NADPH-generating systems in bacteria and archaea

Abstract

Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is an essential electron donor in all organisms. It provides the reducing power that drives numerous anabolic reactions, including those responsible for the biosynthesis of all major cell components and many products in biotechnology. The efficient synthesis of many of these products, however, is limited by the rate of NADPH regeneration. Hence, a thorough understanding of the reactions involved in the generation of NADPH is required to increase its turnover through rational strain improvement. Traditionally, the main engineering targets for increasing NADPH availability have included the dehydrogenase reactions of the oxidative pentose phosphate pathway and the isocitrate dehydrogenase step of the tricarboxylic acid (TCA) cycle. However, the importance of alternative NADPH-generating reactions has recently become evident. In the current review, the major canonical and non-canonical reactions involved in the production and regeneration of NADPH in prokaryotes are described, and their key enzymes are discussed. In addition, an overview of how different enzymes have been applied to increase NADPH availability and thereby enhance productivity is provided.

Keywords: GAPN; NADPH regeneration; ferredoxin:NADP+ oxidoreductase; hydrogenase; isocitrate dehydrogenase; malic enzyme; pentose phosphate pathway; transhydrogenase.

Figure 1

NAD⁺ and NADP⁺ biosynthetic pathways in prokaryotes. (A) De novo and salvage pathways for NAD⁺ biosynthesis. (B) NADP⁺ biosynthesis by NAD kinase. Abbreviations: NaMN, nicotinic acid mononucleotide; NaAD, nicotinic acid adenine dinucleotide; NR, nicotinamide riboside; NMN, nicotinamide mononucleotide; NAD, nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate; NADK, NAD kinase.

Figure 2

NADPH generation by glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). Solid and dashed lines indicate single and lumped reactions, respectively. Pathway abbreviations: ED, Entner–Doudoroff pathway (in green); EMP, Embden–Meyerhof–Parnas pathway (in blue); PPP, pentose phosphate pathway (oxidative phase in red, non-oxidative phase in yellow).

Figure 3

General overview of the TCA cycle and anaplerotic node. Solid and dashed arrows represent single and lumped enzymatic reactions, respectively. Malic enzyme is able to catalyze the decarboxylation of malate and OAA. Abbreviations for metabolites (normal text) and enzymes (bold text): PEP, phosphoenolpyruvate; OAA, oxaloacetate; PEPCK, phosphoenolpyruvate carboxykinase; PEPCx, phosphoenolpyruvate carboxylase; PCx, pyruvate carboxylase; ODx, oxaloacetate decarboxylase; MDH, malate dehydrogenase; ME, malic enzyme; ICL, isocitrate lyase; IDH, isocitrate dehydrogenase.

Figure 4

NADPH generation by GAP dehydrogenases. Solid and dashed arrows represent single and lumped enzymatic reactions, respectively. Abbreviations for metabolites (normal text) and enzymes (bold text): GAP, glyceraldehyde 3-phosphate; 1,3-BPG, 1,3-bisphosphoglycerate; 3-PG, 3-phosphoglycerate; GAPOR, GAP:ferredoxin oxidoreductase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GAPN, non-phosphorylating GAPDH; PGK, phosphoglycerate kinase.

Figure 5

Classical ED pathway compared to modified ED pathways. Solid and dashed arrows represent single and lumped enzymatic reactions, respectively. Shown are the classical ED pathway (in black), the semiphosphorylated ED pathway (in blue), the non-phosphorylated ED pathway (in red), and the branched ED pathway that includes the semi- and non-phosphorylated ED pathways. Only the glucose degradation directions are shown. Abbreviated enzymes (in bold text): G6PDH, glucose-6-phosphate dehydrogenase; GAPN, non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase; GDH, glucose dehydrogenase.

Figure 6

NADPH-generating reactions not coupled to carbon metabolism. Abbreviated enzymes (in bold): H₂-H, hydrogen-evolving hydrogenase; SH, cytosolic NADP⁺-reducing hydrogenase; NfnAB, electron-bifurcating NADH-dependent reduced ferredoxin:NADP⁺ oxidoreductase; FNR, ferredoxin:NADP⁺ oxidoreductase; STH, energy-independent soluble transhydrogenase; H⁺-TH, energy-dependent or proton-translocating, membrane-bound transhydrogenase.