rd-like center, Fe(RS)4, and a diiron-oxo center similar to the ones found in methane monooxygenase (MMO) (40, 41) or ribonucleotide reductase (RNR-R2) (42). After aerobic purification, the UV-visible spectrum shows maxima at 492, 365, and 280 nm, and shoulders at 570 and 350 nm. This spectrum is similar to the ones observed for Rd proteins. From a simple subtraction of a typical Rd UV-vis spectrum (normalized to 492 nm) it is possible to show that the remainder of the spectrum (maxima at 365 nm and a shoulder at 460 nm) strongly resembles the spectrum of met-hemerythrin, another diiron-oxo containing protein. 

Hydroxylase in the mixed-valent Fe(II)Fe(III) oxidation state (Hmv) is readily accessible by one-electron reduction of the dinuclear center. Mossbauer data indicate the presence of one Fe(III) and one Fe(II) (39). Hmv has a rhombic EPR signal with gav = 1.83 (27, 37) and J -30 cm1 (38,39), properties characteristic of other mixed-valent nonheme carboxylate-bridged diiron centers such as that in semimet hemerythrin (J = -15 cm-1) (32). ENDOR spectroscopic studies of Hmv... 

The hemerythrin of Golfingia gouldii consists of eight subunits, each of which contains two iron atoms, in a protein with molecular weight 108,000. Spectral and magnetic data point to an oxo-bridged structure around the non-heme iron atom (99). Protein B2 of ribotide reductase of E. coli has some properties in common with hemerythrin presumably a protein corresponding to that of E. coli reduces ribotides in animal tissues, a conclusion based on probes with inhibitors. 

Hemerythrin Marine invertebrates 02 carrier HX29HX3EXi6HX3H-X23/28HX4D Oxo... 

Detailed kinetic studies of the reaction of Fe(II) in cyclophane hemes with 02 and with CO probed polarity and steric effects the effects of deformation of the porphyrin skeleton from planarity were assessed for one compound (121). Volume profiles have been established for reactions of a lacunar Fe(II) complex with CO (122 for myoglobin with 02 and with CO and for hemerythrin with 02 (123). 

A suitable model for the oxygen carrier protein hemerythrin is [Fe2(Et-HPTB)(OBz)](BF4)2 (Et-HPTB = AWAT,iV -tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-l,3-diaminopropane, OBz = benzoate). It can mimic the formation of a binuclear peroxo iron complex in the natural system (101). The measured value of -12.8 cm3 mol1 for the activation volume of the oxidation reaction together with the negative value of the activation entropy confirm the highly structured nature of the transition state. 

Hemerythrin is a respiratory protein isolated from sipun-culids (marine worms). All sipunculids examined have, in the coelomic fluid, erythrocytes loaded with the protein which in most species so far examined is octameric, but sometimes tri-meric (18, 19) and in one instance dimeric and tetrameric (20, 21). From the retractor muscle of Themiste zostericola, the protein has been characterized as a monomer (22). The monomer (23) and the subunits of the trimer (24) and octamer (25) are remarkably similar in tertiary structure, having a M.W. of about 13,500 daltons. Each subunit contains one binuclear iron site. There is no porphyrin ring and the irons are coordinated only to amino acids, some of which, as well as probably an oxy group, form the binding atoms (26). 

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