Redox-shuttling between chloroplast and cytosol: integration of intra-chloroplast and extra-chloroplast metabolism

Reducing equivalents produced in the chloroplast are essential for many key cellular metabolic enzyme reactions. Two redox shuttle systems transfer reductant out of the chloroplast; these systems consist of metabolite transporters, coupled with stromal and cytosolic dehydrogenase isozymes. The transporters function in the redox shuttle and also operate as key enzymes in carbon/nitrogen metabolism. To maintain adequate levels of reductant and proper metabolic balance, the shuttle systems are finely controlled. Also, in the leaves of C4 plants, cell-specific division of carbon and nitrogen assimilation includes cell-specific localization of the redox shuttle systems. The redox shuttle systems are tightly linked to cellular metabolic pathways and are essential for maintaining metabolic balance between energy and reducing equivalents.

► Two redox shuttle systems across chloroplast envelope maintain metabolic balance.
► The chloroplastic 2-oxoglutarate/malate transporter has multiple roles as malate valve and in carbon/nitrogen metabolism.
► The shuttle systems are finely controlled in response to the stromal redox state.
► In the leaves of C4 plants, the redox shuttle systems are localized cell-specifically and link to cellular metabolic pathways.