Spectral and thermodynamic properties of the two hemes of the D1D2cytochrome b‐559 complex of spinach

Abstract

In agreement with previous work [Shuvalov, Heber and Schreiber (1988) FEBS Lett. 258, 277311 two hemes (low potential (LP) and extra low potential (XLP)) per two pheophytins were found in isolated DlD2Cyt b-559 complexes. Reductive and oxidative redox titrations demonstrate that the E,,, of the LP form is at about +150 mV. It is independent of pH between pH 7.2 and 9.4. The XLP heme is autoxidizable at pH 7.2 and displays. at this pH. an E,,, of -45 mV. Both the LP and XLP hemes show absorption peaks at 559 nm. They are proposed to have bis-histidine ligation of the heme iron. At pH 9.4, the XLP heme splits into two forms. One of them has an E,,, of +40 mV, and absorption peaks at 559 nm showing the bis-histidine ligation. The other displays an E,,, of -220 mV and the peak is shifted to 562 nm. This last form is proposed to be due to the incorporation of OH-which occupies the 6th coordination position of the heme Fe(II1) at high pH. The pK value for the conversion of the XLP heme is close to 7.7. In a structure simulation of the a-helices of a-and /I-polypeptides, the B-polypeptide, but not the a-polypeptide, reveals a distance between the histidine N and the heme Fe which permits stable N-Fe coordination. In the a-polypeptide, OH-can be incorporated between N and Fe. The functional role of the two hemes of cyt b-559 is briefly discussed with respect to water oxidation and cychc electron transfer.

Study of heme Fe(III) ligated by OH– in cytochrome b–559 and its low temperature photochemistry in intact chloroplasts

“…Upon illumination of chloroplasts at 80K bleaching at 557 nm [1] and the formation of a low spin heme Fe(III) signal with g = 3.08 [6] are observed. These data strongly support the idea that this Cyt b-559 form is characterized by bis-histidine ligation (see also [7]). Accordingly, the fiaction of Cyt b-559 with high spin EPR signal is believed to be a result of denaturation of membrane protein [81.…”

“…Optical and EPR studies revealed several forms of Cyt b-559 in D ID2Cyt b-559 complexes with different redox potentials at pH 7.2 and at pH 9.4 [7,9]. The optical and EPR (g = 5.9 and 6.4) properties of one form at pH 9.4 clearly show that its heme is ligated by OH at the 6th coordination position [7,9].…”

“…The optical and EPR (g = 5.9 and 6.4) properties of one form at pH 9.4 clearly show that its heme is ligated by OH at the 6th coordination position [7,9]. Although the 5th ligand of this form was suggested to be histidine [9].…”

“…Since the Cyt b-559 forms are largely variable in RC-2 complexes [7] it seems likely that Cyt b-559 exhibits a high sensitivity to the integrity and state of the thylakoid membrane. In this work we present EPR properties of Cyt b-559 in intact chloroplasts that are functionally competent in 02 evolution and in CO_, fixation [10].…”

Purification and Spectroscopic Characterization of Photosystem II Reaction Center Complexes Isolated with or without Triton X-100

“…This interpretation also explains the low yield of the intact PS II RC complex that usually is observed upon long Triton washing. The possible presence of denatured proteins should also be taken into account in the discussions on the redox state of cytochrome b-559 (Shuvalov et al, 1994).…”