Tryptophan-heme pi-electrostatic interactions in cytochrome f of oxygenic photosynthesis

Cytochrome f of oxygenic photosynthesis has an unprecedented structure, inc luding the N-terminus being a heme ligand. The adjacent N-terminal heme-shi elding domain is enriched in aromatic amino acids. The atomic structures of the chloroplast and cyanobacterial cytochromes f were compared to explain spectral and redox differences between them. The conserved aromatic side ch ain in the N-terminal heme-shielding peptide at position 4, Phe and Tyr in plants and algae, respectively, and Trp in cyanobacteria, is in contact wit h the heme. Mutagenesis of cytochrome from the eukaryotic green alga Chlamy domonas reinhardtii showed that a Phe4 --> Trp substitution in the N-termin al domain was unique in causing a red shift of 1 and 2 nm in the cytochrome Soret (gamma) and Q (alpha) visible absorption bands, respectively. The re sulting alpha band peak at 556 nm is characteristic of the cyanobacterial c ytochrome. Conversely, a Trp4 --> Phe mutation in the expressed cytochrome from the cyanobacterium Phormidium laminosum caused a blue shift to the 554 nm alpha band peak diagnostic of the chloroplast cytochrome. Residue 4 was found to be the sole determinant of this 60 cm(-1) spectral shift, and of approximately one-half of the 70 mV redox potential difference between cyto chrome f of P. laminosum and C. reinhardtii (E-m7 = 297 and 370 mV, respect ively). The proximity of Trp-4 to the heme implies that the spectral and re dox potential shifts arise through differential interaction of its sigma- o r pi-electrostatic potential with the heme ring and of the pi-potential wit h the heme Fe orbitals, respectively. The dependence of the visible spectru m and redox potential of cytochrome f on the identity of aromatic residue 4 provides an example of the use of the relatively sharp cytochrome spectrum as a "spectral fingerprint", and of the novel structural connection betwee n the heme and a single nonliganding residue.