Abstract
Leaf cytosol contains non-phosphorylating and phosphorylating glyceraldehyde 3-phosphate dehydrogenase (np-GAPDH and p-GAPDH, respectively). From the viewpoint of carbon metabolism, np-GAPDH is redundant. However, mutants lacking np-GAPDH show significant metabolic adjustments and decreased growth suggesting that np-GAPDH has central functions in plant metabolism. Here, a cytosolic oxidation-reduction (COR) cycle is proposed. In its forward direction, np-GAPDH supplies NADPH. In its backward direction, phosphoglycerate kinase and p-GAPDH consume ATP and NADH. Thus, COR cycling may constitute a central hub in energy metabolism.
Highlights
In the light, NADPH is primarily synthesized in chloroplasts
Carbon cycling around non-phosphorylating-glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was suggested to provide cytosolic NADPH (Fig. 1; Kelly and Gibbs, 1973a, b; Scagliarini et al, 1990)
In this cycle, (i) dihydroxyacetone phosphate is exported from chloroplasts to the cytosol by the triose phosphate translocator and converted to glyceraldehyde 3-phosphate (GAP) by triose phosphate isomerase, (ii) GAP is oxidized to 3-phosphoglyceric acid (3PGA) and NADP+ is reduced to NADPH by np-GAPDH, and (iii) 3PGA is reimported into chloroplasts by the triose phosphate translocator and reduced to dihydroxyacetone phosphate by chloroplastic phosphoglycerate kinase (PGK) and phosphorylating (p)-GAPDH
Summary
GAP to 3PGA, p-GAPDH together with PGK catalyses the same reaction in a reversible manner (Fig. 1). I argue that growth delays and substantial biochemical compensation effects in np-GAPDH null mutants (Rius et al, 2006) suggest a significant contribution of np-GAPDH to catalysing GAP to 3PGA conversions. Decreasing NADPH due to oxidative stress at low Cc causes np-GAPDH activation and promotes the GAP to 3PGA forward reaction of COR cycling. Rising H2O2 levels are believed to progressively inhibit the reversible p-GAPDH (Hancock et al, 2005; Bedhomme et al, 2012; Piattoni et al, 2013) This is corroborated by reported increases of glycolytic downstream metabolites including 3PGA under oxidative conditions (Baxter et al, 2007; Lehmann et al, 2009, 2012; Rabara et al, 2017). For more information on GAPDH regulation including moonlighting functions see Scheibe et al (2019) and references therein
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