Myoglobin, nitrosyl

The kinetics of reactions of NO with ferri- and ferro-heme proteins and models under ambient conditions have been studied by time-resolved spectroscopic techniques. Representative results are summarized in Table I (22-28). Equilibrium constants determined for the formation of nitrosyl complexes of met-myoglobin (metMb), ferri-cytochrome-c (Cyt111) and catalase (Cat) are in reasonable agreement when measured both by flash photolysis techniques (K= konlkQff) and by spectroscopic titration in aqueous media (22). Table I summarizes the several orders of magnitude range of kon and kQs values obtained for ferri- and ferro-heme proteins. Many k0f[ values were too small to determine by flash photolysis methods and were determined by other means. The small values of kQ result in very large equilibrium constants K for the... 

The reactivity of NO with O2 is dramatically affected upon coordination of one of the diatomic components to a metal center. For example, the second-order reactions of NO with oxyhemoglobin, Hb(02) and oxymyoglobin, Mb(02) (e.g. Eq. (47)) are quite fast and have been used as colorimetric tests for NO (105). The nitrogen product is NO3 rather than N02 that is the product of aqueous autoxidation (106). While the reaction of 02 with nitrosyl myoglobin Mb(NO) (Eq. (48)) might superficially appear similar it is much slower and follows a different rate law (107). Possible mechanisms will be discussed below. 

It is quite evident that the ferrous complexes of porphyrins, both natural and synthetic, have extremely high affinities towards NO. A series of iron (II) porphyrin nitrosyls have been synthesized and their structural data [11, 27] revealed non-axial symmetry and the bent form of the Fe-N=0 moiety [112-116]. It has been found that the structure of the Fe-N-O unit in model porphyrin complexes is different from those observed in heme proteins [117]. The heme prosthetic group is chemically very similar, hence the conformational diversity was thought to arise from the steric and electronic interaction of NO with the protein residue. In order to resolve this issue femtosecond infrared polarization spectroscopy was used [118]. The results also provided evidence for the first time that a significant fraction (35%) of NO recombines with the heme-Fe(II) within the first 5 ps after the photolysis, making myoglobin an efficient N O scavenger. 

Yonetoni and co-workers (1972) have shown that hemoglobin and myoglobin form nitrosyl complexes with different bond angles at 77 K and at room temperature. The high-temperature species has less g tensor anisotropy (g = 2.03, gy = 1.98-1.99) and poorly resolved hyperfine splitting. Addition of glycerol at high concentrations prevented the transition between these forms. 

C(NH2)2+ unit of arginine, via the formation of hydroxylamine. It can be characterized by ESR spectroscopy using deoxy-myoglobin or -haemoglobin with which it reacts to give nitrosyl derivatives having well-defined spectra. Its chief biological role seems to be as a vasorelaxant (Moncada et al., 1988). 

Figure 6 Comparison of NRVS and resonance Raman data on nitrosyl myoglobin (MbNO). (A) Fe VDOS determined from experimental NRVS data at 21 K. The peaks fit to the curve are also shown. Cross-hatched horizontal red bars indicate frequency ranges for Fe-hgand vibrations and for low-frequency vibrations attributed to translation of the entire heme. (B, C) Resonance Raman spectra of MbNO labeled with Fe and Fe, respectively, recorded at ambient temperature. The difference between these two spectra is expanded by a factor of 4. Raman-active vibrations that are sensitive to Fe mass also contribute strongly to the NRVS signal...

The ferriheme protein metmyoglobin (metMb) at the physiological pH 7.4 was reported to bind the NO molecule reversibly yielding the nitrosyl adduct [metMb(NO)] the kinetics of the association and dissociation processes were investigated and a limiting dissociation mechanism was proposed (58,68). 2-His-l-Glu nonheme iron center engineered into myoglobin was reported capable to bind Fe(II) and reduce NO to N2O (69). 

Keywords HNO NO Nitroxyl Azanone Nitroxyl anion Nitrosyl Porphyrin Heme Iron Manganese Ruthenium Cobalt Reductive nitrosylation Kinetics Oxidation Protein Myoglobin NOS. 

Kappl R, Hiittermann J (1989) An ENDOR study of nitrosyl myoglobin single crystals. Isr J Chem 29 73-84... 

There are two main types of NiRs involved in the reduction of nitrites, namely, the heme-containing cytochrome cdj NiR which was obtained and first purified from Thiosphaera pantotropha (261). The second kind of NiR is the copper-containing NiR which was first isolated from Alcaligenes xylosoxidans NCIB 11015, a bacterial isolated from a soil in Japan. Other Cu NiR have been isolated from, Achromohacter cycloclastes, Alcaligenes faecalis S-6, Bacillus halodenitrificans, Haloferax denitrificans, Nitrosomonas europaea, Pseudomonas aureofaciens, Rhodobacter sphaeroides, and Hyphomicrobium sp. (262 and references thereinj. In mammalian systems, nitrites are reduced by deoxyHb (263) and by ferrous myoglobin (264,265) to nitric oxide. In synthetic iron porphyrins. Ford and coworkers have demonstrated how nitrites inhibit the reductive nitrosylation process by forming ferric-nitrites species (266). 

When the nitrosyl-polymer composite Ru-pHEMA was exposed to low-power UV hght (5—10 mW), rapid release of NO was observed (detected by NO-sensitive electrode). The photorelease of NO was also confirmed through the nitrosylation of reduced myoglobin (Figure 11). Covalent attachment of the Ru nitrosyl complex to the organic backbone... 

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