Photosynthesis: Carbon Metabolism Twenty Years of Following Carbon Cycles in Photosynthetic Cells

Abstract

We do not know whether or not the gentleman described above completed his planned 16 years of research on cucumber photosynthesis. If he did, he still would not have matched our efforts for Progress in Botany, in which our reviews of photosynthetic carbon metabolism have now reached their 20th year. We began at a time when the Calvin cycle was well established (although sufficient activities of all of its enzymes were not), and the processes of C₄ photosynthesis and photorespiration had been only recently discovered. During the ensuing 20 years, many details concerning the operation and regulation of these carbon cycles have been revealed. Seven stand out: (1) the demonstration of adequate activities for all Calvin cycle enzymes; (2) the discovery of the chloroplast envelope’s phosphate translocator, (3) the CO₂-plus-Mg²⁺ activation of ribulose-1,5-bisphosphate (RuBP) carboxylase and, more recently, this enzyme’s regulation by the protein “rubisco activase” and a seemingly endless array of novel sugar phosphates; (4) the demonstration of the light-mediated activation of Calvin cycle enzymes, mediated by the small proteins ferredoxin and thioredoxin; (5) the discoveries that the enzymes phosphoenolpyruvate (PEP) carboxylase, pyruvate Pi dikinase, and sucrose-P synthase are regulated by light-mediated phosphorylation/dephosphorylation; (6) the discoveries of the enzyme PPi-phosphofructokinase (PPi-PFK) and the regulatory compound fructose-2,6-P₂ in the cytosol of photosynthetic cells — their roles in regulating sucrose metabolism are still being elucidated; (7) the discovery (reported in this review) that the chloroplast envelope’s adenylate translocator accommodates the uptake of ADP-glucose.

“The first man I saw was of a meagre aspect, with sooty hands and face, his hair and beard long, ragged and singed in several places. His clothes, shirt and skin were all of the same colour. He had been eight years upon a project for extracting sunbeams out of cucumbers, which were to be put into vials hermetically sealed, and let out to warm the air in raw inclement summers. He told me, he did not doubt in eight years more, that he should be able to supply the Governor’s gardens with sunshine at a reasonable rate…” Jonathan Swift in Gulliver’s Travels, ca. 1727