Biochemistry of Tue Violaxanthin Cycle in Higher Plants

The biochemistry of the violaxanthin cycle in relationship to photosynthesis is reviewed. The cycle is a component of the thylakoid and consists of a reaction sequence in which violaxanthin is converted to zeaxanthin (de—epoxidation) and then regenerated (epoxidation) through separate reaction mechanisms. The arrangement of the cycle in the thyla— koid is transmembranous with the de—epoxidation system situated on the loculus side and epoxidation on the outer side of the membrane. Photosynthetic activities affect turnover of the cycle but the cycle it— self consists entirely of dark reactions. Light has at least two roles in de—epoxidation. It establishes through the proton pump the acidic pH in the loculus that is required for de—epoxidase activity and it induces a presumed conformational change in the inner membrane surface which determines the fraction of violaxanthin in the membrane that enters the cycle. De—epoxidation, which requires ascorbate, is presumed to proceed by a reductive—dehydration mechanism. Non—cyclic electron transport can provide the required reducing potential through the dehydroascorbate— ascorbate couple. Whether ascorbate reduces the de—epoxidase system directly or through an intermediate has not been settled. Epoxidation requires NADPH and 02 which suggests a reductive mechanism. In contrast with de—epoxidatlon, it has a pH optimum near neutrality. The coupling of photosynthetically generated NADPH to epoxidation has been shown. Turnover of the cycle under optimal conditions is estimated to be about two orders of magnitude below optimal electron transport rates. This low rate appears to exclude a direct role of the cycle in photosynthesis or a role in significantly affecting photosynthate levels in a back reaction. The fact that the cycle is sensitive to events both before and after Photosysten I suggests a regulatory role, possibly through effects on membrane properties. A model showing the various relationships of the cycle to photosynthesis is presented. The contrasting view that the cycle can participate directly in photosynthesis, such as in oxygen evolution, is discussed. Violaxanthin de—epoxidase has been purified. It is a lipoprotein which contains monogalactosyldiglyceride (MG) exclusively. The enzyme is a mono—de—epoxidase which is specific for 3—OH, 5—6—epoxy carotenoids that are in a 3R, 5S, 6R configuration. In addition, the polyene chain must be all—trans. A model has been presented which depicts enzymic MG in a receptor role and the stereospecific active center situated in a narrow well—like depression that can accommodate only the all—trans structure.