Plant-derived phenolic gallic acid (GA) is an important raw material for antioxidants and food additives. Efforts to ferment GA using microbial processes have aimed at minimizing production costs and environmental load using enzymes that hydroxylate p-hydroxybenzoate and protocatechuate (PCA). Here, we found a p-hydroxybenzoate hydroxylase (PobA) in the bacterium Hylemonella gracilis NS1 (HgPobA) with 1.5-fold more hydroxylation activity than that from Pseudomonas aeruginosa PAO1 and thus converted PCA to GA more efficiently. The PCA hydroxylation activity of HgPobA was improved by introducing the amino acid substitutions L207V/Y393F or T302A/Y393F. These mutants had 2.9- and 3.7-fold lower Kmapp for PCA than wild-type HgPobA. An Escherichia coli strain that reinforces shikimate pathway metabolism and produces HgPobA when cultured for 60 h generated 0.27 g L-1 of GA. This is the first report of fermenting glucose to generate GA using a natural enzyme from the PobA family. The E. coli strain harboring the HgPobA L207V/Y393F mutant increased GA production to 0.56 g L-1. During the early stages of culture, GA was fermented at a 10-fold higher rate by a strain producing either HgPobA L207V/Y393F or T302A/Y393F compared with wild-type HgPobA, which agreed with the high kcatapp/Kmapp PCA values of this mutant. We enhanced a PobA isozyme and its PCA hydroxylating function to efficiently and cost-effectively ferment GA.
Aromatic amines are base materials for generating super-engineering plastics such as polyamides and polyimides. Recombinant Escherichia coli ferments 4-aminocinnamic acid (4ACA) from glucose, and it can be derived to plastics of biomass origin with extreme thermal properties. Here, we scaled-up 4ACA production by optimizing microbial fermentation processes. The initial fermentation of 4-aminophenylalanine (4APhe) using E. coli generated the papABC genes of Pseudomonas fluorescens that produced 4APhe with a volumetric mass transfer coefficient (kLa) of 70 h−1 in 115 L of culture broth, and 334 g of 4APhe at a final concentration of 2.9 g 4APhe L−1. Crude 4APhe prepared from the fermentation broth was bioconverted to 4ACA by an E. coli strain producing phenylalanine ammonia lyase of the yeast Rhodotorula glutinis. The E. coli cells cultured under optimized conditions converted 4APhe to 4ACA at a rate of 0.65 g L−1 4ACA OD600−1. These processes resulted in the final derivation of 4.1 g L−1 of 4ACA from glucose. The 4ACA was purified from the reaction as a hydrochloric acid salt and photodimerized to 4,4'-diaminotruxillic acid, which was polycondensed to produce bioaromatic polyimides. Large-scale 4ACA production will facilitate investigations of the physicochemical properties of biomass-derived aromatic polymers of 4ACA origin.
The aromatic diamine 2-(4-aminophenyl)ethylamine (4APEA) is a potential monomer for polymers and advanced materials. Here, 4APEA was produced by fermentation using genetically engineered Escherichia coli (Masuo et al.2016). Optimizing fed-batch cultures of this strain produced the highest reported yield to date of 4APEA (7.2%; 3.5 g/L versus glucose) within 72 h. Appropriate aeration was important to maximize production and avoid unfavorable 4APEA degradation. Fermented 4APEA was purified from culture medium and polymerized with methylene diphenyldiisocyanate and hexamethylene diisocyanate to produce polyureas PU-1 and PU-2, respectively. The decomposition temperatures for 10% weight loss (Td10) of PU-1 and PU-2 were 276 °C and 302 °C, respectively, and were comparable with that of other thermostable aromatic polyureas. This study is the first to synthesize polyureas from the microbial aromatic diamine. Their excellent thermostability will be useful for the industrial production of heat-resistant polymer materials.
Pyrazines are widespread chemical compounds that include pheromones and odors. Herein, a novel mechanism used by Pseudomonas fluorescens SBW25 to biosynthesize monocyclic pyrazines is reported. Heterologous expression of the papABC genes that synthesize the natural α-amino acid 4-aminophenylalanine (4APhe), together with three adjacent papDEF genes of unknown function, in Escherichia coli resulted in the production of 2,5-dimethyl-3,6-bis(4-aminobenzyl)pyrazine (DMBAP), which comprised two symmetrical aminobenzyl moieties derived from 4APhe. It is found that PapD is a novel amino acid C-acetyltransferase, which decarboxylates and transfers acetyl residues to 4APhe, to generate an α-aminoketone, which spontaneously dehydrates and condenses to give dihydro DMBAP. PapF is a novel oxidase in the amine oxidase superfamily that oxidizes dihydro DMBAP to yield the pyrazine ring of DMBAP. These two enzymes constitute a unique mechanism for synthesizing monocyclic pyrazines and might serve as a novel strategy for the enzymatic synthesis of pyrazine derivatives from natural α-amino acids.
Abstract The uncontrolled oxidative decomposition of electrolyte while operating at high potential (> 4.2 V vs Li/Li + ) severely affects the performance of high-energy density transition metal oxide-based materials as cathodes in Li-ion batteries. To restrict this degradative response of electrolyte species, the need for functional molecules as electrolyte additives that can restrict the electrolytic decomposition is imminent. In this regard, bio-derived molecules are cost-effective, environment friendly, and non-toxic alternatives to their synthetic counter parts. Here, we report the application of microbially synthesized 2,5-dimethyl-3,6-bis(4-aminobenzyl)pyrazine (DMBAP) as an electrolyte additive that stabilizes high-voltage (4.5 V vs Li/Li + ) LiNi 1/3 Mn 1/3 Co 1/3 O 2 cathodes. The high-lying highest occupied molecular orbital of bio-additive (DMBAP) inspires its sacrificial in situ oxidative decomposition to form an organic passivation layer on the cathode surface. This restricts the excessive electrolyte decomposition to form a tailored cathode electrolyte interface to administer cyclic stability and enhance the capacity retention of the cathode.
