Could structural similarity of specific domains between animal globins and plant antenna proteins provide hints important for the photoprotection mechanism?

Non photochemical quenching is a fundamental mechanism in photosynthesis, which protects plants against excess excitation energy and is of crucial importance for their survival and fitness. In the last decades hundreds of papers have appeared that describe the role of antenna regulation in protection or the so called qE response. However, the exact quenching site is still obscure. Previously overlooked features of the antenna may provide hints towards the elucidation of its functionality and of the quenching mechanism. Recently it was demonstrated that the catalytic domain of human myoglobin that binds the pigment (i.e. heme) is similar in structure to the domain of the light harvesting complex II of pea that binds Chl a 614 (former known as b3). In addition, it is well accepted that conformational changes of the chlorophyll macrocycle result in reversible changes of fluorescence (the lowest fluorescence corresponds to non planar macrocycle). Here we put forward a hypothesis regarding the molecular mechanism that leads to the formation of a quenching center inside the antenna proteins. Our main suggestion is that a conformational change of helix H5 (known also as helix D) forces conformational changes in the macrocycle of Chl a 614 is implicated in the ΔA535 absorbance change and quenching during photoprotective qE. The specific features (some of them similar to those of heme domain of globins) of the b3 domain account for these traits. The model predicts that antenna proteins having b3 pigments (i.e. LHCII, CP29, CP26) can act as potential quenchers. A working model is suggested for the site and mechanism of qE. [Display omitted] •A conformational two-state mechanism for qE is proposed.•The mechanism relies on conformational changes of helix D and b3.•Helix D is a sensor of lumen energization.•Chlorophyll b3 is a quenching center.