Structure-based simulation of linear optical spectra of the CP43 core antenna of photosystem II

Abstract

The linear optical spectra (absorbance, linear dichroism, circular dichroism, fluorescence) of the CP43 (PsbC) antenna of the photosystem II core complex (PSIIcc) pertaining to the S(0) → S(1) (Q(Y)) transitions of the chlorophyll (Chl) a pigments are simulated by applying a combined quantum chemical/electrostatic method to obtain excitonic couplings and local transition energies (site energies) on the basis of the 2.9 Å resolution crystal structure (Guskov et al., Nat Struct Mol Biol 16:334-342, 2009). The electrostatic calculations identify three Chls with low site energies (Chls 35, 37, and 45 in the nomenclature of Loll et al. (Nature 438:1040-1044, 2005). A refined simulation of experimental spectra of isolated CP43 suggests a modified set of site energies within 143 cm(-1) of the directly calculated values (root mean square deviation: 80 cm(-1)). In the refined set, energy sinks are at Chls 37, 43, and 45 in agreement with earlier fitting results (Raszewski and Renger, J Am Chem Soc 130:4431-4446, 2008). The present structure-based simulations reveal that a large part of the redshift of Chl 37 is due to a digalactosyldiacylglycerol lipid. This finding suggests a new role for lipids in PSIIcc, namely the tuning of optical spectra and the creation of an excitation energy funnel towards the reaction center. The analysis of electrostatic pigment-protein interactions is used to identify amino acid residues that are of potential interest for an experimental approach to an assignment of site energies and energy sinks by site-directed mutagenesis.

The quest for energy traps in the CP43 antenna of photosystem II

“…Our electrostatic approach to this problem [36][37][38] is approximate, but turned out to give reasonable results, when applied to the Fenna-Matthews-Olson (FMO) protein of green sulfur bacteria [42][43][44][45], photosystem I [46], the antenna system of plant photosystem II [47][48][49], and the BLUF photoreceptor [50]. In the present paper, we apply it to the CP43 core antenna of PSIIcc, updating earlier work [51].…”

“…An overview of different numbering schemes for the Chls is given in Table 1. In this paper, we use the numbering of Chls designated as "2AXT" in Table 1 for the sake of continuity with earlier work [8,15,23,51,52], when we refer to specific Chls in CP43, but use a consecutive numbering following the order of the Chls in the PDB files (designated with "m" in Table 2). Besides harboring antenna Chls in its membrane-spanning part, CP43 is significantly extended into the lumen, where it is in direct contact with the water-oxidizing catalyst [2,6,32,34,35].…”

Structure-Based Calculation of Pigment–Protein and Excitonic Pigment–Pigment Coupling in Photosynthetic Light-Harvesting Complexes

“…This is done in the Poisson-Boltzmann/quantum chemical (PB/QC) method [37,42,44,45]. The name of this method originates from the fact that besides the atomic partial charges originating from quantum chemistry, one also needs to calculate variants of the electrostatic potentials ϕ ðmÞ 00 (x) and ϕ ðmÞ 11 (x) of the ground and excited state of the pigment that take into account the polarization induced by these charges in the dielectric environment.…”

“…The computational methods described herein have been applied so far only to LHCII [37,74,75]. In addition, high-resolution structures of the PSII core complex from cyanobacteria have been published [2,11], but rigorous structure-based calculations of site energies are in an early stage [45]. Therefore, we concentrate on LHCII in the following overview.…”

Heat-induced unfolding of apo-CP43 studied by fluorescence spectroscopy and CD spectroscopy

“…Interaction among pigment proteins is related to the apoprotein. In addition, with the improvement in the resolution of the crystal structures of PS II complexes (Zouni et al 2001;Bricker and Frankel 2002;Loll et al 2005;Umena et al 2011), the energy transfer mechanism has become one of the focuses of recent studies, and the pigment plays a key role in energy transfer (Müh et al 2012). However, apo-CP43 provided the rack for the pigment binding, so its structure largely affected the energy transfer among pigments.…”