Haem orientation

H NMR spectrum of reconstituted protein demonstrated that one form is converted into the other, thermodynamically favored, form and the rate at which the equilibrium mixture is attained is on the hour or day rather than the millisecond time scale as originally thought. Therefore, the presence of the haem orientation disorder as well as the dynamics of the haem reorientation reaction need to be taken into account in the structural and functional study of reconstituted proteins. 

The F NMR spectra of various forms of Mb(7-PF), Mb(3,7-DF), and Mb(2-MF) are illustrated in Figs. 12-14. Two well-separated signals were observed in each spectrum and the unequal intensities of the signals in the spectra of Mb(2-MF) as well as Mb(7-PF) reflected the fractions of the two isomers due to the haem orientation disorder. The shifts and spread of the signals depended crucially upon the type of the F labeling, i.e., 7-PF vs. 3,7-DF, 2-MF, and their paia values tend to exhibit larger low-frequency shifts with an increasing number of unpaired electrons. The spectra of the reconstituted Mbs in five different states are compared... 

The C2 symmetry of the electronic structure of 3,7-DF bis-cya.no complex along the 5-H-15-H mcio-proton axis is clearly reflected in its and F NMR spectra (Figs. 4-7). Upon incorporation of 3,7-DF into apoMb, the degeneracy of the two F signals is removed, as indicated in Fig. 13. The observation of two signals in the spectrum of MbCO(3,7-DF) is not due to the presence of haem orientation... 

The nature of the molecular recognition between the haem and proteins in Z -type haemoproteins is not sufficiently specific to force a single orientation of the prosthetic group within its protein matrix. Although their crystal structures have invariably shown that the haem possesses a unique orientation within the protein, a solution NMR study has clearly demonstrated that the haem in these proteins is seated in two different orientations that differ by 180° rotation of the haem plane, about the 5,15-H we.so-proton axis, relative to the protein matrix (Fig. 3), and that the haem orientational disorder is present in almost all native Z -type haemopro-The dominant component in Mb possesses the same haem orientation... 

The functional consequence of the haem orientation appears to be dependent on the particular protein. The influence of the haem orientation on the O2 affinity is negligible in sperm whale Mb, but is relatively significant in insect Chiwnomus thummi thummi) Hbs. Similarly, the Bohr effect of the insect Hb, the autoxidation rate of oxy-Mb and tetrameric human adult oxy-Hb, the redox potential of ferricyto-chrome bs, and the thermal spin equilibrium for both met-azido insect Hb and ferricytochrome have been reported to be influenced by the haem orientation. Thus, an understanding of the molecular mechanism underlying the modulation of functional properties for Z -type haemoproteins through the interaction of the prosthetic haem periphery with the protein matrix demands detailed scrutiny of the influence of the haem orientation on the molecular and electronic structures of the haem-active site. 

A distinct difference in the pAa value has been detected between the normal and reversed forms of metMb(7-PF) (Fig. 29). The characterisation of the dynamics of the transition in the reversed form indicates that the kj value is diffusion-controlled (see below), and therefore the 2 values of the normal and reversed forms should be similar. Consequently, the difference in value between the two forms can be attributed to the effect of the haem orientation, with respect to the protein, on the ki value, and hence the stability of the acidic form. In the acidic form, the hydrogen bond between the coordinated ligand and His E7 contributes not only to stabilization... 

Since Ci-symmetric 3,7-DF does not possess the haem orientation disorder, the equivalency of the two fluorine atoms of 3,7-DF is abolished primarily by bonding interactions between the haem iron and the axial ligands, and the asymmetry in the chemical environment around the haem. X-ray crystal studies ... 

Figure 2.7 (a) A front view of the nitrite reductase dimer with the five haems in each monomer in white, a bound Ca2+ ion in grey and Lys-133, which coordinates the active site iron of haem 1, in yellow, (b) The haem arrangement. The overall orientation corresponds to (a), with the active site located at haem 1. Reprinted with permission from Einsle et al., 1999. Copyright (1999), Macmillan Magazines Limited. 

