cells Review Pentose Phosphate Pathway Reactions in Photosynthesizing Cells Thomas D. Sharkey MSU-DOE Plant Research Laboratory, Plant Resilience Institute, Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48823, USA; [email protected] Abstract: The pentose phosphate pathway (PPP) is divided into an oxidative branch that makes pentose phosphates and a non-oxidative branch that consumes pentose phosphates, though the non-oxidative branch is considered reversible. A modified version of the non-oxidative branch is a critical component of the Calvin–Benson cycle that converts CO2 into sugar. The reaction se- quence in the Calvin–Benson cycle is from triose phosphates to pentose phosphates, the opposite of the typical direction of the non-oxidative PPP. The photosynthetic direction is favored by replac- ing the transaldolase step of the normal non-oxidative PPP with a second aldolase reaction plus sedoheptulose-1,7-bisphosphatase. This can be considered an anabolic version of the non-oxidative PPP and is found in a few situations other than photosynthesis. In addition to the strong association of the non-oxidative PPP with photosynthesis metabolism, there is recent evidence that the oxida- tive PPP reactions are also important in photosynthesizing cells. These reactions can form a shunt around the non-oxidative PPP section of the Calvin–Benson cycle, consuming three ATP per glucose 6-phosphate consumed. A constitutive operation of this shunt occurs in the cytosol and gives rise to an unusual labeling pattern of photosynthetic metabolites while an inducible shunt in the stroma may occur in response to stress. Citation: Sharkey, T.D. Pentose Keywords: 13C; 14C; aldol; Calvin-Benson cycle; light respiration; photosynthesis; isotope labeling Phosphate Pathway Reactions in Photosynthesizing Cells. Cells 2021, 10, 1547. https://doi.org/10.3390/ cells10061547 1. Introduction Academic Editor: Suleyman The PPP consists of two reaction sequences, often referred to as the oxidative and Allakhverdiev non-oxidative branches [1] (Figure1). In the oxidative branch, glucose 6-phosphate (G6P) is converted to ribulose 5-phosphate (Ru5P) and CO2, with the reduction of two molecules + Received: 30 May 2021 of NADP to NADPH. The NADPH is needed for anabolic reactions, especially synthesis Accepted: 11 June 2021 of lipids, and for ROS scavenging systems. The Ru5P and other pentose phosphates are Published: 18 June 2021 used for nucleic acid synthesis, among other things. In the non-oxidative branch of the PPP, three molecules of Ru5P are converted to two molecules of fructose 6-phosphate (F6P) Publisher’s Note: MDPI stays neutral and one molecule of glyceraldehyde 3-phosphate (GAP) [1]. The pathway can also supply with regard to jurisdictional claims in erythrose 4-phosphate (E4P) for synthesis of phenylpropanoids, including several amino published maps and institutional affil- acids. The PPPs are important in nearly all biological organisms. The PPPs, like glycolysis, iations. are considered to be evolutionarily ancient, perhaps preceding the first living organisms [2]. Photosynthetic carbon metabolism, specifically the Calvin–Benson cycle of CO2 uptake and reduction, is considered a variant of the non-oxidative branch of the PPP. It is not uncommon for photosynthetic carbon metabolism to be referred to as the “reductive PPP.” Copyright: © 2021 by the author. However, the reduction step of the Calvin–Benson cycle is not related to pentose phosphate Licensee MDPI, Basel, Switzerland. metabolism but rather to gluconeogenesis (the reversal of glycolysis) [3,4]. The Calvin– This article is an open access article Benson cycle has more gluconeogenic enzymes and reactions than PPP reactions (Table1, distributed under the terms and Figure2). conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Cells 2021, 10, 1547. https://doi.org/10.3390/cells10061547 https://www.mdpi.com/journal/cells Cells 2021, 10, 1547 2 of 18 Cells 2021, 10, 1547 2 of 17 FigureFigure 1.1. PentosePentose phosphate phosphate pathways. pathways. The The oxidative oxidative branch branch (top) (top) and andcatabolic catabolic non-oxidative non-oxidative branch branch (middle) (middle) can be 2 canshown be shown as one aspathway one pathway releasing releasing one CO onemolecule CO2 moleculeper glucose per 6-phosphate glucose 6-phosphate consumed. consumed.At the bottom At, the the catabolic bottom, non the - catabolicoxidative non-oxidative PPP, as found PPP, in the as Calvin found– inBenson the Calvin–Benson cycle, is shown. cycle, DHAP is = shown. dihydroxyacetone DHAP = dihydroxyacetone phosphate, E4P = phosphate,erythrose 4- phosphate, F6P = fructose 6-phosphate, G6P = glucose 6-phosphate, GAP = glyceraldehyde 3-phosphate, PRPP = 5-O- E4P = erythrose 4-phosphate, F6P = fructose 6-phosphate, G6P = glucose 6-phosphate, GAP = glyceraldehyde 3-phosphate, phosphono- ribose 1-diphosphate, R5P = ribose 5-phosphate, Ru5P = ribulose 5-phosphate, RuBP = ribulose 1,5-bisphos- PRPP = 5-O-phosphono- ribose 1-diphosphate, R5P = ribose 5-phosphate, Ru5P = ribulose 5-phosphate, RuBP = ribulose phate, S7P = sedoheptulose 7-phosphate, SBP = sedoheptulose 1,7-bisphosphate, SBPase = sedoheptulose 1,7-bisphospha- 1,5-bisphosphate,tase, and Xu5P = xylulose S7P = sedoheptulose 5-phosphate. 