Influence of subunit structure on the oligomerization state of light-harvesting complexes: A free energy calculation study

Optimal fold symmetry of LH2 rings on a photosynthetic membrane

An intriguing observation of photosynthetic light-harvesting systems is the N-fold symmetry of light-harvesting complex 2 (LH2) of purple bacteria. We calculate the optimal rotational configuration of N-fold rings on a hexagonal lattice and establish two related mechanisms for the promotion of maximum excitation energy transfer (EET). (i) For certain fold numbers, there exist optimal basis cells with rotational symmetry, extendable to the entire lattice for the global optimization of the EET network. (ii) The type of basis cell can reduce or remove the frustration of EET rates across the photosynthetic network. We find that the existence of a basis cell and its type are directly related to the number of matching points S between the fold symmetry and the hexagonal lattice. The two complementary mechanisms provide selection criteria for the fold number and identify groups of consecutive numbers. Remarkably, one such group consists of the naturally occurring 8-, 9-, and 10-fold rings. By considering the inter-ring distance and EET rate, we demonstrate that this group can achieve minimal rotational sensitivity in addition to an optimal packing density, achieving robust and efficient EET. This corroborates our findings i and ii and, through their direct relation to S, suggests the design principle of matching the internal symmetry with the lattice order.

In silico predictions of LH2 ring sizes from the crystal structure of a single subunit using molecular dynamics simulations

Most of the currently known light-harvesting complexes 2 (LH2) rings are formed by 8 or 9 subunits. As of now, questions like "what factors govern the LH2 ring size?" and "are there other ring sizes possible?" remain largely unanswered. Here, we investigate by means of molecular dynamics (MD) simulations and stochastic modeling the possibility of predicting the size of an LH2 ring from the sole knowledge of the high resolution crystal structure of a single subunit. Starting with single subunits of two LH2 rings with known size, that is, an 8-ring from Rs. moliscianum (MOLI) and a 9-ring from Rps. acidophila (ACI), and one with unknown size (referred to as X), we build atomic models of subunit dimers corresponding to assumed 8-, 9-, and 10-ring geometries. After inserting each of the dimers into a lipid-water environment, we determine the preferred angle between the corresponding subunits by three methods: (1) energy minimization, (2) free MD simulations, and (3) potential of mean force calculations. We find that the results from all three methods are consistent with each other, and when taken together, it allows one to predict with reasonable level of confidence the sizes of the corresponding ring structures. One finds that X and ACI very likely form a 9-ring, while MOLI is more likely to form an 8-ring than a 9-ring. Finally, we discuss both the merits and limitations of all three prediction methods.

Forces guiding assembly of light-harvesting complex 2 in native membranes

Interaction forces of membrane protein subunits are of importance in their structure, assembly, membrane insertion, and function. In biological membranes, and in the photosynthetic apparatus as a paradigm, membrane proteins fulfill their function by ensemble actions integrating a tight assembly of several proteins. In the bacterial photosynthetic apparatus light-harvesting complexes 2 (LH2) transfer light energy to neighboring tightly associated core complexes, constituted of light-harvesting complexes 1 (LH1) and reaction centers (RC). While the architecture of the photosynthetic unit has been described, the forces and energies assuring the structural and functional integrity of LH2, the assembly of LH2 complexes, and how LH2 interact with the other proteins in the supramolecular architecture are still unknown. Here we investigate the molecular forces of the bacterial LH2 within the native photosynthetic membrane using atomic force microscopy single-molecule imaging and force measurement in combination. The binding between LH2 subunits is fairly weak, of the order of k(B)T, indicating the importance of LH2 ring architecture. In contrast LH2 subunits are solid with a free energy difference of 90 k(B)T between folded and unfolded states. Subunit α-helices unfold either in one-step, α- and β-polypeptides unfold together, or sequentially. The unfolding force of transmembrane helices is approximately 150 pN. In the two-step unfolding process, the β-polypeptide is stabilized by the molecular environment in the membrane. Hence, intermolecular forces influence the structural and functional integrity of LH2.

The effect of correlated bath fluctuations on exciton transfer

Excitation dynamics of various light harvesting systems have been investigated with many theoretical methods including various non-Markovian descriptions of dissipative quantum dynamics. It is typically assumed that each excited state is coupled to an independent thermal environment, i.e., that fluctuations in different environments are uncorrelated. Here the assumption is dropped and the effect of correlated bath fluctuations on excitation transfer is investigated. Using the hierarchy equations of motion for dissipative quantum dynamics it is shown for models of the B850 bacteriochlorophylls of LH2 that correlated bath fluctuations have a significant effect on the LH2→LH2 excitation transfer rate. It is also demonstrated that inclusion of static disorder is crucial for an accurate description of transfer dynamics.

Architecture of the native photosynthetic apparatus of Phaeospirillum molischianum

The ubiquity and importance of photosynthetic organisms in nature has made the molecular mechanisms of photosynthesis a widely studied subject at both structural and functional levels. A current challenge is to understand the supramolecular assembly of the proteins involved in photosynthesis in native membranes. We have used atomic force microscopy to study the architecture of the photosynthetic apparatus and analyze the structure of single molecules in chromatophores of Phaeospirillum molischianum. Core complexes are formed by the reaction center enclosed by an elliptical light harvesting complex 1. LH2 are octameric rings, assembled either with cores or in hexagonally packed LH2 antenna domains. The symmetry mismatch caused by octameric LH2 packing in a hexagonal lattice, that could be avoided in a square lattice, suggests lipophobic effects rather than specific inter-molecular interactions drive protein organization. The core and LH2 complexes are organized to form a supramolecular assembly reminiscent to that found in Rhodospirillum photometricum, and very different from that observed in Rhodobacter sphaeroides, Rb. blasticus, and Blastochloris viridis.