Experimental and Theoretical Mutation of Exciton States
on the Smallest Type‑I Photosynthetic Reaction Center Complex
of a Green Sulfur Bacterium Chlorobaclum tepidum
The
exciton states on the smallest type-I photosynthetic reaction
center complex of a green sulfur bacterium Chlorobaculum
tepidum (GsbRC) consisting of 26 bacteriochlorophylls a (BChl a) and four chlorophylls a (Chl a) located on the homodimer of two
PscA reaction center polypeptides were investigated. This analysis
involved the study of exciton states through a combination of theoretical
modeling and the genetic removal of BChl a pigments
at eight sites. (1) A theoretical model of the pigment assembly exciton
state on GsbRC was constructed using Poisson TrESP (P-TrESP) and charge
density coupling (CDC) methods based on structural information. The
model reproduced the experimentally obtained absorption spectrum,
circular dichroism spectrum, and excitation transfer dynamics, as
well as explained the effects of mutation. (2) Eight BChl a molecules at different locations on the GsbRC were selectively
removed by genetic exchange of the His residue, which ligates the
central Mg atom of BChl a, with the Leu residue on
either one or two PscAs in the RC. His locations are conserved among
all type-I RC plant polypeptide, cyanobacteria, and bacteria amino
acid sequences. (3) Purified mutant-GsbRCs demonstrated distinct absorption
and fluorescence spectra at 77 K, which were different from each other,
suggesting successful pigment removal. (4) The same mutations were
applied to the constructed theoretical model to analyze the outcomes
of these mutations. (5) The combination of theoretical predictions
and experimental mutations based on structural information is a new
tool for studying the function and evolution of photosynthetic reaction
centers.