A MIXED-LIGAND IRON-SULFUR CLUSTER (C556S(PSAB) OR C565S(PSAB)) IN THE F-X-BINDING SITE LEADS TO A DECREASED QUANTUM EFFICIENCY OF ELECTRON-TRANSFER IN PHOTOSYSTEM

The proposed structure of Photosystem I depicts two cysteines on the P saA polypeptide and two cysteines on the PsaB polypeptide in a symmetr ical environment, each providing ligands for the interpolypeptide F-X cluster. We studied the role of F-X in electron transfer by substituti ng serine for cysteine (C565S(PsaB) and C556S(PsaB)), thereby introduc ing the first example of a genetically engineered, mixed-ligand [4Fe-4 S] cluster into a protein, Optical kinetic spectroscopy shows that aft er a single-turnover flash at 298 K, the contribution of A(1)(-) (life time of 10 mu s, 40% of total and lifetime of 100 mu s, 20% of total) and F-X(-) (lifetime of 500-800 mu s, 10-15% of total) to the overall P-700(+) back reaction have increased in C565S(PsaB) and C556S(PsaB) a t the expense of the back reaction from [F-A/F-B](-). The electron par amagnetic resonance spectrum of F-A shows g-values of 2.04, 1.94, acid 1.81 in both mutants acid a similarly decreased amount of F-A and F-B reduced at 15 K after a single-turnover flash. These results indicate that the mixed-ligand (3 cysteines, 1 serine) F-X cluster is an ineff icient electron carrier, but that a small leak through F-X still permi ts F-A and F-B to be reduced quantitatively when the samples are froze n during continuous illumination. The data confirm that F-X is a neces sary intermediate in the electron transfer pathway from A(1) to F-A an d F-B in Photosystem I.