From red to blue to far-red in Lhca4: How does the protein modulate the spectral properties of the pigments?

Abstract

The first event of photosynthesis is the harvesting of solar energy by a large array of pigments. These pigments are coordinated to proteins that organize them to assure efficient excitation energy transfer. The protein plays an essential role in tuning the spectroscopic properties of the pigments, by determining their site energy and/or by favoring pigment-pigments interactions. Here we investigate how the protein modulates the pigment properties by using a single-point-mutation approach. We monitor changes in the low-energy absorption/emission band of Lhca4, which is well separated from the bulk absorption and thus represents an attractive model system. Moreover, it was recently shown that Lhca4 exists in at least two conformations, a dominating one emitting at 720 nm and a second one emitting at 685 nm (Kruger et al. PNAS 2011). Here we show that a single amino-acid substitution (from Asn to Gln, which are both chlorophyll-binding residues and only differ for one C-C bond), moves the equilibrium almost completely towards the 685-nm conformation. This indicates that small changes in the protein can have a large effect on the properties of the pigments. We show that His99, which was suggested to coordinate a red-absorbing chlorophyll (Melkozernov and Blankenship, JBC 2003), is not a chlorophyll ligand. We also show that single amino-acid substitutions nearby the chlorophylls allow to tune the emission spectrum of the pigments over a wide range of wavelengths and to modulate the excited-state lifetimes of the complex. These findings are discussed in the light of previously proposed non-photochemical quenching models.

Structure of the plant photosystem I supercomplex at 2.6 Å resolution

“…Remarkably, the core pigment organization is conserved across kingdoms despite this diversity in connected antenna (Amunts et al, 2007;Busch and Hippler, 2011;Croce and van Amerongen, 2013), which suggests that the connection points between the core and the antennae are conserved. The existence of red-absorbing pigments (or 'red traps') is a general property of PSI (Morosinotto et al, 2005;Wientjes et al, 2012). These pigments affect the rate of trapping in PSI and can affect the path of excitation migration in the complex.…”

Photosynthetic light harvesting: excitons and coherence

“…Energetically detuned chromophores are commonplace in light-harvesting complexes. For example, differences in the local protein environment of otherwise identical molecules can give rise to differences in transition energies [90][91][92][93][94]. Slow environmental motions, for example conformational changes in the protein, cause random fluctuations in electronic parameters over long timescales (compared with the timescale of exciton dynamics), giving rise to what is known as static disorder.…”

Single-Molecule Identification of Quenched and Unquenched States of LHCII

“…The most likely molecular mechanism behind the three states of LHCII is three different structural conformations, as small conformational changes can have a significant effect on the excited state energies and dynamics 46 . These states correspond unchanged.…”