The oxidative pentose phosphate pathway: structure and organisation Nicholas J Kruger  and Antje von Schaewen y The oxidative pentose phosphate pathway is a major source of reducing power and metabolic intermediates for biosynthetic processes. Some, if not all, of the enzymes of the pathway are found in both the cytosol and plastids, although the precise distribution of their activities varies. The apparent absence of sections of the pathway from the cytosol potentially complicates metabolism. These complications are partly offset, however, by exchange of intermediates between the cytosol and the plastids through the activities of a family of plastid phosphate translocators. Molecular analysis is confirming the widespread presence of multiple genes encoding each of the enzymes of the oxidative pentose phosphate pathway. Differential expression of these isozymes may ensure that the kinetic properties of the activity that catalyses a specific reaction match the metabolic requirements of a particular tissue. This hypothesis can be tested thanks to recent developments in the application of 13 C-steady-state labelling strategies. These strategies make it possible to quantify flux through metabolic networks and to discriminate between pathways of carbohydrate oxidation in the cytosol and plastids. Addresses  Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK e-mail: nick.kruger@plants.ox.ac.uk y Institut fu ¨ r Botanik, Westfa ¨ lische Wilhelms-Universita ¨ t Mu ¨ nster, Schlossgarten 3, 48149 Mu ¨ nster, Germany e-mail: schaewen@uni-muenster.de Correspondence: Nicholas J Kruger Current Opinion in Plant Biology 2003, 6:236–246 This review comes from a themed issue on Physiology and metabolism Edited by Alison Smith and Mary Lou Guerinot 1369-5266/03/$ – see front matter ß 2003 Elsevier Science Ltd. All rights reserved. DOI 10.1016/S1369-5266(03)00039-6 Abbreviations G6PDH glucose 6-phosphate dehydrogenase GC gas chromatography MS mass spectroscopy NMR nuclear magnetic resonance spectroscopy oxPPP oxidative pentose phosphate pathway 6PGDH 6-phosphogluconate dehydrogenase TA transaldolase TK transketolase XPT xylulose phosphate/phosphate translocator Introduction Recent years have witnessed a resurgence of interest in the oxidative pentose phosphate pathway (oxPPP), which has been brought about by an increasing appreciation of the central role of this pathway in metabolism. In non- photosynthetic cells, the oxPPP is a major source of reductant (i.e. NADPH) for biosynthetic processes such as fatty-acid synthesis and the assimilation of inorganic nitrogen [1], and maintains the redox potential necessary to protect against oxidative stress [2]. The reversible non- oxidative section of the pathway is also the source of carbon skeletons for the synthesis of nucleotides, aro- matic amino acids, phenylpropanoids and their deriva- tives [3]. Although the basic features of the oxPPP are well-established [4], details of how the pathway operates in plants and how it influences other processes remain largely conjecture. This article focuses on the impact of recent studies on our understanding of the structure and organisation of the oxPPP in higher plants. Structure of the oxidative pentose phosphate pathway The oxPPP is commonly considered to operate as depicted in Figure 1a. Varying proportions of fructose 6-phosphate, and even triose phosphate, are potentially recycled through the pathway following their conversion to glucose 6-phosphate by glucose-6-phosphate isomerase [5,6]. However, as recently re-emphasised, both transketolase (TK) and transaldolase (TA) possess broad substrate spe- cificities [7  ]. This has led to alternative schemes for the non-oxidative section of the pathway that involve addi- tional metabolites such as octulose 8-phosphate [8]. Sup- port for such schemes is based on the identification of proposed novel intermediates and the apparent failure of the conventional pathway to account for the observed labelling pattern in pathway products. However, both of these lines of evidence are open to alternative explanations and the proposed alternative schemes remain strongly contested [9]. Nevertheless, even if we consider only recognised pathway intermediates containing up to seven carbon atoms, it is possible to draw a different but equally valid pathway, as shown in Figure 1b. This raises the question of whether it is appropriate to regard the oxPPP as a rigid formal sequence. The two schemes depicted in Figure 1 result in the same distribution of carbon atoms within the pathway intermediates, and so there is no definitive evidence favouring one scheme over the other. Indeed, in the absence of any compelling evidence to suggest that the contributing enzymes are organised in a way that limits the free diffusion of intermediates between them, it is difficult to see how they could operate as a fixed sequence. This view is reinforced by the realisa- tion that varying amounts of ribose 5-phosphate and ery- throse 4-phosphate are withdrawn from the pathway for 236 Current Opinion in Plant Biology 2003, 6:236–246 www.current-opinion.com