7-phosphate, SBP = sedoheptulose 1,7-bisphosphate, SBPase = sedoheptulose 1,7-bisphosphatase, and Xu5P = xylulose 5-phosphate. Photosynthetic carbon metabolism, specifically the Calvin–Benson cycle of CO2 up- take and reduction, is considered a variant of the non-oxidative branch of the PPP. It is not uncommon for photosynthetic carbon metabolism to be referred to as the “reductive PPP.” However, the reduction step of the Calvin–Benson cycle is not related to pentose Cells 2021, 10, 1547 3 of 18 phosphate metabolism but rather to gluconeogenesis (the reversal of glycolysis) [3,4]. The Cells 2021, 10, 1547 Calvin–Benson cycle has more gluconeogenic enzymes and reactions than PPP reactions3 of 17 (Table 1, Figure 2). Table 1. Number of enzymes and reaction of the Calvin–Benson cycle which are gluconeogenic or Table 1. Number of enzymes and reaction of the Calvin–Benson cycle which are gluconeogenic or pentose phosphate pathway reactions. pentose phosphate pathway reactions. Pathway Enzymes Reactions Pathway Enzymes Reactions Gluconeogenesis 5 16 GluconeogenesisPentose phosphate pathway 54 16 6 Pentose phosphate pathway 4 6 Figure 2. The Calvin–Benson cycle. Two reactions unique (or nearly so) to photosynthetic carbon metabolism link catabolic Figure 2. The Calvin–Benson cycle. Two reactions unique (or nearly so) to photosynthetic carbon metabolism link catabolic non-oxidative pentose phosphate reactions with gluconeogenic reactions to form the Calvin–Benson cycle. DHAP = non-oxidative pentose phosphate reactions with gluconeogenic reactions to form the Calvin–Benson cycle. DHAP = dihy- dihydroxyacetonedroxyacetone phosphate, phosphate, E4P E4P= erythrose = erythrose 4-phosphate, 4-phosphate, F6P F6P = =fructose fructose 6- 6-phosphate,phosphate, FBP FBP = = fructose fructose 1,6 1,6-bisphosphate,-bisphosphate, FBPase = FBP phosphatase, GAP = glyceraldehyde 3-phosphate,3-phosphate, PGA = 3-phosphoglycerate,3-phosphoglycerate, R5P = ribose 5 5-phosphate,-phosphate, Ru5P = ribulose 5-phosphate,5-phosphate, RuBP = ribulose 1,5-bisphosphate,1,5-bisphosphate, S7P == sedoheptulose 7-phosphate,7-phosphate, SBP == sedoheptulose 1,7-bisphosphate,1,7-bisphosphate, SBPaseSBPase == SBP phosphatase, ThDP = thiamine diphosphate diphosphate,, and Xu5P = xylulose 5 5-phosphate.-phosphate. 14 Using radioactive radioactive 14COCO2,2 it, itwas was quickly quickly determined determined that that PGA PGA is isthe the first first product product of COof CO2 fixation2 fixation by algae by algae and andplants plants [5]. There [5]. There remained remained two mysteries two mysteries about the about path the of path car- bonof carbon in photosynthesis in photosynthesis that was that blocking was blocking its discovery its discovery [3]. First [ 3was]. Firstthe nature was the of naturethe re- ductionof the reduction step and stepsecond and was second the nature was the of naturethe “two of-carbon the “two-carbon acceptor”, acceptor”,known to knownbe nec- essaryto be necessary in order for in order3-PGA for to 3-PGAbe the tofirst be product. the first Answers product. to Answers both mysteries to both were mysteries pro- posedwere proposed in a landmark in a landmark 1954 paper 1954 [4];paper the reduction [4]; the reductionstep was the step reversal was the of reversal the oxidation of the oxidation step in glycolysis and the “two-carbon acceptor” was the top two carbons of RuBP, which had three different sources related to the PPP. Ribose 5-phosphate (R5P) is made by successive aldol additions of carbon to GAP (Figure3). The carbon donor is DHAP. After the aldol addition of DHAP to GAP, two of the carbons of DHAP are transferred to Cells 2021, 10, 1547 4 of 18 step in glycolysis and the “two-carbon acceptor” was the top two carbons of RuBP, which Cells 2021, 10, 1547 had three different sources related to the PPP. Ribose 5-phosphate (R5P) is made by4 suc- of 17 cessive aldol additions of carbon to GAP (Figure 3). The carbon donor is DHAP. After the aldol addition of DHAP to GAP, two of the carbons of DHAP are transferred to thiamine diphosphate (ThDP) as a glycolaldehyde fragment, leaving one carbon added to GAP. The resultingthiamine diphosphateE4P