Conserved residue PsaB-Trp673 is essential for high-efficiency electron transfer between the phylloquinones and the iron-sulfur clusters in Photosystem I

Despite the high level of symmetry between PsaA and PsaB polypeptides in Photosystem I, some amino acids pairs are strikingly different, such as PsaA-Gly693 PsaB-Trp673, which located near a cluster 11 water molecules A1A A1B quinones FX iron-sulfur cluster. In this work, we changed PsaB-Trp673 to PsaB-Phe673 Synechocystis sp. PCC 6803. The variant contains ~ 85% wild-type (WT) levels I but is unable grow photoautotrophically. Both time-resolved steady-state optical measurements show that PsaB-W673F less than 50% electrons reach terminal clusters FA FB; majority recombine from A1A− A1B−. However, those reaction centers pass forward transfer heterogeneous: minor population shows electron rates A1B− slightly slower WT, whereas major slowed 10 µs time range. Competition relatively similar backward account for low yield long-lived charge separation variant. A higher content its increased mobility observed MD simulations interquinone cavity shifts pK PsaB-Asp575 allows deprotonation situ. heterogeneity found may be rooted protonation state PsaB-Asp575, controls whether can proceed beyond phylloquinone cofactors.