Ferrous Heme

By resonance Raman methods, Wang and coworkers105 showed NO bound to both ferric and ferrous heme of bNOS. Hurshman and Marietta106 used iNOS and spectrophotometric methods to show similar reactions, although the physiological effects will depend on the effects of arginine and oxygen in vivo. 

The NO in this structure appears to be bound equally in two orientations, a component that is linear (Fe-N distance of 1.6 A and Fe-N-0 angle of 170°, Fig. 17), and a second component that is bent (Fe-N 2.0 A, Fe-N-0 110°). To account for this, the NO molecule has been refined as a mixture of both orientations. The roughly linear orientation is indicative of a ferric (Fe 0 NO complex (2, 65), indicating that the NP4-NO structure represents the first ferric heme-NO complex for any protein. However, the bent orientation is similar in geometry to a ferrous heme-NO complex (2, 65). The bend directs the NO toward a... 

The interaction of dioxygen has been observed in several systems, mostly due to autooxidation of ferrous hemes with dioxygen, but only characterized in a few instances. Sakamoto et al. (119) have illustrated peroxidase-type activity using a helix-disulfide-helix system that binds a single heme as shown in Fig. 13. The initial communication illustrated that the addition of an organic cosolvent, trifiuoroethanol, increases the helical content of the peptide, the affinity for heme (1.7 DM IQ at maximal affinity, 15% TFE), and the peroxidase activity (conversion of... 

Table II shows the same results for the nitrosyl ferrous heme complex at Fe-NO 1.743 and 2.143 A. The singly occupied orbital (79) has primarily NO n and a character with both d and d 2 contributions. Although this orbital is deeply buried, as can be seen from its energy, it is shown between the doubly occupied and virtual orbitals for convenience. For comparison, the ground-state molecular orbitals of carbonyl-(44) and oxyheme (45) complexes are shown in Table III.

Table II. Ground State Orbital Description of Nitrosyl Ferrous Heme at Fe NO Distances of 1.743 A and 2.143 A...

Figure 2. Simplified, scaled, dlagramatlc representation of the excited-state energies of nitrosyl ferrous heme complex at iron-nitrogen distances of 1.743, 1,943, 2.143 and 2.343 A showing correlation between similar photodissociating states. The 71 71 transition of the Soret and Q bands are also identified.

The major conclusion of the present study, as can be seen in Figures 1 and 2, is that the primary photodissociating states, in both nitrosyl ferrous and ferric heme complexes correspond to the d - d 2 excitations. The calculated energies also indicate that this dissociative channel can be activated independent of the excitation frequency in the range of Q to Soret band energies. This is the same type of excitation previously identified as the photoactive state involved in CO and O2 photodissociation from ferrous heme complexes. 

Ground-state molecular orbitals carbonyl-carbonylheme oxyheme complexes, 5,11-12t nitrosyl ferrous heme complexes, 5... 

Nitrosyl ferrous heme complexes See also Nitrosoheme complexes geometry, 4... 

Hemes, see also Ferric hemes Ferrous hemes in bacfer, 36 429... 

Both ferric and ferrous hemes have been encapsulated in micelles with different spin states, and coordination geometries and oxidation states have been stabilized. Their electronic and structural properties are reviewed below. 

The ferrous complex of octaethyl porphyrin in SDS micelles has been characterized as four coordinated (S = 1) ferrous heme species and is similar to that observed for the ferrous protoheme complex in CTAB. It is noted that ferrous complexes of natural porphyrins cannot be stabilized in aqueous SDS micelles, and much larger aqueous micelles like CTAB were needed to stabilize various ferrous protohemes. This indicates that the environment around the octaethyl porphyrin complex in aqueous SDS is more hydrophobic than that of the analogous natural heme species, suggesting that the OEP moiety is embedded much deeper inside the micellar hydrophobic cavity than the protoporphyrin analogue. 

A general observation from the HNMR studies on micelle-encapsulated heme is that the line width of the heme protons increases significantly in the micellar solutions compared to those in simple solutions. The change in linewidths of heme methyl protons in micelles is not as large in low-spin Fe(III) hemes as in the high-spin ones. The linewidths in four-coordinate and five-coordinate complexes of ferrous hemes in aqueous micellar solutions are also quite broad compared to those observed in benzene solutions [12, 61], but are similar to those of hemoproteins [2,62]. We examine below the origin and implications of the linewidth change in micellar solutions. 

These observations suggest that the environment of heme inside the micellar cavity is more asymmetric than that in simple solution this asymmetry is much larger inside the protein cavity [45]. An unsymmetrical structural disposition of the heme complex inside the micellar cavity has been proposed [67]. Besides, the increase in the spread of the heme methyl signals in the five-coordinate 2-methyl imidazole ferrous heme complex might also arise from the decrease in symmetry... 

The results presented in this article on the ferric and ferrous hemes in different aqueous detergent micelles have brought out several interesting features. In general, it can be observed that ironporphyrin complexes exist in monomeric form inside the micellar cavity. This is a unique advantage since it provides an opportunity to study these hemes in monomeric form in an aqueous medium without any of the usual complications of aggregation. The ferric and ferrous... 

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