Recent Advances in Metabolic Engineering for the Biosynthesis of Phosphoenol Pyruvate-Oxaloacetate-Pyruvate-Derived Amino Acids

The phosphoenol pyruvate-oxaloacetate-pyruvate-derived amino acids (POP-AAs) comprise native intermediates in cellular metabolism, within which the phosphoenol pyruvate-oxaloacetate-pyruvate (POP) node is the switch point among the major metabolic pathways existing in most living organisms. POP-AAs have widespread applications in the nutrition, food, and pharmaceutical industries. These amino acids have been predominantly produced in Escherichia coli and Corynebacterium glutamicum through microbial fermentation. With the rapid increase in market requirements, along with the global food shortage situation, the industrial production capacity of these two bacteria has encountered two bottlenecks: low product conversion efficiency and high cost of raw materials. Aiming to push forward the update and upgrade of engineered strains with higher yield and productivity, this paper presents a comprehensive summarization of the fundamental strategy of metabolic engineering techniques around phosphoenol pyruvate-oxaloacetate-pyruvate node for POP-AA production, including L-tryptophan, L-tyrosine, L-phenylalanine, L-valine, L-lysine, L-threonine, and L-isoleucine. Novel heterologous routes and regulation methods regarding the carbon flux redistribution in the POP node and the formation of amino acids should be taken into consideration to improve POP-AA production to approach maximum theoretical values. Furthermore, an outlook for future strategies of low-cost feedstock and energy utilization for developing amino acid overproducers is proposed.

Elongation factor P is required for EIIGlc translation in Corynebacterium glutamicum due to an essential polyproline motif

Translating ribosomes require elongation factor P (EF-P) to incorporate consecutive prolines (XPPX) into nascent peptide chains. The proteome of Corynebacterium glutamicum ATCC 13032 contains a total of 1,468 XPPX motifs, many of which are found in proteins involved in primary and secondary metabolism. We show here that synthesis of EII^Glc , the glucose-specific permease of the phosphoenolpyruvate (PEP): sugar phosphotransferase system (PTS) encoded by ptsG, is strongly dependent on EF-P, as an efp deletion mutant grows poorly on glucose as sole carbon source. The amount of EII^Glc is strongly reduced in this mutant, which consequently results in a lower rate of glucose uptake. Strikingly, the XPPX motif is essential for the activity of EII^Glc , and substitution of the prolines leads to inactivation of the protein. Finally, translation of GntR2, a transcriptional activator of ptsG, is also dependent on EF-P. However, its reduced amount in the efp mutant can be compensated for by other regulators. These results reveal for the first time a translational bottleneck involving production of the major glucose transporter EII^Glc , which has implications for future strain engineering strategies.

The glucose uptake systems in Corynebacterium glutamicum: a review

The phosphoenolpyruvate-dependent glucose phosphotransferase system (PTS^Glc) is the major uptake system responsible for transporting glucose, and is involved in glucose translocation and phosphorylation in Corynebacterium glutamicum. For the longest time, the PTS^Glc was considered as the only uptake system for glucose. However, some PTS-independent glucose uptake systems (non-PTS^Glc) were discovered in recent years, such as the coupling system of inositol permeases and glucokinases (IPGS) and the coupling system of β-glucoside-PTS permease and glucokinases (GPGS). The products (e.g. lysine, phenylalanine and leucine) will be increased because of the increasing intracellular level of phosphoenolpyruvate (PEP), while some by-products (e.g. lactic acid, alanine and acetic acid) will be reduced when this system become the main uptake pathway for glucose. In this review, we survey the uptake systems for glucose in C. glutamicum and their composition. Furthermore, we summarize the latest research of the regulatory mechanisms among these glucose uptake systems. Detailed strategies to manipulate glucose uptake system are addressed based on this knowledge.

Metabolic engineering of glucose uptake systems in Corynebacterium glutamicum for improving the efficiency of l-lysine production

Traditional amino acid producers typically exhibit the low glucose uptake rate and growth deficiency, resulting in a long fermentation time because of the accumulation of side mutations in breeding of strains. In this study, we demonstrate that the efficiency of l-lysine production in traditional l-lysine producer Corynebacterium glutamicum ZL-9 can be improved by rationally engineering glucose uptake systems. To do this, different bypasses for glucose uptake were investigated to reveal the best glucose uptake system for l-lysine production in traditional l-lysine producer. This study showed that overexpression of the key genes in PTSGlc or non-PTSGlc increased the glucose consumption, growth rate, and l-lysine production. However, increasing the function of PTSGlc in glucose uptake led to the increase of by-products, especially for plasmid-mediated expression system. Increasing the participation of non-PTSGlc in glucose utilization showed the best glucose uptake system for l-lysine production. The final strain ZL-92 with increasing the expression level of iolT1, iolT2 and ppgK could produce 201.6 ± 13.8 g/L of l-lysine with a productivity of 5.04 g/L/h and carbon yield of 0.65 g/(g glucose) in fed-batch culture. This is the first report of a rational modification of glucose uptake systems that improve the efficiency of l-lysine production through increasing the participation of non-PTSGlc in glucose utilization in traditional l-lysine producer. Similar strategies can be also used for producing other amino acids or their derivatives.

Rational modification of tricarboxylic acid cycle for improving L-lysine production in Corynebacterium glutamicum

BACKGROUND

Oxaloacetate (OAA) and L-glutamate are essential precursors for the biosynthesis of L-lysine. Reasonable control of all potentially rate-limiting steps, including the precursors supply rate, is of vital importance to maximize the efficiency of L-lysine fermentation process.

RESULTS

In this paper, we have rationally engineered the tricarboxylic acid (TCA) cycle that increased the carbon yield (from 36.18 to 59.65%), final titer (from 14.47 ± 0.41 to 23.86 ± 2.16 g L-1) and productivity (from 0.30 to 0.50 g L-1 h-1) of L-lysine by Corynebacterium glutamicum in shake-flask fermentation because of improving the OAA and L-glutamate availability. To do this, the phosphoenolpyruvate-pyruvate-oxaloacetate (PEP-pyruvate-OAA) node's genes ppc and pyc were inserted in the genes pck and odx loci, the P1 promoter of the TCA cycle's gene gltA was deleted, and the nature promoter of glutamate dehydrogenase-coding gene gdh was replaced by Ptac-M promoter that resulted in the final engineered strain C. glutamicum JL-69Ptac-M gdh. Furthermore, the suitable addition of biotin accelerates the L-lysine production in strain JL-69Ptac-M gdh because it elastically adjusts the carbon flux for cell growth and precursor supply. The final strain JL-69Ptac-M gdh could produce 181.5 ± 11.74 g L-1 of L-lysine with a productivity of 3.78 g L-1 h-1 and maximal specific production rate (qLys, max.) of 0.73 ± 0.16 g g-1 h-1 in fed-batch culture during adding 2.4 mg L-1 biotin with four times.

CONCLUSIONS

Our results reveal that sufficient biomass, OAA and L-glutamate are equally important in the development of L-lysine high-yielding strain, and it is the first time to verify that fed-batch biotin plays a positive role in improving L-lysine production.