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The 2-Methoxy Group Orientation Regulates the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of Type II Bacterial Photosynthetic Reaction Centers.

de Almeida WB, Taguchi AT, Dikanov SA, Wraight CA, O'Malley PJ - J Phys Chem Lett (2014)

Bottom Line: A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference.This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔE m of ∼160-195 mV.The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

View Article: PubMed Central - PubMed

Affiliation: LQC-MM, Departamento de Química, ICEx, Universidade Federal de Minas Gerais (UFMG) , Campus Pampulh, Belo Horizonte, MG 31.910-270, Brazil.

ABSTRACT
Recent studies have shown that only quinones with a 2-methoxy group can act simultaneously as the primary (QA) and secondary (QB) electron acceptors in photosynthetic reaction centers from purple bacteria such as Rb. sphaeroides. (13)C HYSCORE measurements of the 2-methoxy group in the semiquinone states, SQA and SQB, were compared with DFT calculations of the (13)C hyperfine couplings as a function of the 2-methoxy dihedral angle. X-ray structure comparisons support 2-methoxy dihedral angle assignments corresponding to a redox potential gap (ΔE m) between QA and QB of 175-193 mV. A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference. This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔE m of ∼160-195 mV. The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

No MeSH data available.


QA and QB quinones in the Rb. sphaeroides RC (coordinates from PDB ID: 3I4D). Hydrogen bondacceptance by the O4 atom of each quinone from the imidazole groupNδ of His-M219 (QA) and His-L190 (QB) is illustrated, as well as the Fe(II) atom that bridgesthe imidazoles. The 2-methoxy group of each quinone is circled inred.
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fig1: QA and QB quinones in the Rb. sphaeroides RC (coordinates from PDB ID: 3I4D). Hydrogen bondacceptance by the O4 atom of each quinone from the imidazole groupNδ of His-M219 (QA) and His-L190 (QB) is illustrated, as well as the Fe(II) atom that bridgesthe imidazoles. The 2-methoxy group of each quinone is circled inred.

Mentions: Type II reaction centers (RCs)from anoxygenic and oxygenic photosynthetic RCs contain two quinonesQA and QB that function in series as electronacceptors (Figure 1).1,2 Followingcharge separation in the RC, QA is one-electron-reduced,generating the semiquinone radical form SQA, which thentransfers the electron to QB, forming SQB. Inthe anoxygenic species Rhodobacter (Rb.) sphaeroides, and many others, QA and QB are chemicallyidentical ubiquinones, and yet forward electron transfer is thermodynamicallyfavorable by 60–75 mV.3


The 2-Methoxy Group Orientation Regulates the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of Type II Bacterial Photosynthetic Reaction Centers.

de Almeida WB, Taguchi AT, Dikanov SA, Wraight CA, O'Malley PJ - J Phys Chem Lett (2014)

QA and QB quinones in the Rb. sphaeroides RC (coordinates from PDB ID: 3I4D). Hydrogen bondacceptance by the O4 atom of each quinone from the imidazole groupNδ of His-M219 (QA) and His-L190 (QB) is illustrated, as well as the Fe(II) atom that bridgesthe imidazoles. The 2-methoxy group of each quinone is circled inred.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4126703&req=5

fig1: QA and QB quinones in the Rb. sphaeroides RC (coordinates from PDB ID: 3I4D). Hydrogen bondacceptance by the O4 atom of each quinone from the imidazole groupNδ of His-M219 (QA) and His-L190 (QB) is illustrated, as well as the Fe(II) atom that bridgesthe imidazoles. The 2-methoxy group of each quinone is circled inred.
Mentions: Type II reaction centers (RCs)from anoxygenic and oxygenic photosynthetic RCs contain two quinonesQA and QB that function in series as electronacceptors (Figure 1).1,2 Followingcharge separation in the RC, QA is one-electron-reduced,generating the semiquinone radical form SQA, which thentransfers the electron to QB, forming SQB. Inthe anoxygenic species Rhodobacter (Rb.) sphaeroides, and many others, QA and QB are chemicallyidentical ubiquinones, and yet forward electron transfer is thermodynamicallyfavorable by 60–75 mV.3

Bottom Line: A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference.This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔE m of ∼160-195 mV.The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

View Article: PubMed Central - PubMed

Affiliation: LQC-MM, Departamento de Química, ICEx, Universidade Federal de Minas Gerais (UFMG) , Campus Pampulh, Belo Horizonte, MG 31.910-270, Brazil.

ABSTRACT
Recent studies have shown that only quinones with a 2-methoxy group can act simultaneously as the primary (QA) and secondary (QB) electron acceptors in photosynthetic reaction centers from purple bacteria such as Rb. sphaeroides. (13)C HYSCORE measurements of the 2-methoxy group in the semiquinone states, SQA and SQB, were compared with DFT calculations of the (13)C hyperfine couplings as a function of the 2-methoxy dihedral angle. X-ray structure comparisons support 2-methoxy dihedral angle assignments corresponding to a redox potential gap (ΔE m) between QA and QB of 175-193 mV. A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference. This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔE m of ∼160-195 mV. The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

No MeSH data available.