Critical analysis of the extinction coefficient of chloroplast cytochrome f

In oxygenic photosynthesis the cytochrome bf complex links electron transport between photosystem II and photosystem I. The largest subunit of the complex is cytochrome f, a 32-kDa polypeptide that is anchored in the membrane by a transmembrane alpha helix located near the carboxyl end. The three-dimensional structure of the soluble domain of cytochrome f isolated from turnip has been determined by X-ray crystallography to 1.96 A resolution. The structure revealed several novel features compared to previously solved soluble c-type cytochrome structures, including a predominant beta-strand motif, the N-terminal alpha-amino group of a tyrosyl residue as an orthogonal ligand, and a bound internal water chain. Here we report a novel and unprecedented extinction coefficient for cytochrome f. Using the pyridine hemochrome assay, the reduced minus oxidized extinction coefficient for the soluble domain of turnip cytochrome f was 26 +/- 1 mM-1 cm-1 for the alpha-band wavelength peak at 554 nm relative to the isosbestic wavelengths (534, 543.5 and 560.5 nm), and 25 +/- 1 mM-1 cm-1 for spinach cytochrome f relative to the isosbestic wavelengths (533.5, 543.3 and 560.2). The extinction coefficients reported here are significantly higher than previously published values for cytochrome f. We believe earlier determinations underestimated the cytochrome f extinction coefficient and that the same is likely true for commonly used extinction coefficients of cytochrome b6. The cytochrome f extinction coefficient is large compared to most other c-type cytochromes, which could be due to the unique axial ligand of the cytochrome f heme. Polarographic measurements show the midpoint potential of soluble turnip cytochrome f to be 362 +/- 5 mV at pH 7.5. The midpoint potential was pH-independent from 5.0 to 8.5, and pH-dependent from pH 8.5 to 10.5 (-58 mV/pH unit) with a pK on the oxidized from near 9. Storage of some samples of purified turnip and spinach cytochrome f at -20 degrees C modified the heme environment in a fraction of the protein, shifting the midpoint potential to near -165 mV (pH 7.5) and the peak of the alpha-band absorption spectrum from 554 nm to 552 nm.