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Structural and kinetic properties of Rhodobacter sphaeroides photosynthetic reaction centers containing exclusively Zn-coordinated bacteriochlorophyll as bacteriochlorin cofactors
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DOI：10.1016/j.bbabio.2013.11.015.png)
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Abstract
En 中文
The Zn-BChl-containing reaction center (RC) produced in a bchD (magnesium chelatase) mutant of Rhoclobacter sphaeroides assembles with six Zn-bacteriochlorophylls (Zn-BChls) in place of four Mg-containing bacteriochlorophylls (BChls) and two bacteriopheophytins (BPhes). This protein presents unique opportunities for studying biological electron transfer, as Zn-containing chlorins can exist in 4-, 5-, and (theoretically) 6-coordinate states within the RC. In this paper, the electron transfer perturbations attributed exclusively to coordination state effects are separated from those attributed to the presence, absence, or type of metal in the bacteriochlorin at the H-A pocket of the RC. The presence of a 4-coordinate Zn2+ ion in the HA bacteriochlorin instead of BPhe results in a small decrease in the rates of the P* -> P+H-A(-) -> P+ Q(A)(-) electron transfer, and the charge separation yield is not greatly perturbed; however coordination of the Zn2+ by a fifth ligand provided by a histidine residue results in a larger rate decrease and yield loss. We also report the first crystal structure of a Zn-BChl-containing RC, Confirming that the H-A Zn-BChl was either 4- or 5-coordinate in the two types of Zn-BChl-containing RCs studied here. Interestingly, a large degree of disorder, in combination with a relatively weak anomalous difference electron density was found in the H-B pocket. These data, in combination with spectroscopic results, indicate partial occupancy of this binding pocket. These findings provide insights into the use of BPhe as the bacteriochlorin pigment of choice at H-A in both BChl- and Zn-BChl-containing RCs found in nature. (C) 2013 Elsevier B.V. All rights reserved.
Keywords:
Bacteriochlorophyll
Reaction center
Bacteriopheophytin
Electron transfer
Rhodobacter sphaeroides
Spectroscopy
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