Journal:Angew Chem Int Ed:1

Fine tuning of chlorophyll spectra by protein-induced ring deformation

Dominika Bednarczyk, Orly Dym, Yoav Peleg, Vadivel Prabahar, and Dror Noy ^[1]

Molecular Tour
The ability to tune the light absorption properties of chlorophylls by their protein environment is the key to the high efficiency, robustness, and adaptability of photosynthetic light harvesting proteins. Unfortunately, the intricacy of the natural complexes makes it very difficult to identify and isolate specific protein-pigment interactions that underlie the spectral tuning mechanisms and to quantify their effect on a pigment’s spectral properties. Here we identify and demonstrate the tuning mechanism of chlorophyll spectra in type II water soluble chlorophyll binding proteins from Brassicaceae (WSCPs). By comparing the molecular structures of two natural WSCPs we correlate a shift in the chlorophyll red absorption band with deformation of its tetrapyrrole macrocycle that is induced by changing the position of a nearby tryptophan residue. We show by a set of reciprocal point mutations that this change accounts for up to 2/3 of the observed spectral shift between the two natural variants.

We constructed, purified, and solved the crystal structure of water soluble chlorophyll binding protein from cauliflower complex with chlorophyll a (CaWSCP-Chl a). (PDB entry 5hpz). Chains A, B, C, and D are coloured purple, pink, grey, and blue. Chlorophylls are shown in ball-and-stick representation with carbons in cyan colour. The structure is highly homologous to the previously elucidated structure of native WSCP from Lepidium virginicum (LvWSCP; PDB ID: 2dre). Overall, the structure of recombinant CaWSCP and . LvWSCP shown in brown with Chl carbons in orange. Both assemble as symmetric homotetramers in which each monomeric subunit binds a single Chl. This results in an . Overall, the protein structures and Chl arrangements of the and subunits of LvWSCP and CaWSCP are highly homologous. But, the interfaces between the . Aligning the dimeric subunits of CaWSCP and LvWSCP reveals about 60º of the adjacent dimer. . Dimers AB of CaWSCP and LvWSCP both colored in deeppink, dimer CD of CaWSCP is in yellow and dimer CD of LvWSCP is in salmon. A view from chains A and B toward chains C and D of CaWSCP and LvWSCP revealing the . For clarity, protein chains, and Chl phytyl chains are not presented. from chain C and D toward chains A an B.

reveals that in both CaWSCP and LvWSCP the . The , and there are no charged residues within the Chl binding sites of both proteins. The most significant difference between LvWSCP and CaWSCP appears to be around Chl rings IV and I. An asparagine at position 38 (N38) in LvWSCP is replaced by alanine at position 34 (A34) in CaWSCP. This causes a . Consequently, the bulky tryptophan sidechain moves away from Chl ring D and closer to Chl ring A, which results in bending the ring out of the Chl macrocycle plain. In addition, the vinyl group of ring A rotates 180º about the C3-C3 bond. , respectively in CaWSCP accommodate to the different vinyl group position. Carbons of CaWSCP-Chl complex are in cyan and Carbons of LvWSCP-Chl complex are in orange.

PDB reference: Type II water soluble Chl binding proteins, 5hpz.