Molecular studies on structural changes and oligomerisation of violaxanthin de-epoxidase associated with the pH-dependent activation

Violaxanthin de-epoxidase (VDE) is a conditionally soluble enzyme located in the thylakoid lumen and catalyses the conversion of violaxanthin to antheraxanthin and zeaxanthin, which are located in the thylakoid membrane. These reactions occur when the plant or algae are exposed to saturating light and the zeaxanthin formed is involved in the process of non-photochemical quenching that protects the photosynthetic machinery during stress. Oversaturation by light results in a reduction of the pH inside the thylakoids, which in turn activates VDE and the de-epoxidation of violaxanthin. To elucidate the structural events responsible for the pH-dependent activation of VDE, full length and truncated forms of VDE were studied at different pH using circular dichroism (CD) spectroscopy, crosslinking and small angle X-ray scattering (SAXS). CD spectroscopy showed the formation of α-helical coiled-coil structure, localised in the C-terminal domain. Chemical crosslinking of VDE showed that oligomers were formed at low pH, and suggested that the position of the N-terminal domain is located near the opening of lipocalin-like barrel, where violaxanthin has been predicted to bind. SAXS was used to generate models of monomeric VDE at high pH and also a presumably dimeric structure of VDE at low pH. For the dimer, the best fit suggests that the interaction is dominated by one of the domains, preferably the C-terminal domain due to the lost ability to oligomerise at low pH, shown in earlier studies, and the predicted formation of coiled-coil structure.