Metmyoglobin complex

The NO dissociation rate constants are summarized in Table HI (50) and are smaller than those seen with NO-metmyoglobin complexes. NP2 and NP3 (A ofr 0.1 s ) release NO approximately 10 times more slowly than NPl and NP4 kofs 2-3 s ) at pH 8.0, and the NO release rate for all nitrophorins decreases as the pH is lowered to 5.0. The NO release curves cannot be fit with a single exponential, indicating two off rates at each pH, as previously noted for both recombinant and insect-derived NPl (46), and which has also been recently reported for recombinant NP2 (145). The biphasic kinetics suggest the presence of slowly interconverting conformations. The values obtained are pH and protein dependent, ranging from 2.6 to 0.05 (Table HI) (50), values that are considerably slower than found for sperm whale metmyoglobin... 

In a more recent series of experiments, George (97) has shown that the secondary hydrogen peroxide complexes of horseradish peroxidase and cytochrome-c peroxidase can be titrated with ferroevanide or ferrous ions and also appear to take part in a one oxidizing equivalent reduction to the ferric form of the enzyme. In this important chemical property they thus resemble the metmyoglobin complex in spite of marked spectroscopic differences in the visible region of the spectrum (Keilin and Hartree, 48). If the peroxide molecule is not a component part of the structure it... 

The most elegant example which demonstrates this aspect of proton transfer equilibrium of heme proteins is the study reported on six coordinated aqua (pyridine) iron(III) porphyrin complexes encapsulated in aqueous micelles [27]. The ferric ion in these complexes is axially co-ordinated to a water and a pyridine molecule, thus having a coordination geometry similar to that of the heme in metmyoglobin. The pH dependence of the absorption spectra of the... 

Table 5. EPR g values of metmyoglobin (Mb) and met-hemoglobin (Hb) complexes with sulphur ligands (43)...

In contrast, complexation of HNO tends to result in reductive nitrosylation. Doyle et al. (39, 170) first demonstrated the experimental utility of this reaction with iron proteins such as methemoglobin (metHb) and metmyoglobin (metMb). 

The hyperfine-shifted proton resonance of metmyoglobin and methemoglobin complexes with imidazoles, in particular, 4-nitroimidazole, was studied in order to obtain an insight into the structural features of the iron-bound imidazole [352], The structure of l-(l,3-dihydroxy-2-propyl)-4-nitroimidazoles, so called acyclic nucleosides, has been established by II and 13C NMR [327],... 

The spectra of ferric porphyrins are much more complex than those of the "typical metalloporphyrins discussed in Section II. Fig. 6 shows the spectra of metmyoglobin (MbH20) and ferrimyoglobin cyanide (MbCN). The latter is seen to have a fairly typical metalloporphyrin spectrum, although the a-band is rather weak and there is a new, weak band in the near infra-red. The spectrum of MbH20 is completely different the bands between 17 kK and 19 kK are similar in position to the a- and /3-bands of the cyanide, but there are new bands at around 10 kK, 16 kK, and 20 kK. 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

Figure 4 Low-temperature UV-visible spectra (a) and resonance Raman spectra of the same samples (b) recorded at 77 K. The 95-qs and 245-qs MHQ samples were obtained after mixing of 1 M azide and 0.35 mM metmyoglobin at a calibrated distance from the nozzle and the precooled cylinder. The azide-fi ee metmyoglobin sample and a pre-incubated metmyoglobin azido complex were used as reference samples. (b) Trace B and C represent the residual spectra of the 95-qs MHQ (B ) and 245-qs MHQ samples (C ) after maximal subtraction of the contribution fi om the starting material (trace A) and the end-product (trace D). Traces B and C were multiplied by four to allow easy comparison with those of metMb (A) and metMb pre-incubated with azide (D). (Reproduced from Lu, Wiertz, de Vries and Moenne-Loccoz. 2005 John Wiley Sons, Ltd)...

These results illustrate the complexity of ligand binding in a simple two-component system. The results further indicate the time resolution of the MHQ technique. The reaction between metmyoglobin and azide is a useful molecular timer, but the kinetic s are not proportional to [N3 ] observation of the intermediate described here can be used to argue that the particular rapid-freezing instrument operates below the 100 ps time scale. 

From a mechanistic perspective, the nitrosyl complex of metmyoglobin is of particular interest because both association and dissociation of NO are conveniently observable rmder... 

The fundamental simplicity of the powder diffraction experiment coupled with the sophistication of the available numerical data processing techniques and unsurpassed visual elegance, with which the results of the full profile refinement are represented, have transformed the powder diffraction technique into a modem and exceptionally important experimental tool in structure determination. Without a doubt, the powder method is poised to play a role in stmcture determinations with ever-increasing complexity. A recent example of the successful refinement of the sperm whale Physeter catodon) metmyoglobin stmcture from synchrotron powder diffraction data is an excellent illustration of how far the technique has been already advanced. ... 

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