Mehler-peroxidase cycle

Fig. 13. The Mehler-peroxidase cycle. ASC - ascorbate MDHA - monodehydroascorbate Fd - ferredoxin PC - plastocyanin PS I Photosystem I PS II - Photosystem II SOD - superoxide dismutase tAPX - thylakoid bound ascorbate peroxidase.

While ascorbate is transported into isolated intact chloroplasts (Anderson et al., 1983b Beck et al., 1983 Foyer and Lelandais, 1996), the thylakoid membranes appear to have no carrier system to transport ascorbate into the thylakoid lumen (Foyer and Lelandais, 1996). This is surprising since the enzyme VDE, located inside the thylakoid, requires ascorbate to convert violaxanthin to zeaxanthin (Fig. 2). Upon illumination the pH ofthe lumen falls and VDE binds to the lumenal side of the thylakoid membrane and becomes active (Hager and Holocher, 1994 Rockholmand Yamamoto, 1996). The affinity of VDE for ascorbate is strongly dependent on pH, because ascorbic acid is the true substrate of the enzyme (Bratt et al., 1995). The addition ofHjOj to illuminated thylakoid membrane preparations induces a transient inhibition of zeaxanthin formation (Neubauer and Yamamoto, 1992), suggesting competition for ascorbate between the Mehler-peroxidase cycle and the VDE reaction. Since the ascorbate pool in the chloroplast stroma is substantial (10-50 rnM), sufficient amounts of ascorbic acid may be able to aoss the membrane by diffusion (Eoyer et al., 1983 Foyer, 1993 Foyer and Lelandais,... 

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