Heme complex

NO-sensitive GC represents the most important effector enzyme for the signalling molecule NO, which is synthesised by NO synthases in a Ca2+-dependent manner. NO-sensitive GC contains a prosthetic heme group, acting as the acceptor site for NO. Formation of the NO-heme complex leads to a conformational change, resulting in an increase of up to 200-fold in catalytic activity of the enzyme [1]. The organic nitrates (see below) commonly used in the therapy of coronary heart disease exert their effects via the stimulation of this enzyme. 

Complexes III and IV have Fe-porphyrin prosthetic groups (hemes), complex IV also contains copper atoms which are involved in electron transport. Complexes I, III, and IV use the energy of electron transport to pump protons out of the matrix so as to maintain a pH gradient and an electrical potential difference across the inner membrane required for ATP synthesis (see below and Appendix 3). It is important to remember that all dehydrogenations of metabolic substrates remove two protons as well as two electrons and that a corresponding number of protons are consumed in the final reduction of dioxygen (Figures 5, 6). 

Allcock, H. R., Neenan, T. X., and Boso, B., Synthesis, oxygen-binding behavior, and Mossbauer spectroscopy of covalently-bound polyphosphaene heme complexes, Inorg. Chem.. 

Iron is an essential element for the proper function of nearly all known biological systems. In living organism, iron is generally stored in the center of metallopro-teins, most important is the incorporation into heme complexes. These complexes... 

Heme complexes and heme proteins have also been the subject of NIS studies. Of specific interest have been three features the in-plane vibrations of iron, which have not been reported by Resonance Raman studies [108], the iron-imidazole stretch, which has not been identified in six-coordinated porphyrins before, and the heme-doming mode, which was assumed to be a soft mode. 

The NIS investigation of heme complexes includes various forms of porphyrins (deuteroporphyrin IX, mesoporphyrin IX, protoporphyrin IX, tetraphenylpor-phyrin, octaethylporphyrin, and picket fence porphyrin) and their nitrosyl (NO) and carbonyl (CO) derivatives, and they have been the subject of a review provided by Scheidt et al. [109]. 

Reactions at the exocyclic position of cyclophosphazene derivatives represent a means for widely expanding the range of available phosphazenes. The synthesis of covalently bound cyclophosphazene heme complexes starts with N3P3 (OPh), -Cl which is converted to N3P3(OPh)5NMeCH2CH2CN. Following reduction of the... 

The reddish metal was already known in prehistoric times. It occasionally occurs as a native metal, but mostly in conspicuous green ores, from which it is extracted relatively easily. It is convenient to work, but not very hard. Not very optimal as a tool ("Otzi the Iceman" had a copper axe with him). Only through the addition of tin is the more useful bronze obtained. Its zinc alloy is the versatile and widely used brass. Copper is one of the coinage metals. Water pipes are commonly made of copper. Its very good thermal and electrical conductivity is commonly exploited (cable ), as well as its durability (roofs, gutters), as the verdigris (basic copper carbonate) protects the metal. Cu phthalocyanines are the most beautiful blue pigments. Seems to be essential to all life as a trace element. In some molluscs, Cu replaces Fe in the heme complex. A 70-kg human contains 72 mg. 

A.M. Raitsimring, P. Borbat, T. Kh. Shokjireva and F.A. Walker, Magnetic field (g-value) dependence of proton hyperfine couplings obrai-ned from ESEEM measurements Determination of the orientation of the magnetic axes of model heme complexes in glassy media, J. Phys. Chem., 1996, 100, 5235. 

Taylor, C.P.S. 1977. The EPR of low spin heme complexes. Relation of the fg hole model to the directional properties of the g tensor, and a new method for calculating the ligand field parameters. Biochimica et Biophysica Acta 491 137-149. 

The large and positive Avalues and, particularly the large and positive AV values obtained (21) for kon and represent signatures for a substitution mechanism dominated by ligand dissociation, for the ferri-heme complexes, i.e.,... 

Activation parameters for the reaction of NO with metMb, Eq. (15), were determined in this laboratory and in collaboration with van Eldik and Stochel (Table II) (23). Comparison of these activation parameters with those determined for reactions of NO with the water soluble ferri-heme complexes Fem(TPPS)(H20)2 and Feni(TMPS)(H20)2 (Table II) demonstrate that the latter compounds represent reasonable models for the kinetics for the analogous reaction with metMb. For example, the kon step would appear to be defined largely by the H20 lability of metMb(H20), although it is clear that the diffusion through protein channels, the distal residues and the proximal histidine binding to the Fe(III) center must all influence the NO binding kinetics (23,24). These properties may indeed be reflected in the lower AS values for both the on and off reactions on metMb. In a related study, Cao et al. recently... 

