THE SOLUTION STRUCTURE OF OXIDIZED RAT MICROSOMAL CYTOCHROME B(5)

The solution structure of oxidized rat microsomal cytochrome bs has be en obtained from H-1 NMR spectra measured at 800 MHz. Tile available a ssignment has been extended to 78% of the total protons and 95% of the residues. From 1372 meaningful NOEs, a family of 40 structures has be en obtained through the program DYANA; 235 pseudocontact shifts have b een then added as further constraints, obtaining an essentially simila r family of structures. This latter family has been further refined th rough restrained energy minimization. The final RMSD values with respe ct to the average structure are 0.58 +/- 0.10 Angstrom and 1.05 +/- 0. 11 Angstrom for backbone and heavy atoms, respectively. The high quali ty of the structure allows meaningful comparisons with the solution st ructure of the reduced protein, with the X-ray and solution structures of the oxidized bovine isoenzyme, and with the solution structure of the apoprotein. Upon loss of one electron, the heme plane undergoes a change in its orientation, possibly due to the change of the total cha rge. Propionate 7 appears to have a conformation which is dependent on the oxidation state of the iron. Helices alpha 2 and alpha 4 also exp erience changes in their average positions in the two oxidation states . Finally, the backbone NHs experience different exchange properties i n the two oxidation states. While those present in the beta sheets for ming the basis of the heme pocket are nonexchanging in both oxidation states, the NHs in the helices forming the heme-binding pocket are exc hanging with the bulk solvent in the oxidized form, indicating larger local mobility in this state, This observation could suggest that, to optimize the electron transfer process, the local mobility should be p roperly tuned.