It would appear that the determination of the haem plane orientations by paramagnetic resonance is much more accurate than that by any other method so far applied. 

Cyt 6-559 is closely associated with the PS II reaction centre. A structural model has recently appeared in which the haem, which is oriented perpendicular to the membrane, is liganded to two histidines each on different membrane-spanning polypeptides (9 kDa). Changes of the relative orientation of the imidazole rings from parallel to perpendicular have been proposed to be responsible for the high-potential to low-potential redox form transition [212]. It is still not clear whether 1 or 2 cytochromes are present per reaction centre. The conflicting reports may be... 

Winter et al. [140] suggested that haem a is bound to subunit II. If so, it may be uniquely located because it is a bisimidazole complex and the number of conserved histidines is limited. It would then be expected to be near the C domain, sandwiched in part between transmembranous helices [92] (Fig. 3.4). This would agree with the perpendicularity between the haem and membrane planes observed by polarised spectroscopy of orientated enzyme specimens [141-143]. 

The reactive properties of the haem group and Compound I may be affected by the particular protein environment of a specific cytochrome P450. Different P450 isoenzymes show very different substrate specificity, and hydroxylation patterns. These could be the result of orientation or binding effects,188 or the... 

Figure 2-4. A simplified scheme of the proposed water-gated mechanism of proton translocation. Each numbered state shows haem a and the binuclear site (left and right rectangles, respectively) the A-propionate of haem is shown schematically. Three water molecules (oxygen in red hydrogen in yellow) are shown to mediate Grotthuss proton transfer from the glutamic acid (GLU-OH) to the propionate or the binuclear site, respectively. In state 1, an electron is transferred to haem a. The formed electric field between the redox sites orientates the water molecules towards the propionate (state 2). In state 3, electron transfer to the binuclear site is accompanied by proton transfer via the propionate a proton is deposited above haem and the glutamate is reprotonated via the D-pathway (state 4). The switch of electric field orientation reorientates the water array towards the binuclear site (state 5). Finally, a proton is transferred to this site, and the first proton is ejected (state 6). Reprotonation of the glutamate transfers the system back to state 1. For details, see the text and ref. 17.

Fig. 28.21 The Cua, Cub, haem a and haem sites in cytochrome c oxidase extracted from bovine Bos taurus) heart muscle. The lower right-hand diagram shows the relative positions and orientations of the metal sites within the protein an enlargement of each site shows details of the ligand spheres. Hydrogen atoms have been omitted colour code Cu, brown Fe, green S, yellow N, blue C, grey O, red.

Fig. 3. Schematic representation of preparation of apoprotein (1), reconstitution (2), and haem reorientation reaction (3). Two possible orientations of haem relative to His F8 are illustrated at the bottom normal form (left) represents the orientation found in the crystal structure of sperm whale Mb and reversed one (right) that with the haem rotated 180° about the 5,15-H me o-proton axis from that of the normal form.

The electronic structures of the porphyrins were compared through the analysis of the H NMR spectra of their dimethyl ester derivatives (Fig. 4). The fluorinated porphyrins exhibited typical spectral patterns similar to those of protoporphyrin and mesoporphyrin. In general, the shifts of the signals are affected by the ring current of the porphyrin macrocycle and the electronic inductive effect of nearby substituents and the ring-current shift is also modulated by the electronic contribution of the substituents. Since four meso protons, namely, protons bound to carbons at the 5, 10, 15, and 20 positions occupy the same orientation relative to the porphyrin macrocycle, in-plane asymmetry of the haem electronic structure is reflected directly... 

The spread of the two signals in the spectra of MbCO and met-cyano Mb are compared with those of 7-PFP, 3,7-DFP, and 2-MFP in Figs. 20-22. Due to the dpara valuc, thc sprcads of the signals in the spectra of met-cyano Mbs are increased considerably, compared with those in the spectra of MbCOs. In the case of met-cyano Mb, the orientation of His F8 imidazole, relative to the haem, modulates the haem electronic structure through the interaction of the /, j-orbital of imidazole with the highest energy unpaired electron resides (Fig. 23). ... 

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