Kinetics of oxidation of the bound cytochromes in reaction centers from Rhodopseudomonas viridis

Abstract

The initial oxidized species in the photochemical charge separation in reaction centers from Rps. viridis is the primary donor, P(+), a bacteriochlorophyll dimer. Bound c-type cytochromes, two high potential (Cyt c 558) and two low potential (Cyt c 553), act as secondary electron donors to P(+). Flash induced absorption changes were measured at moderate redox potential, when the high potential cytochromes were chemically reduced. A fast absorption change was due to the initial oxidation of one of the Cyt c 558 by P(+) with a rate of 3.7×10(6)s(-1) (τ=270nsec). A slower absorption change was attributable to a transfer, or sharing, of the remaining electron from one high potential heme to the other, with a rate of 2.8×10(5)s(-1) (τ=3.5 μsec). The slow change was measured at a number of wavelengths throughout the visible and near infrared and revealed that the two high potential cytochromes have slightly different differential absorption spectra, with α-band maxima at 559 nm (Cyt c 559) and 556.5 nm (Cyt c 556), and dissimilar electrochromic effects on nearby pigments. The sequence of electron transfers, following a flash, is: Cyt c 556→Cyt c 559→P(+). At lower redox potentials, a low midpoint potential cytochrome, Cyt c 553, is preferentially oxidized by P(+) with a rate of 7×10(6)s(-1) (τ=140 nsec). The assignment of the low and high potential cytochromes to the four, linearly arranged hemes of the reaction center is discussed. It is concluded that the closest heme to P must be the high potential Cyt c 559, and it is suggested that a likely arrangement for the four hemes is: c 553 c 556 c 553 c 559P.

Electron Transfer in Natural Proteins Theory and Design

“…Using crystallographic distances, reported redox midpoint potentials and assuming a typical reaction center reorganization energy of 0.7eV for heme to heme electron transfer, the estimates for the individual electron transfer rates from heme 1 to BChl 2 + is 4.1 10 6 s ñ1 compares with 5.4 10 6 s ñ1 measured (Shopes et al, 1987;Dohse et al, 1995). The estimated rate for the multistep, mixed endergonic transfers from the heme 3 to heme 1 rate of 1.8 10 5 s ñ1 compares with 2.8 10 5 s ñ1 (Shopes et al, 1987), while the overall chain rate from heme c 2 to BChl2 of 1.1 10 4 s ñ1 compares with 1.4 10 4 s ñ1 (Ortega et al, 1999, Meyer et al, 1993. Other examples of chains abound.…”

Kinetics of photo-induced electron transfer from high-potential iron-sulfur protein to the photosynthetic reaction center of the purple phototroph Rhodoferax fermentans.

“…The four hemes are arranged in an almost linear fashion, (2,5,6) and are aligned in the sequence P/c-559/c-552/c-556/c-554, where P indicates the bacteriochlorophyll special pair (4,7). The highest potential heme (c-559) is oxidized the most rapidly (t/2 -300 ns) by the photooxidized P (P+), followed by a slower oxidation of heme c-556 (ti/2 2 ,us) (4,8,9). The quinol generated by RC photooxidation is utilized by the bc1 complex to reduce cytochrome c2 (10), a soluble protein closely related to mitochondrial cytochrome c (11,12).…”

Orientation and assignment of the four cytochrome hemes in Rhodopseudomonas viridis reaction centers

“…It is clear from the comparison of experimental and theoretical LD/A values that heme C556, which presents two resolved optical transitions of nearly identical LD/A magnitudes and of opposite sign, can be unequivocally correlated with heme 2, the only heme presenting these characteristics in the theoretical LD/A values. This assignment leads us to ascribe [8,9]. Under redox potential conditions where both high potential hemes are reduced, C559 is oxidized in about 300 ns and subsequently rereduced in 3/zs by C556.…”

“…In particular, the respective assignment of each of the two high and the two low potential hemes in the reaction center structure is still uncertain. Several attempts have been made based upon: (i) kinetics of photoinduced cytochrome oxidation [8,9]; (ii) linear dichroism study of oriented monolayers of reaction centers at room temperature [10]; (iii) nature of the heme ligands [11]; (iv) electrogenicity of the electron transfer from the cytochrome hemes to the primary donor [12] and (v) analysis of ESR spectra of oriented chromatophores (Nitschke, W. and Rutherford, A.W., personal communication).…”