The pentose phosphate pathway in industrially relevant fungi: crucial insights for bioprocessing

Abstract

The pentose phosphate pathway (PPP) is one of the most targeted pathways in metabolic engineering. This pathway is the primary source of NADPH, and it contributes in fungi to the production of many compounds of interest such as polyols, biofuels, carotenoids, or antibiotics. However, the regulatory mechanisms of the PPP are still not fully known. This review provides an insight into the current comprehension of the PPP in fungi and the limitations of this current understanding. It highlights how this knowledge contributes to targeted engineering of the PPP and thus to better performance of industrially used fungal strains. KEY POINTS: • Type of carbon and nitrogen source as well as oxidative stress influence the PPP. • A complex network of transcription factors regulates the PPP. • Improved understanding of the PPP will allow to increase yields of bioprocesses.

Keywords: Filamentous fungi; Industrially relevant fungi; Pentose phosphate pathway; Yeast.

Fig. 1

Schematic drawing of the pentose phosphate pathway, its enzymes, and its connection to glycolysis. Single arrows indicate a non-reversible reaction, and double arrows indicate a reversible reaction. The oxidative pentose phosphate pathway and the non-oxidative pentose phosphate pathway are identified by purple or yellow background, respectively. Red arrows indicate links to other biochemical pathways. Abbreviations of glycolysis enzymes: HK, hexokinase; GPI, glucose-6-phosphate isomerase; PFK, phosphofructokinase 1; FBA, fructose-bisphosphate aldolase; TPI, triosephosphate isomerase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; PK, pyruvate kinase (Stincone et al. 2015)

Fig. 2

Schematic drawings of the proposed nitrogen regulation in Aspergillus and Saccharomyces. In Aspergillus nitrate is reduced by consecutive action of two enzymes. Nitrate is reduced to nitrite by the nitrate reductase NiaD, and then nitrite is reduced to ammonium by the nitrite reductase NiiA, also called Nir. Ammonium is further metabolized to glutamine and glutamate. The nir product, which is currently not characterized, presumably activates the PPP enzymes through an unknown mechanism. Glutamine and ammonium repress the transcription of s niaD, of niiA, and of the nitrate permease-encoding gene. The transcription factors NirA and AreA as well as nitrate activate the transcription of the genes involved in nitrate metabolism (Soderberg ; Oost and van der Siebers 2007). In Saccharomyces, glutamine is produced from different sources, ammonium is one of them. When present, glutamine inhibits the TOR protein, which itself represses Rtg1, Rtg3, and Gln3 expression. When inhibited by TOR, Gln3 cannot translocate in the nucleus and act as a transcription activator of the nitrogen-regulated genes. Rtg1 and Rtg3 have been shown to have an impact on activity of the PPP enzymes through an unknown mechanism (Soderberg ; Oost and van der Siebers 2007). Consequently, in the presence of glutamine, TOR is not expressed resulting in active Rtg1 and Rtg3 as well as Gln3 and the latter to activation of nitrogen metabolism. A hypothesis to explain the effect of nitrate on the PPP enzymes in Aspergillus could be a mechanism similar to what has been observed in Saccharomyces. It can be hypothesized that the nir product is glutamine, and it impacts the PPP through a pathway involving TOR, Rtg1, Rtg3, and Gln3