Abstract:
:The (1)H NMR resonances of the heme substituents of the low-spin Fe(III) form of nitrophorin 2, as its complexes with N-methylimidazole (NP2-NMeIm) and imidazole (NP2-ImH), have been assigned by a combination of (1)H homonuclear two-dimensional NMR techniques and (1)H-(13)C HMQC. Complete assignment of the proton and partial assignment of the (13)C resonances of the heme of these complexes has been achieved. Due to favorable rates of ligand exchange, it was also possible to assign part of the (1)H resonances of the high-spin heme via saturation transfer between high- and low-spin protein forms in a partially liganded NP2-NMeIm sample; additional resonances (vinyl and propionate) were assigned by NOESY techniques. The order of heme methyl resonances in the high-spin form of the protein over the temperature range of 10-37 degrees C is 8 = 5 > 1 > 3; the NMeIm complex has 5 > 1 > 3 > 8 as the order of heme methyl resonances at <30 degrees C, while above that temperature, the order is 5 > 3 > 1 > 8, due to crossover of the closely spaced 3- and 1-methyl resonances of the low-spin complex at higher temperatures. This crossover defines the nodal plane of the heme orbital used for spin delocalization as being oriented 162 +/- 2 degrees clockwise from the heme N(II)-Fe-N(IV) axis for the heme in the B orientation. For the NP2-ImH complex, the order of heme methyl resonances is 3 > 5 > 1 > 8, which defines the orientation of the nodal plane of the heme orbital used for spin delocalization as being oriented approximately 150-155 degrees clockwise from the heme N(II)-Fe-N(IV) axis. In both low-spin complexes, the results are most consistent with the exogenous planar ligand controlling the orientation of the nodal plane of the heme orbital. In the high-spin form of NP2, the proximal histidine plane is shown to be oriented 135 degrees clockwise from the heme N(II)-Fe-N(IV) axis, again for the B heme orientation. A correlation between the order of heme methyl resonances in the high-spin form of NP2 and several other ferriheme proteins and an apparent 90 degrees shift in the nodal plane of the orbital involved in spin delocalization from that expected on the basis of the orientation of the axial histidine imidazole nodal plane have been explained in terms of bonding interactions between Fe(III), the axial histidine imidazole nitrogen, and the porphyrin pi orbitals of the high-spin protein.
journal_name
Biochemistryjournal_title
Biochemistryauthors
Shokhireva TKh,Shokhirev NV,Walker FAdoi
10.1021/bi026765wsubject
Has Abstractpub_date
2003-01-28 00:00:00pages
679-93issue
3eissn
0006-2960issn
1520-4995journal_volume
42pub_type
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