Hemerythrin active site

The binuclear iron unit consisting of a (p,-oxo(or hydroxo))bis(p.-carboxylato)diiron core is a potential common structural feature of the active sites of hemerythrin, ribonucleotide reductase, and the purple acid phosphatases. Synthetic complexes having such a binuclear core have recently been prepared their characterization has greatly facilitated the comparison of the active sites of the various proteins. The extent of structural analogy among the different forms of the proteins is discussed in light of their spectroscopic and magnetic properties. It is clear that this binuclear core represents yet another stractural motif with the versatility to participate in different protein functions. 

Figure 16-20 (A) The active site of hemerythrin showing the two iron atoms (green) and their ligands which include the (X oxo bridge and two bridging car-boxylate groups. From Lukat et al.193 The active site is between four parallel helices as shown in Fig. 2-22. (B) Stereoscopic view of the backbone structure of a A9 stearoyl-acyl carrier protein desaturase which also contains a diiron center.

Figure 11 Idealized structural representations for the Fe2-02 active site in hemerythrin...

Pulver, S., Proland, W. A., Pox, B. G., Lipscomb, J. D., and Solomon, E. 1., 1993, Spectroscopic studies of the coupled binuclear non-heme iron active site in the fully reduced hydroxylase component of methane monooxygenase Comparison to deoxy and deoxy-azide hemerythrin, J. Am. Chem. Soc. 115 12409912422. 

Hemerythrin (Hr) is an 02-carrying protein found in a few phyla of marine invertebrates where it is thought to function as an O2 storage reservoir. The diiron active site reversibly binds the O2. Several reviews that discuss various aspects of Hr structure and function are available.Hr most often occurs as an octamer of essentially identical 02-binding subunits, although tetrameric, trimeric, and dimeric Hrs are also known. A monomeric counterpart, myoHr, is confined to muscle tissues of the same marine invertebrates. The structure of the myoHr subunit and active site are both very similar to those of Hr. The following discussion, therefore, applies, with very few exceptions, equally well to both Hr and myoHr. 

Hemerythrin is the best studied example and high-resolution X-ray data are available for the deoxy (176), oxy (176), met (177), and azido-met (177) forms. Extensive spectroscopic studies have also been carried out on these forms and on the semi-met Fe" Fe form 178). The active site structure of hemerythrin is illustrated in Fig. 40. The two iron atoms are bridged by an oxygen donor (OH or 0 ) and by two carbox-ylates, one aspartate and one gluamate. The remaining protein donors are five histidine residues, three coordinated to one iron and two to the other. In the deoxy form one iron atom is five coordinate and this vacant site can be occupied by dioxygen in oxyhemerythrin or by other... 

Fig. 40. The active sites of some /x-oxo-diiron proteins, hemerythrin (top), ribonucleo-...

Peroxo-diiron(III) complexes can undergo not only redox but also ligand substitution reactions. Liberation of H202 was observed in the reactions with phenols and carboxylic acids leading also to the respective phenolate or carboxylate iron(III) complexes.86 Hydrolysis of a peroxo-diiron(III) complex results in an oxo-diiron(III) species and hydrogen peroxide. Such reaction is responsible for the autoxidation of hemerythrin, but is very slow for the native protein due to hydrophobic shielding of the active site (Section 4.2.3).20 The hydrolysis of iron(III) peroxides is reversible, and the reverse reaction, the formation of peroxo intermediates from H202 and the (di)iron(III), is often referred to as peroxide shunt and is much better studied for model complexes. 

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