Fig. 1
From: Protein-based biorefining driven by nitrogen-responsive transcriptional machinery
Scheme of the protein-to-fuel conversion driven by the nitrogen-responsive transcriptional machinery. a The protein-to-fuel biosynthetic pathways. A total of seven genes are overexpressed for the biosynthesis of higher alcohols from protein biomass. Acetolactate synthase, ketol–acid reductoisomerase, and dihydroxy acid dehydratase are encoded by alsS (UniProt: Q04789), ilvC (UniProt: P05793), and ilvD (UniProt: P05791), respectively, together, these enzymes convert pyruvate to 2-ketoisovalerate (KIV) and 2-keto-3-methyl-valerate (KMV), which are the direct precursors of valine and isoleucine, respectively. A substantial proportion of the produced KIV, KMV, and the leucine precursor 2-ketoisocaproate (KIC) are forced to undergo decarboxylation catalyzed by the 2-ketoisovalerate decarboxylase (encoded by kivd (UniProt: Q684J7)) and are then reduced to the corresponding higher alcohols by the alcohol dehydrogenase (encoded by yqhD (UniProt: Q46856)), producing isobutanol, 2-methyl-1-butanol, and 3-methyl-1-butanol, respectively. The leuDH (UniProt: Q60030), which encodes the leucine dehydrogenase and the avtA (UniProt: P09053), encoding the valine–pyruvate aminotransferase, are also overexpressed to drive the amino acid flux into fuel production. With the continuous consumption of the keto acids by the decarboxylase, the reversible reactions catalyzed by LeuDH and AvtA would proceed toward the release of carbon skeletons from amino acids. Other amino acids could be directly transformed into pyruvate through transamination and deamination, or indirectly through the tricarboxylic acid (TCA) cycle, and finally being channeled into biofuel synthesis. b Transcription of the σ54-dependent promoters mediated by the nitrogen regulatory proteins. Under nitrogen starvation, the phosphorylated NtrB (NtrB-P) transfers its phosphoryl group to NtrC, which interacts with Eσ54 and activates the transcription. c Assumed performance of the σ54-mediated biofuel production throughout the whole growth phase and under stress conditions. Rsd regulator of σD, (p)ppGpp guanosine pentaphosphate or tetraphosphate