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Spectral and thermodynamic properties of the two hemes of the D1D2cytochrome b‐559 complex of spinach
Author(s) -
Shuvalov V.A.,
Schreiber U.,
Heber U.
Publication year - 1994
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/0014-5793(94)80196-7
Subject(s) - spinach , chemistry , thermodynamics , spectral properties , physics , biochemistry , computational chemistry
In agreement with previous work [Shuvalov, Heber and Schreiber (1988) FEBS Lett. 258, 27‐31] two hemes (low potential (LP) and extra low potential (XLP )) per two pheophytins were found in isolated D1D2Cyt b‐559 complexes. Reductive and oxidative redox titrations demonstrate that the E m 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 m 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 m of +40 mV, and absorption peaks at 559 nm showing the bis‐histidine ligation. The other displays an E m 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(III) at high pH. The p K value for the conversion of the XLP heme is close to 7.7. In a structure simulation of the α‐helices of α‐ and β‐polypeptides, the β‐polypeptide, but not the α‐polypeptide, reveals a distance between the histidine N and the heme Fe which permits stable N‐Fe coordination. In the α‐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 cyclic electron transfer.