An unusual role for the phytyl chains in the photoprotection of the chlorophylls bound to Water-Soluble Chlorophyll-binding Proteins

Abstract

Water-Soluble Chlorophyll Proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins which tetramerize upon binding four chlorophyll (Chl) molecules. The bound Chls are highly photostable, despite the lack of bound carotenoids known, in Chl-containing photosynthetic proteins, to act as singlet oxygen and Chl triplet (3Chl) quenchers. Although the physiological function of WSCPs is still unclear, it is likely to be related to their biochemical stability and their resistance to photodegradation. To get insight into the origin of this photostability, the properties of the 3Chl generated in WSCPs upon illumination were investigated. We found that, unlike the excited singlet states, which are excitonic states, the triplet state is localized on a single Chl molecule. Moreover, the lifetime of the 3Chl generated in WSCPs is comparable to that observed in other Chl-containing systems and is reduced in presence of oxygen. In contrast to previous observations, we found that WSCP actually photosensitizes singlet oxygen with an efficiency comparable to that of Chl in organic solvent. We demonstrated that the observed resistance to photooxidation depends on the conformation of the phytyl moieties, which in WSCP are interposed between the rings of Chl dimers, hindering the access of singlet oxygen to the oxidizable sites of the pigments.

Water-soluble chlorophyll-binding proteins from Brassica oleracea allow for stable photobiocatalytic oxidation of cellulose by a lytic polysaccharide monooxygenase.

“…There was, however, evidence to con rm the abundant production of 1 O 2 by WSCP-Chl α (Fig. S1) as demonstrated by Agostini et al 29 . Based on this information, a proposed reaction scheme from WSCP-Chl α can be seen in Fig.…”

“…This was con rmed by uorescence detection using Singlet Oxygen Sensor Green (SOSG) (Fig. S1), and was performed according to Agostini et al 29 (Fig. 2b).…”

New homologues of Brassicaceae water‐soluble chlorophyll proteins shed light on chlorophyll binding, spectral tuning, and molecular evolution

“…The assembly of the natural WSCP homologues with Chl b and the spectral characteristics of the resulting complexes followed the same trend as Chl a , with the exception of CaWSCP and Ol1WSCP, both of which featured additional blue‐ and redshifted bands around the main Qy peak at 656 nm. These were likely due to a Chl b by‐product formed in water‐in‐oil emulsions, since the additional bands were not observed in other reconstitution protocols .…”

Designed for life: biocompatible de novo designed proteins and components

“…Natural proteins often use accessory proteins to obfuscate this problem, but a less complex approach would be beneficial to the assembly of de novo proteins. Successful methods in incorporating such hydrophobic molecules into soluble proteins have included the use of detergents [ 108 ] or water-in-oil emulsions [ 109 ], although other strategies would need to be developed to achieve this in vivo . One approach used in previous work regarding the incorporation of chlorophyll (Chl) and bacteriochlorophyll (BChl) into soluble de novo proteins involved the removal of their hydrophobic tails to improve solubility and prevent aggregation [ 106 , 107 , 110 ].…”