A soluble monoheme c-type cytochrome (cytochrome c,) has been isolated from the green alga Monoraphidium braunii. It has a molecular mass of 9.3 kDa, an isoelectric point of 3.6 and a reduction potential of 358 mV at pH 7. The determined amino acid sequence allows its classification as a class-I c-type cytochrome. The femc and ferrous cytochrome forms and their pH equilibria have been studied using 'H-NMR, ultraviolet/visible, EPR and Mossbauer spectroscopies. The pH equilibria are complex, several pK, values and pH-dependent forms being observed. The amino acid sequence, the reduction-potential value and the visible and NMR spectroscopies data in the pH range 4-9 indicate that the heme iron has a methionine-histidine axial coordination. However, the EPR and Mossbauer data obtained for the ferricytochrome show that in this pH range two distinct forms are present: form I, g, = 3.27, g, = 2.05 and g, = 1.05; form 11, g, = 2.95, g, = 2.29 and g, = 1.43. While form I has crystal-field parameters typical of a methionine-histidine coordination, those associated with form I1 would suggest a histidine-histidine axial ligation. This possibility was extensively analyzed by spectroscopic methods and by chemical modification of a histidine residue. It was concluded that form I1 actually corresponds to an unusual type of methionine-histidine axial coordination. Straightforward examples of this type of coordination have recently been found in other c-type hemeproteins [
A bacterioferritin was isolated from the anaerobic bacterium Desulfovibrio desulfuricans ATCC 27774, grown with nitrate as the terminal electron acceptor, which is the first example of a bacterioferritin from a strict anaerobic organism. This new bacterioferritin was isolated mainly as a 24-mer of 20 kDa identical subunits, containing 0.5 noncovalently bound heme and 2 iron atoms per monomer. Although its N-terminal sequence is significantly homologous with ferritins from other microorganisms and the ligands to the di-iron ferroxidase center are conserved, it is one of the most divergent bacterioferritins so far characterized. Also, in contrast to all other known bacterioferritins, its heme is not of the B type; its chromatographic behavior is identical to that of iron uroporphyrin. Thus, D. desulfuricans bacterioferritin appears to be the second example of a protein unexpectedly containing this heme cofactor, or a closely related porphyrin, after its finding in Desulfovibrio gigas rubredoxin:oxygen oxidoreductase ¿Timkovich, R., Burkhalter, R. S., Xavier, A. V., Chen, L., and Le Gall, J. (1994) Bioorg. Chem. 22, 284-293. The oxidized form of the protein has a visible spectrum characteristic of low-spin ferric hemes, exhibiting a weak absorption band at 715 nm, indicative of bis-methionine heme axial coordination; upon reduction, the alpha-band appears at 550 nm and a splitting of the Soret band occurs, with two maxima at 410 and 425 nm. The heme center has a reduction potential of 140 +/- 10 mV (pH 7.6), a value unusually high compared to that of other bacterioferritins (ca. -200 mV).
Bacteriophage SPP1 portal protein is a large cyclical homo-oligomer composed of 13 subunits. The solution structure and assembly behavior of this protein with high-point rotational symmetry was characterized. The purified protein was present as a monodisperse population of 13-mers, named gp6 H , at univalent salt concentrations in the hundred millimolar range ($ 250 mm NaCl) or in the presence of bivalent cations in the millimolar range ($ 5 mm MgCl 2 ). Gp6 H had a slightly higher sedimentation coefficient, a smaller shapedependent frictional ratio, and a higher rate of intersubunit cross-linking in the presence of magnesium than in its absence. In the absence of bivalent cations and at univalent salt concentrations below 250 mm, the 13-mer molecules dissociated partially into stable monomers, named gp6 L . The monomer had a somewhat different shape from the subunit present in the 13-mer, but maintained a defined tertiary structure. The association±dissociation equilibrium was mainly between the monomer and the 13-mer with a minor population of intermediate oligomers. Their interconversion was strongly influenced by the ionic environment. Under physiological conditions, the concentration of Mg 2+ found in the Bacillus subtilis cytoplasm (10±50 mm) probably promotes complete association of gp6 into 13-mer rings with a compact conformation.
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