Heme Complex Subunit/Heme-Protein Connection Complex Subunit/Fe Ligands (in Addition to Porphyrin N Atoms) Bond Distance (A)... 

Fig. 15. Modelled structure of MOPl bis-heme complex. Reproduced with permission from J AmChemSoc (1998) 120 468. ( 1998 ACS)...

Formation of the bis-histidyl-heme complex also produces characteristic alterations in the protein s conformation, particularly in the environment of aromatic amino acids, notably tryptophan. Exposure of Trp residues to solvent decreases (113), Trp fluorescence is quenched (111), and an unusual band of positive ellipticity at 230 nm attributable to Trp is nearly doubled in intensity (Fig. 4) (104, 111, 124). 

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).

The affinity of hemopexin for heme (estimated to be pM 12)) is among the highest reported for a noncovalent heme complex, and direct examinations of heme binding support the effectiveness of hemopexin... 

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).

Temperature and pH effects on hemopexin, its domains, and the respective heme complexes have also been examined using absorbance and CD spectroscopy, which reflect stability of the heme iron-bis-histidyl coordination of hemopexin and of the conformation of protein, rather than overall thermodynamic unfolding of the protein. Using these spectral methods to follow temperature effects on hemopexin stability yielded results generally comparable to the DSC findings, but also revealed interesting new features (Fig. 14) (N. Shipulina et al., unpublished). Melting experiments showed that apo-hemopexin loses tertiary... 

Fig. 14. Effects of temperature on the absorbance of hemopexin and the N-domain of hemopexin. The unfolding of hemopexin and N-domain in 25 mM sodium phosphate, pH 7.4, was examined using absorbance spectroscopy (N. Shipulina et al., unpublished). The second derivative UV absorbance spectra of the protein moieties were used to follow protein unfolding and the Soret and visible region spectra to monitor the integrity of the heme complexes, as done with cytochrome 6502 (166). The ferri-heme complex is more stable than the apo-protein moiety, but the is slightly lower than that assessed by DSC, indicating that changes in conformation occur before thermodynamic unfolding. Reduction causes a large decrease in heme-complex stabihty, which is proposed to be a major factor in heme release from hemopexin by its cell membrane receptor, and addition of 150 mM sodium chloride enhanced the stabihty of ah forms of hemopexin.

Fig. 15. Effects of pH on apo- and heme-hemopexin. The Soret region absorbance (filled squares) of rabbit heme-hemopexin was monitored in two separate titrations, from pH 7.4 to 11.8 in one and from pH 7.4 to 3.8 in the other. Similarly, theellipticity at 231 nm of apo-hemopexin (open circles) and of heme-hemopexin (filled circles) was assessed from pH 7.4 to 11.8 and from pH 7.4 to 1.7 111). The heme complex and the tertiary structure are unaffected by pH in the region from pH 6 to 9, and other values are normalized to these.

Fig. 8. Mossbauer spectra of a sample containing 9 mM [OEP PeCl] and 30 mM Al-methylimidazole in jyjV-dimethylacetamide, into which NO gas was bubbled for 5 minutes, taken at 4.2 K in a magnetic field of (a) 5.34 T parallel and (b) 20 mT perpendicular to the 7-beam. The dotted line corresponds to the spectrum of the low-spin ferric heme complex [OEP Fe(NMelm)2]+Cl (39% relative contribution) and the dashed line to the heme-NO complex [OEP Fe(NMelm)(NO)]+Cl (61% relative contribution). Reproduced with permission from Ref. (86).

The conversion of a-meso-hydroxyheme to verdoheme is an oxygen-dependent process because the HO-l a-meso-hydroxyheme complex, whether obtained by reconstitution of the apoenzyme with synthetic a-meso-hydroxyheme or from oxidation of the heme complex with H2O2, is stable under anaerobic conditions 104, 105). EPR analysis of the... 

However, in contrast to the human His25Ala HO-l heme complex, which has no detectable activity in the absence of imidazole (78), the His20Ala Hmu O rheme complex in the presence of NAD PH and NADPH-cytochrome P450 reductase was foimd to catalyze the initial meso-hydroxylation of the heme (151). The product of the reaction was Fe verdoheme, as judged by the electronic absorption spectrum and the detection of carbon monoxide as a product of the reaction. Hydrolytic conversion of the verdoheme product to biliverdin and subsequent HPLC analysis confirmed that the oxidative cleavage of the porphyrin macrocycle was specific for the a-meso-carbon. 

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