Enzyme Regulation in C 4 Photosynthesis

Abstract

" -Homogeneous preparations of thioredoxin-linked fructose-l,6-bisphosphatase and sedoheptulose-l,7-bis-phosphatase were prepared from leaves of corn (Zea mays), a classical C4 plant. Fructose-1,6-bisphosphatase had a molecular weight of 184,000 and consisted of four apparently identical subunits of M, = 46,000. Sedohep-tulose-1,7-bisphosphatase, by contrast, was composed of two identical subunits of M, = 38,000 (total M, = 76,000). Antibody experiments revealed that the two enzymes were immunologically distinct. The fructose-1,6-bisphosphatase and sedoheptulose-l,7-bisphosphatase were substrate-specific and re-quired M&' for activity. Both enzymes were also acti-vated specifically by thioredoxin f , which could be re-duced either photochemically by the ferredoxin/thio-redoxin system or chemically by dithiothreitol. The substrates, fructose l,(r-bisphosphate and sedoheptu-lose l,7-bisphosphate, acted synergistically with re-duced thioredoxin f in the activation of fructose-1,6-bisphosphatase and sedoheptulose-l,7-bisphosphatase, respectively. Activation by reduced thioredoxin f ef-fected a shift of the pH optimum for fructose-1,6-bis-phosphatase (pH 8.2 + pH 7.7), but not for sedoheptu-lose-l,7-bisphosphatase. Sedoheptulose-1,7-bisphos-phatase, but not fructose-1,6-bisphosphatase, showed an absolute requirement for a dithiol, Le., reduced thio-redoxin f o r dithiothreitol. Selected oxidants, including dehydroascorbate and oxidized glutathione, deacti-vated both enzymes following activation by reduced thioredoxin f. Both enzymes showed hysteretic behav-ior in activation as well as deactivation. The results demonstrate that fructose-1,6-bisphos-phatase and sedoheptulose-l,7-bisphosphatase from corn leaves are distinct enzymes, which, like their spin-ach counterparts, share in common the capacity for photoactivation by the ferredoxin/thioredoxin system. Activation of each of the enzymes by reduced thiore-doxin fis enhanced by the presence of the appropriate sugar bisphosphate substrate. The results support the earlier expressed view that the ferredoxin/thioredoxin system functions jointly with effector metabolites in light-actuated enzyme regulation during photosyn-thesis. During the past several years, it has become increasingly apparent that light functions in C3 plants in the regulation of