Stereoselective syntheses of L-pipecolic acid and (2S,3S)-3-hydroxypipecolic acid from a chiral N-imino-2-phenyl-1,2-dihydropyridine intermediate

Systems metabolic engineering upgrades Corynebacterium glutamicum for selective high-level production of the chiral drug precursor and cell-protective extremolyte L-pipecolic acid

The nonproteinogenic cyclic metabolite l-pipecolic acid is a chiral precursor for the synthesis of various commercial drugs and functions as a cell-protective extremolyte and mediator of defense in plants, enabling high-value applications in the pharmaceutical, medical, cosmetic, and agrochemical markets. To date, the production of the compound is unfavorably fossil-based. Here, we upgraded the strain Corynebacterium glutamicum for l-pipecolic acid production using systems metabolic engineering. Heterologous expression of the l-lysine 6-dehydrogenase pathway, apparently the best route to be used in the microbe, yielded a family of strains that enabled successful de novo synthesis from glucose but approached a limit of performance at a yield of 0.18 mol mol^-1. Detailed analysis of the producers at the transcriptome, proteome, and metabolome levels revealed that the requirements of the introduced route were largely incompatible with the cellular environment, which could not be overcome after several further rounds of metabolic engineering. Based on the gained knowledge, we based the strain design on l-l-lysine 6-aminotransferase instead, which enabled a substantially higher in vivo flux toward l-pipecolic acid. The tailormade producer C. glutamicum PIA-7 formed l-pipecolic acid up to a yield of 562 mmol mol^-1, representing 75% of the theoretical maximum. Ultimately, the advanced mutant PIA-10B achieved a titer of 93 g L^-1 in a fed-batch process on glucose, outperforming all previous efforts to synthesize this valuable molecule de novo and even approaching the level of biotransformation from l-lysine. Notably, the use of C. glutamicum allows the safe production of GRAS-designated l-pipecolic acid, providing extra benefit toward addressing the high-value pharmaceutical, medical, and cosmetic markets. In summary, our development sets a milestone toward the commercialization of biobased l-pipecolic acid.

Expanding the lysine industry: biotechnological production of l-lysine and its derivatives

l-lysine is an essential amino acid that contains various functional groups including α-amino, ω-amino, and α-carboxyl groups, exhibiting high reaction potential. The derivatization of these functional groups produces a series of value-added chemicals, such as cadaverine, glutarate, and d-lysine, that are widely applied in the chemical synthesis, cosmetics, food, and pharmaceutical industries. Here, we review recent advances in the biotechnological production of l-lysine and its derivatives and expatiate key technological strategies. Furthermore, we also discuss the existing challenges and potential strategies for more efficient production of these chemicals.

Structural Basis for Recognition of L-lysine, L-ornithine, and L-2,4-diamino Butyric Acid by Lysine Cyclodeaminase

L-pipecolic acid is a non-protein amino acid commonly found in plants, animals, and microorganisms. It is a well-known precursor to numerous microbial secondary metabolites and pharmaceuticals, including anticancer agents, immunosuppressants, and several antibiotics. Lysine cyclodeaminase (LCD) catalyzes β-deamination of L-lysine into L-pipecolic acid using β-nicotinamide adenine dinucleotide as a cofactor. Expression of a human homolog of LCD, μ-crystallin, is elevated in prostate cancer patients. To understand the structural features and catalytic mechanisms of LCD, we determined the crystal structures of Streptomyces pristinaespiralis LCD (SpLCD) in (i) a binary complex with NAD+, (ii) a ternary complex with NAD+ and L-pipecolic acid, (iii) a ternary complex with NAD+ and L-proline, and (iv) a ternary complex with NAD+ and L-2,4-diamino butyric acid. The overall structure of SpLCD was similar to that of ornithine cyclodeaminase from Pseudomonas putida. In addition, SpLCD recognized L-lysine, L-ornithine, and L-2,4-diamino butyric acid despite differences in the active site, including differences in hydrogen bonding by Asp236, which corresponds with Asp228 from Pseudomonas putida ornithine cyclodeaminase. The substrate binding pocket of SpLCD allowed substrates smaller than lysine to bind, thus enabling binding to ornithine and L-2,4-diamino butyric acid. Our structural and biochemical data facilitate a detailed understanding of substrate and product recognition, thus providing evidence for a reaction mechanism for SpLCD. The proposed mechanism is unusual in that NAD+ is initially converted into NADH and then reverted back into NAD+ at a late stage of the reaction.

Expanding metabolic pathway for de novo biosynthesis of the chiral pharmaceutical intermediate L-pipecolic acid in Escherichia coli

BACKGROUND

The six-carbon circular non-proteinogenic compound L-pipecolic acid is an important chiral drug intermediate with many applications in the pharmaceutical industry. In the present study, we developed a metabolically engineered strain of Escherichia coli for the overproduction of L-pipecolic acid from glucose.

RESULTS

The metabolic pathway from L-lysine to L-pipecolic acid was constructed initially by introducing lysine cyclodeaminase (LCD). Next, L-lysine metabolic flux from glucose was amplified by the plasmid-based overexpression of dapA, lysC, and lysA under the control of the strong trc promoter to increase the biosynthetic pool of the precursor L-lysine. Additionally, since the catalytic efficiency of the key enzyme LCD is limited by the cofactor NAD⁺, the intracellular pyridine nucleotide concentration was rebalanced by expressing the pntAB gene encoding the transhydrogenase, which elevated the proportion of LCD with bound NAD⁺ and enhanced L-pipecolic acid production significantly. Further, optimization of Fe²⁺ and surfactant in the fermentation process resulted in 5.33 g/L L-pipecolic acid, with a yield of 0.13 g/g of glucose via fed-batch cultivation.

CONCLUSIONS

We expanded the metabolic pathway for the synthesis of the chiral pharmaceutical intermediate L-pipecolic acid in E. coli. Using the engineered E. coli, a fast and efficient fermentative production of L-pipecolic acid was achieved. This strategy could be applied to the biosynthesis of other commercially and industrially important chiral compounds containing piperidine rings.

Synthesis, utility and medicinal importance of 1,2- & 1,4-dihydropyridines

The present review aims to describe various methodologies that have been used for the synthesis of 1,2- & 1,4-dihydropyridines (DHPs) and also highlight their medicinal significance.

Directed functionalization of 1,2-dihydropyridines: stereoselective synthesis of 2,6-disubstituted piperidines

A practical and highly stereoselective approach to access 2,6-disubstituted piperidines using an amidine auxiliary is reported. These were reduced to the saturated piperidine rings with high diastereoselectivity.

Synthesis of (+)-L-733,060, (+)-CP-99,994 and (2S,3R)-3-hydroxypipecolic acid: application of an organocatalytic direct vinylogous aldol reaction

The γ-butenolide obtained from an organocatalyzed, direct vinylogous aldol reaction of γ-crotonolactone and benzaldehyde serves as the key starting material in the expedient synthesis of a 3-hydroxy-2-phenyl piperidine intermediate which is converted to the target 2,3-disubstituted piperidines.

Copper-catalyzed Perkin-acyl-Mannich reaction of acetic anhydride with pyridine: expeditious entry to unconventional piperidines

A regioselective introduction of a methoxycarbonyl methyl group at the C(2) position of unsubstituted pyridine has been accomplished with catalytic amounts of copper(II) triflate in mild reaction conditions. The N-acetyl-1,2-dihydropyridyl acetic acid methyl ester obtained is a valuable building block for the synthesis of new polyfunctionalized piperidine derivatives bearing unconventional substitution patterns.