How does light regulate chloroplast enzymes? Structure–function studies of the ferredoxin/thioredoxin system

Abstract

1. Introduction 682. Ferredoxin reduction by photosystem I 723. Ferredoxins 734. Ferredoxin[ratio ]thioredoxin reductase 734.1 Spectroscopic investigations of FTR 764.2 The three-dimensional structure of FTR from the cyanobacterium Synechocystis sp. PCC6803 774.2.1 The variable subunit 774.2.2 The catalytic subunit 814.2.3 The iron–sulfur center and active site disulfide bridge 824.2.4 The dimer 844.3 Thioredoxin f and m 854.4 Ferredoxin and thioredoxin interactions 864.5 Mechanism of action 884.6 Comparison with other chloroplast FTRs 925. Target enzymes 955.1 NADP-dependent malate dehydrogenase 955.1.1 Regulatory role of the N-terminal extension 975.1.2 Regulatory role of the C-terminal extension 995.1.3 Thioredoxin interactions 1015.2 Fructose-1,6-bisphosphatase 1015.3 Redox regulation of chloroplast target enzymes 1036. Conclusion 1037. Acknowledgements 1048. References 104A pre-requisite for life on earth is the conversion of solar energy into chemical energy by photosynthetic organisms. Plants and photosynthetic oxygenic microorganisms trap the energy from sunlight with their photosynthetic machinery and use it to produce reducing equivalents, NADPH, and ATP, both necessary for the reduction of carbon dioxide to carbohydrates, which are then further used in the cellular metabolism as building blocks and energy source. Thus, plants can satisfy their energy needs directly via the light reactions of photosynthesis during light periods. The situation is quite different in the dark, when these organisms must use normal catabolic processes like non-photosynthetic organisms to obtain the necessary energy by degrading carbohydrates, like starch, accumulated in the chloroplasts during daylight. The chloroplast stroma contains both assimilatory enzymes of the Calvin cycle and dissimilatory enzymes of the pentose phosphate cycle and glycolysis. This necessitates a strict, light-sensitive control that switches between assimilatory and dissimilatory pathways to avoid futile cycling (Buchanan, 1980, 1991; Buchanan et al. 1994; Jacquot et al. 1997; Schürmann & Buchanan, 2000).