Mesoheme

Fig. 3. Absorbance spectra of ferri- and ferro-heme-hemopexin in the Soret and visible regions. The spectra of rabbit ferri- and ferro-mesoheme-hemopexin (solid hne and dashed line, respectively) are shown and are typical of low-spin bis-histidyl heme proteins. Other species display similar spectra, and a variety of other 2,4-substituted heme analogs are also bound by hemopexin.

Fig. 8. Crystal structure of heme-hemopexin. The crystal structure of the rabbit mesoheme-hemopexin complex (PDB accession number IQHU) (11) showed heme to be bound in a relatively exposed site between the N- and C-domains with one axial His ligand being contributed by the hinge or linking region between the domains and the other by the C-domain. Also noteworthy is the disposition of the heme with its propionate residues pointing inward and neutralized by positive charges in the binding site.

Fig. 9. EPR spectra of heme-hemopexin and heme-N-domain. X-band EPR spectra at 4 K of ferri-mesoheme-hemopexin (a) and ferri-mesoheme-N-domain (b) are shown. The concentration of both heme complexes was 0.15 mM in 50 50 (v/v) 10 mM sodium phosphate/150 mM NaCl (pH 7.2) glycerol. The g-value scale is noted at the top and the -values observed are noted in each spectrum. Although both complexes are low-spin (some adventitious high-spin iron is present), the differences in g-values indicate nonidentical heme environments in the two complexes (.114).

Fig. 10. H NMR spectra of rabbit hemopexin and domain. The 400 MHz H NMR spectra at 298 K of rabbit apo- and ferri-mesoheme-hemopexin are shown in panels A and B, and the spectra of apo- and mesoheme-N-domain in panels C and D, respectively. The spectra demonstrate that the heme environment in the intact protein is distinct from that in the N-domain. The heme resonances are broadened in the N-domain, consistent with a greater accessibility to solvent 114).

The biphasic reaction with CO points to the existence of multiple heme-hemopexin conformers, and this is borne out by spectral analyses. The absorbance spectra of rabbit ferri-, ferro-, and CO-ferro-mesoheme-hemopexin are entirely analogous to those of other bis-histidyl heme proteins such as cytochrome 65 142), but the CD spectra exhibit unusual features (Fig. 11). Of particular interest are the weak signal of the ferro complex and the bisignate signal of the CO-ferro complex (also seen in the NO-ferro-mesoheme-hemopexin complex (140) and in human ferri-protoheme—hemopexin (139)). 

Fig. 11. Absorbance and CD spectra of ferri-, ferro-, and CO-ferro-heme complexes of hemopexin and its domains. Panels A, C, and E show the Soret region absorbance spectra, and panels B, D, and F the corresponding CD spectra. Rabbit hemopexin (panels A and B), N-domain (panels C and D), and N-domain-C-domain (panels E and F) complexes with mesoheme in the ferri- (solid lines), ferro- (dashed lines), and CO-ferro- (dash-double dot lines) states in phosphate-buffered saline, pH 7.4, are presented. The differences between the spectra of hemopexin and the N-domain point to multiple heme binding modes in the protein (139).

Fig. 10. a-meso-Methyl-substituted heme groups and their oxidation to biliverdin products. The substituent R in the heme structure is a methyl in the symmetric porphyrin and an ethyl in mesoheme. 

Fig. 3. Chemical structures of top) the peptide sandwiched mesoheme (PSM) and bottom) its monopeptide analog. Reprinted with permission from Ref. 108). Copyright 1995 American Chemical Society.

Fig. 2. Structure of some porphyrin-iron complexes. Protoheme IX (Proto) R = —CH=CH2 Deuteroheme IX (Deut) R = —H Mesoheme IX (Meso) R =...

Table 1 gives the names and abbreviations used for the hemoproteins. The reconstituted cyanoferrimyoglobins, where protoheme IX was replaced by deuteroheme IX or mesoheme IX, will be referred to as DeutMbmCN and MesoMbinCN. For mixed state hemoglobins the states of the individual subunits will be indicated. For example in Hb l1 jSspCN) the a-chains would be in the deoxy-form, and the /9-chains in the cyanoferri-form. 

R = ethyl chelated mesoheme R = vinyl chelated protoheme... 

Figure 4. Strained chelated mesoheme derivatives. See the text for the meanings of A, n, and G.

Figure 5. Rate constants for reactions of CO with mesoheme dimethyl ester and the indicated imidazoles in 2% aqueous CTAB at pH 7.3. On-rates and in M-1 sec f off-rates in sec 1. Data is from Ref. 20. Similar steric effects on kinetic constants were observed in benzene.

Benson DR, Hart BR, Zhu X et al (1995) Design, synthesis and circular dichroism investigation of a peptide-sandwiched mesoheme. J Am Chem Soc 117 8501-8510... 

One of the variables in the structures of the porphyrins present in heme proteins is the presence or absence of vinyl substituents on the periphery of the macrocycle. For example, b hemes have vinyl substituents whereas c hemes do not. Because of the sensitivity of such vinyl substituents during synthetic transformations, it has often been desirable to use octa-alkyl porphyrins in model studies of the spectroscopic properties of heme systems. The development of improved methods for the preparation of octa-alkyl porphyrins has likewise increased the availability of such porphyrins for model studies (20, 21). To assess the effect that replacement of the two vinyl substituents in protoporphyrin IX with alkyl (ethyl) groups has on the MCD properties of the heme system, an extensive and systematic study of the MCD properties of mesoheme IX-reconstituted myoglobin and horseradish peroxidase in comparison with the spectra of the native protoheme-bound proteins has been carried out (22). The structures of these two porphyrins are shown in Figure 3. 

Figure 3. Structures of iron protoporphyrin IX (protoheme IX) (left) and iron mesoporphyrin IX (mesoheme IX) (right).

---