Ramesh V M, Gibasiewicz Krzysztof, Lin Su, Bingham Scott E, Webber Andrew N
School of Life Sciences, Department of Chemistry and Biochemistry, and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287, USA.
Biochemistry. 2004 Feb 10;43(5):1369-75. doi: 10.1021/bi0354177.
Photosystem I contains two potential electron transfer pathways between P(700) and F(X). These branches are made up of the electron transfer chain components A, A(0), and A(1). The primary electron acceptor A(0) is a chlorophyll a monomer that could be one or both of the two chlorophyll molecules, eC-A(3)/eC-B(3), identified in the 2.5 A resolution structure. The eC-A(3)/eC-B(3) chlorophylls are both coordinated by the sulfur atom of a methionine. This coordination is highly unusual, as interactions between the acid Mg(2+) and the soft base sulfur are weak. The eC-A(3)/eC-B(3) chlorophylls also are located close to one of the connecting chlorophylls that may link the antenna and the electron transfer chain chlorophylls. Due to their location in the structure, the eC-A(3)/eC-B(3) chlorophylls may play a role in both excitation energy transfer and electron transfer. To test the role of the eC-A(3)/eC-B(3) chlorophylls in electron transfer, Met-684 of PsaA and Met-664 of PsaB have been changed to His, Ser, and Leu. Replacement of either M(A684) or M(B664) results in a significant alteration in growth phenotype. The His and Leu mutants are very light sensitive in the presence of oxygen. Growth is impaired to a greater extent in the B-side mutants. However, all of the mutants are able to grow anaerobically at comparable rates. The His and Ser mutants all accumulate PSI at a level similar to that of wild type, whereas the Leu mutants have reduced amounts of PSI. Ultrafast transient absorbance measurements show that the (A(0)(-) - A(0)) difference signal accumulates in the MH(A684) and MH(B664) mutants under neutral conditions, demonstrating that electron transfer between A(0)(-) and A(1) is blocked or significantly slowed. The results show that both the A-branch and the B-branch of the ETC are active in PSI from Chlamydomonas reinhardtii.
光系统I在P(700)和F(X)之间包含两条潜在的电子传递途径。这些分支由电子传递链组分A、A(0)和A(1)组成。初级电子受体A(0)是叶绿素a单体，它可能是在2.5埃分辨率结构中鉴定出的两个叶绿素分子eC - A(3)/eC - B(3)中的一个或两个。eC - A(3)/eC - B(3)叶绿素均由甲硫氨酸的硫原子配位。这种配位非常不寻常，因为酸性的Mg(2+)与软碱硫之间的相互作用很弱。eC - A(3)/eC - B(3)叶绿素也位于可能连接天线和电子传递链叶绿素的连接叶绿素之一附近。由于它们在结构中的位置，eC - A(3)/eC - B(3)叶绿素可能在激发能量传递和电子传递中都起作用。为了测试eC - A(3)/eC - B(3)叶绿素在电子传递中的作用，已将PsaA中的Met - 684和PsaB中的Met - 664分别替换为His、Ser和Leu。替换M(A684)或M(B664)中的任何一个都会导致生长表型的显著改变。His和Leu突变体在有氧存在下对光非常敏感。B侧突变体的生长受到更大程度的损害。然而，所有突变体都能够以相当的速率在厌氧条件下生长。His和Ser突变体积累PSI的水平与野生型相似，而Leu突变体中PSI的量减少。超快瞬态吸收测量表明，在中性条件下，(A(0)(-) - A(0))差异信号在MH(A684)和MH(B664)突变体中积累，表明A(0)(-)和A(1)之间的电子传递被阻断或显著减慢。结果表明，衣藻光系统I中电子传递链的A分支和B分支均具有活性。
