Light-harvesting in photosystem I

Abstract

This review focuses on the light-harvesting properties of photosystem I (PSI) and its LHCI outer antenna. LHCI consists of different chlorophyll a/b binding proteins called Lhca's, surrounding the core of PSI. In total, the PSI-LHCI complex of higher plants contains 173 chlorophyll molecules, most of which are there to harvest sunlight energy and to transfer the created excitation energy to the reaction center (RC) where it is used for charge separation. The efficiency of the complex is based on the capacity to deliver this energy to the RC as fast as possible, to minimize energy losses. The performance of PSI in this respect is remarkable: on average it takes around 50 ps for the excitation to reach the RC in plants, without being quenched in the meantime. This means that the internal quantum efficiency is close to 100% which makes PSI the most efficient energy converter in nature. In this review, we describe the light-harvesting properties of the complex in relation to protein and pigment organization/composition, and we discuss the important parameters that assure its very high quantum efficiency. Excitation energy transfer and trapping in the core and/or Lhcas, as well as in the supercomplexes PSI-LHCI and PSI-LHCI-LHCII are described in detail with the aim of giving an overview of the functional behavior of these complexes.

Fig. 1

Structure of PSI-LHCI from pea (Amunts et al. 2010). Top view from the stromal side. The main subunits of core and antenna are indicated in figure. The Chls responsible for the red forms in Lhca4 and Lhca3 are presented in space-filled style

Fig. 2

Structure of the cyanobacteria core (Jordan et al. 2001). Top protein organization. Left, top view from the stomal side. Right, side view the main proteins are indicated in figure, the color code for left and right is identical. Bottom pigment organization. Chlorophylls are in green with the exception of P700 which is in red. Carotenoids are in yellow. Left and right as in the top panel

Fig. 3

Absorption (solid) and fluorescence emission (dot) spectra of Lhca1/4 (red) and Lhca2/3 (black) native dimers at 77 K (Wientjes et al. 2011a)

Fig. 4

Schematic presentations of energy transfer and trapping in PSI-LHCI based on Wientjes et al. (2011b). Increasing thickness of the arrows indicates increasing rates. The transfer rate between Lhca2 and Lhca4 could not be estimated from the target analysis in that study, but based on structural data, it has been suggested to be similar to the intradimer transfer rates