The de-epoxidase and epoxidase reactions of Mantoniella squamata (Prasinophyceae) exhibit different substrate-specific reaction kinetics compared to spinach

Abstract

In vivo the prasinophyceaen alga Mantoniella squamata Manton et Parke uses an incomplete violaxanthin (Vx) cycle, leading to a strong accumulation of antheraxanthin (Ax) under conditions of high light. Here, we show that this zeaxanthin (Zx)-depleted Vx/Ax cycle is caused by an extremely slow second de-epoxidation step from Ax to Zx, and a fast epoxidation from Ax back to Vx in the light. The rate constant of Ax epoxidation is 5 to 6 times higher than the rate constant of Zx formation, implying that Ax is efficiently converted back to Vx before it can be de-epoxidated to Zx. It is, however, only half the rate constant of the first de-epoxidation step from Vx to Ax, thus explaining the observed net accumulation of Ax during periods of strong illumination. When comparing the rate constant of the second de-epoxidation step in M. squamata with Zx formation in spinach (Spinacia oleracea L.) thylakoids, we find a 20-fold reduction in the reaction kinetics of the former. This extremely slow Ax de-epoxidation, which is also exhibited by the isolated Mantoniella violaxanthin de-epoxidase (VDE), is due to a reduced substrate affinity of M. squamata VDE for Ax compared with the VDE of higher plants. Mantoniella VDE, which has a similar Km value for Vx, shows a substantially increased Km for the substrate Ax in comparison with spinach VDE. Our results furthermore explain why Zx formation in Mantoniella cells can only be found at low pH values that represent the pH optimum of VDE. A pH of 5 blocks the epoxidation reaction and, consequently, leads to a slow but appreciable accumulation of Zx.

Influence of thylakoid membrane lipids on the structure and function of the plant photosystem II core complex

“…Pigment extraction and HPLC separation were performed according to Frommolt et al (2001). Conversion of Chl a to pheophytin was performed in a medium which consisted of 10 mM NaCl, 5 mM MgCl 2 and 10 mM Hepes (pH 3.5).…”

One step closer to eliminating the nomenclatural problems of minute coccoid green algae: Pseudochloris wilhelmii, gen. et sp. nov. (Trebouxiophyceae, Chlorophyta)

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A Structural Basis for the pH-Dependent Xanthophyll Cycle in Arabidopsis thaliana

“…This drop in lumen pH leads to activation of the violaxanthin deepoxidase (VDE), thus shifting the xanthophyll balance from violaxanthin (V; a di-epoxide), which acts as a lightharvesting pigment, toward Z (no epoxide group), which favors dissipation of the excitation energy through heat. This reaction comes with a transient accumulation of antheraxanthin, a monoepoxide intermediate (Hartel et al, 1996;Frommolt et al, 2001). When the lumen pH increases, Z is converted back to V by a stromal enzyme zeaxanthin epoxidase.…”