Abstract
The FX electron acceptor in Photosystem I (PS I) is a highly electronegative (Em = -705 mV) interpolypeptide [4Fe-4S] cluster ligated by cysteines 556 and 565 on PsaB and cysteines 574 and 583 on PsaA in Synechocystis sp. PCC 6803. An aspartic acid is adjacent to each of these cysteines on PsaB and adjacent to the proline-proximal cysteine on PsaA. We investigated the effect of D566PsaB and D557PsaB on electron transfer through FX by changing each aspartate to the neutral alanine or to the positively charged lysine either singly (D566APsaB, D557APsaB, D566KPsaB, and D557KPsaB) or in pairs (D557APsaB/D566APsaB and D557KPsaB/D566APsaB). All mutants except for D557KPsaB/D566APsaB grew photoautotrophically, but the growth of D557KPsaB and D557APsaB/D566APsaB was impaired under low light. The doubling time was increased, and the chlorophyll content per cell was lower in D557KPsaB and D557APsaB/D566APsaB relative to the wild type and the other mutants. Nevertheless, the rates of NADP+ photoreduction in PS I complexes from all mutants were no less than 75% of that of the wild type. The kinetics of back-reaction of the electron acceptors on a single-turnover flash showed efficient electron transfer to the terminal acceptors FA and FB in PS I complexes from all mutants. The EPR spectrum of FX was identical to that in the wild type in all but the single and double D566APsaB mutants, where the high-field resonance was shifted downfield. We conclude that the impaired growth of some of the mutants is related to a reduced accumulation of PS I rather than to photosynthetic efficiency. The chemical nature and the charge of the amino acids adjacent to the cysteine ligands on PsaB do not appear to be significant factors in the efficiency of electron transfer through FX.
Highlights
The FX electron acceptor in Photosystem I (PS I) is a highly electronegative (Em ؍؊705 mV) interpolypeptide [4Fe-4S] cluster ligated by cysteines 556 and 565 on PsaB and cysteines 574 and 583 on PsaA in Synechocystis sp
Unlike the non-heme iron that bridges the two subunits of the quinone-type reaction centers and that does not participate in the electron transfer, FX serves as an indispensable component of the electron transfer chain of PS I [6, 7]
Relationship between Growth and Electron Transfer—Our studies of D557PsaB and D566PsaB show that the ability of Synechocystis sp
Summary
The FX electron acceptor in Photosystem I (PS I) is a highly electronegative (Em ؍؊705 mV) interpolypeptide [4Fe-4S] cluster ligated by cysteines 556 and 565 on PsaB and cysteines 574 and 583 on PsaA in Synechocystis sp. The EPR spectral properties of FX are unusual in that the average of the g-values 2.096, 1.853, and 1.757 is lower than in typical low potential cubane clusters, and its line widths are broad relative to the other two [4Fe-4S] clusters (FA and FB), which serve as the terminal electron acceptors in PS I. The EPR spectrum of FX shows a low temperature optimum (ϳ8 K) and a relatively high microwave half-saturation power (P1/2 Ͼ 200 mW at 8 K), which indicates an efficient spin relaxation mechanism [5]. It is not known which features of the protein are responsible for conferring the unusual redox and spectroscopic properties to FX. An additional interesting feature of FX is that an aspartate residue is adjacent to each conserved
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