TY - JOUR
T1 - Mutations in algal and cyanobacterial Photosystem I that independently affect the yield of initial charge separation in the two electron transfer cofactor branches
AU - Badshah, Syed Lal
AU - Sun, Junlei
AU - Mula, Sam
AU - Gorka, Mike
AU - Baker, Patricia
AU - Luthra, Rajiv
AU - Lin, Su
AU - van der Est, Art
AU - Golbeck, John H.
AU - Redding, Kevin
N1 - Funding Information:
Rachel Johnson and Caileen Eckerson assisted PB in performing algal transformations and characterization of the initial transformants. Christopher Gisriel made Fig. 1 . This work was supported by NSF grant MCB-1052573 to KR, by NSF grants MCB-1021725 and MCB-1613022 to JHG and by an NSERC Discovery Grant to AvdE.
Publisher Copyright:
© 2017 Elsevier B.V.
PY - 2018/1
Y1 - 2018/1
N2 - In Photosystem I, light-induced electron transfer can occur in either of two symmetry-related branches of cofactors, each of which is composed of a pair of chlorophylls (ec2A/ec3A or ec2B/ec3B) and a phylloquinone (PhQA or PhQB). The axial ligand to the central Mg2 + of the ec2A and ec2B chlorophylls is a water molecule that is also H-bonded to a nearby Asn residue. Here, we investigate the importance of this interaction for charge separation by converting each of the Asn residues to a Leu in the green alga, Chlamydomonas reinhardtii, and the cyanobacterium, Synechocystis sp. PCC6803, and studying the energy and electron transfer using time-resolved optical and EPR spectroscopy. Nanosecond transient absorbance measurements of the PhQ to FX electron transfer show that in both species, the PsaA-N604L mutation (near ec2B) results in a ~ 50% reduction in the amount of electron transfer in the B-branch, while the PsaB-N591L mutation (near ec2A) results in a ~ 70% reduction in the amount of electron transfer in the A-branch. A diminished quantum yield of P700+ PhQ− is also observed in ultrafast optical experiments, but the lower yield does not appear to be a consequence of charge recombination in the nanosecond or microsecond timescales. The most significant finding is that the yield of electron transfer in the unaffected branch did not increase to compensate for the lower yield in the affected branch. Hence, each branch of the reaction center appears to operate independently of the other in carrying out light-induced charge separation.
AB - In Photosystem I, light-induced electron transfer can occur in either of two symmetry-related branches of cofactors, each of which is composed of a pair of chlorophylls (ec2A/ec3A or ec2B/ec3B) and a phylloquinone (PhQA or PhQB). The axial ligand to the central Mg2 + of the ec2A and ec2B chlorophylls is a water molecule that is also H-bonded to a nearby Asn residue. Here, we investigate the importance of this interaction for charge separation by converting each of the Asn residues to a Leu in the green alga, Chlamydomonas reinhardtii, and the cyanobacterium, Synechocystis sp. PCC6803, and studying the energy and electron transfer using time-resolved optical and EPR spectroscopy. Nanosecond transient absorbance measurements of the PhQ to FX electron transfer show that in both species, the PsaA-N604L mutation (near ec2B) results in a ~ 50% reduction in the amount of electron transfer in the B-branch, while the PsaB-N591L mutation (near ec2A) results in a ~ 70% reduction in the amount of electron transfer in the A-branch. A diminished quantum yield of P700+ PhQ− is also observed in ultrafast optical experiments, but the lower yield does not appear to be a consequence of charge recombination in the nanosecond or microsecond timescales. The most significant finding is that the yield of electron transfer in the unaffected branch did not increase to compensate for the lower yield in the affected branch. Hence, each branch of the reaction center appears to operate independently of the other in carrying out light-induced charge separation.
KW - Electron transfer
KW - Photosynthesis
KW - Photosystem I
KW - Primary charge separation
KW - Quantum yield
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U2 - 10.1016/j.bbabio.2017.10.003
DO - 10.1016/j.bbabio.2017.10.003
M3 - Article
C2 - 29066393
AN - SCOPUS:85032979545
SN - 0005-2728
VL - 1859
SP - 42
EP - 55
JO - Biochimica et Biophysica Acta - Bioenergetics
JF - Biochimica et Biophysica Acta - Bioenergetics
IS - 1
ER -