Light/Dark Modulation: Regulation of Chloroplast Metabolism in a New Light

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Abstract
Light‐dark modulation of chloroplast enzymes is achieved by covalent redox‐modification of protein thiols/disulfides mediated by ferredoxin/thioredoxin reductase and thioredoxins. Light‐dependent electron flow leads to reduction of particular chloroplast proteins, while photosynthetically evolved oxygen effects their continuous reoxidation. The oxidized and the reduced forms, respectively, differ greatly in their catalytic properties. The rate of reduction of each target enzyme is specifically fine‐controlled by metabolites. By this combined mode of producing a defined ratio of active to inactive enzyme during steady‐state each of the enzymes is adjusted to the immediate requirements of the chloroplast. Upon changes of the metabolic situation the system can respond in a flexible manner as is known from comparable regulatory mechanisms such as protein phosphorylation/dephosphorylation in animals and bacteria. From sequence comparisons between various light‐dark modulated chloroplast enzymes and their non‐regulated counterparts from other organelles or non‐photosynthetic organisms, the presence of extra‐peptides in the otherwise highly homologous sequences has been estabüshed for the chloroplast enzymes. However, no general pattern in the primary structure of those extra‐sequences can be recognized. By the acquisition of “regulatory peptides” during evolution a new type of metabolic control was created in a compartment uniquely occurring in organisms performing oxygenic photosynthesis.

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