Figure 3.

Representative scheme of the d-mannose/l-galactose pathway of ascorbic acid synthesis and its connections with the mETC. The yellow boxes represent the enzymes involved in the 10 steps of the pathway. The black box has the GDP-l-galactose phosphorylase, the enzyme that catalyzes the limiting step for AsA synthesis. The substrates of the enzymatic reactions are described between the boxes. The dotted black arrows represent the path of electrons in the mETC. In green are the respiratory complexes indicated by Roman numerals corresponding to the I-IV Complexes. The shaded green is indicative of respiratory protein components not directly engaged with electron flux during the synthesis of ascorbic acid in light. In purple are the alternative NAD(P)H dehydrogenases, highlighting the external NAD(P)H dehydrogenase as the location of the potential regeneration of NAD⁺. The alternative oxidase is in red and Cytc is in pink. The ubiquinone pool is represented in magenta. NAD⁺ and NADH are represented by spiked outlines. The substrate l-galactone-1,4-lactone (l-GalL) (in light orange) is oxidized by l-GalLDH using Cytc as the electron acceptor to produce ascorbate (Asc) (in dark orange). GPI: Glucose-6-phosphate isomerase. MPI: Mannose-6-phosphate isomerase. PMM: Phosphomannose mutase. GMPP, GDP-mannose pyrophosphorylase; GME, GDP-mannose-3’-5’-epimerase; GGP, GDP-l-galactose phosphorylase; GPP, l-galactose-1-phosphate phosphatase; l-GalDH, l-galactose dehydrogenase; l-GalLDH, l-galactono-1,4-lactone dehydrogenase; l-GalL, l-Galactono-1,4-lactone; Asc, Ascorbate; NAD⁺, Nicotinamide adenine dinucleotide oxidized; NADH, Nicotinamide adenine dinucleotide reduced; Cyt c^ox, Cytochrome c in oxidated state; Cyt c^red, Reduced cytochrome c; AOX, Alternative oxidase; UQ, ubiquinone; I, NADH-ubiquinone oxidoreductase; II, succinate-ubiquinone oxidoreductase; III, ubiquinol-cytochrome c oxidoreductase; IV, cytochrome c oxidase; H⁺, proton; e^-, electron.