In vivo and in vitro characterization of hydrophilic protein tag-fused Ralstonia eutropha polyhydroxyalkanoate synthase

Characterization and Homology Modeling of Catalytically Active Recombinant PhaCAp Protein from Arthrospira platensis

Polyhydroxybutyrate (PHB) is a biocompatible and biodegradable polymer that has the potential to replace fossil-derived polymers. The enzymes involved in the biosynthesis of PHB are β-ketothiolase (PhaA), acetoacetyl-CoA reductase (PhaB), and PHA synthase (PhaC). PhaC in Arthrospira platensis is the key enzyme for PHB production. In this study, the recombinant E. cloni^®10G cells harboring A. platensis phaC (rPhaC_Ap) was constructed. The overexpressed and purified rPhaC_Ap with a predicted molecular mass of 69 kDa exhibited V_max, K_m, and k_cat values of 24.5 ± 2 μmol/min/mg, 31.3 ± 2 µM and 412.7 ± 2 1/s, respectively. The catalytically active rPhaC_Ap was a homodimer. The three-dimensional structural model for the asymmetric PhaC_Ap homodimer was constructed based on Chromobacterium sp. USM2 PhaC (PhaC_Cs). The obtained model of PhaC_Ap revealed that the overall fold of one monomer was in the closed, catalytically inactive conformation whereas the other monomer was in the catalytically active, open conformation. In the active conformation, the catalytic triad residues (Cys151-Asp310-His339) were involved in the binding of substrate 3HB-CoA and the CAP domain of PhaC_Ap involved in the dimerization.

Enhancing heterologous expression of a key enzyme for the biosynthesis of 2'-fucosyllactose

BACKGROUND

2'-Fucosyllactose (2'-FL) is the most abundant human milk oligosaccharide (HMO) in human milk and has important physiological functions. The market demand of 2'-FL is continuing to grow, but high production cost has limited its availability. To solve the dilemma, biosynthesis of 2'-FL has been proposed and is considered the most promising pathway for massive production. α-1,2-Fucosyltransferase is one of the key elements involved in its biosynthesis, but the limited intracellular accumulation and unstable properties of α-1,2-fucosyltransferases when expressed in host strains have become a major hurdle for the effective biosynthesis of 2'-FL.

RESULTS

A combinatorial engineering strategy of synergic modification of ribosome binding site, fusion peptide and enzyme gene was leveraged to enhance the soluble expression of α-1,2-fucosyltransferases and promote enzyme activity. The preferable combination was to employ an optimized ribosome binding site region to drive 3 × FLAG as a fusion partner along with the α-1,2-fucosyltransferase for expression in Escherichia coli (DE3) PlySs, and protein yield and enzyme activity were remarkably improved by 11.51-fold and 13.72-fold, respectively.

CONCLUSION

After finely tuning the synergy among different elements, the abundant protein yield and high enzyme activity confirmed that the drawbacks of heterologous expression in α-1,2-fucosyltransferase had been properly addressed. A suitable external environment further drives the efficient synthesis of α-1,2-fucosyltransferases. To our knowledge, this is the first report of a systematic and effective modification of α-1,2-fucosyltransferase expression, which could potentially serve as a guideline for industrial application. © 2022 Society of Chemical Industry.

Strategies for Poly(3-hydroxybutyrate) Production Using a Cold-Shock Promoter in Escherichia coli

The present study attempted to increase poly(3-hydroxybutyrate) (PHB) production by improving expression of PHB biosynthesis operon derived from Cupriavidus necator strain A-04 using various types of promoters. The intact PHB biosynthesis operon of C. necator A-04, an alkaline tolerant strain isolated in Thailand with a high degree of 16S rRNA sequence similarity with C. necator H16, was subcloned into pGEX-6P-1, pColdI, pColdTF, pBAD/Thio-TOPO, and pUC19 (native promoter) and transformed into Escherichia coli JM109. While the phaC_A–04 gene was insoluble in most expression systems tested, it became soluble when it was expressed as a fusion protein with trigger factor (TF), a ribosome associated bacterial chaperone, under the control of a cold shock promoter. Careful optimization indicates that the cold-shock cspA promoter enhanced phaC_A–04 protein expression and the chaperone function of TF play critical roles in increasing soluble phaC_A–04 protein. Induction strategies and parameters in flask experiments were optimized to obtain high expression of soluble PhaC_A–04 protein with high Y_P/S and PHB productivity. Soluble phaC_A–04 was purified through immobilized metal affinity chromatography (IMAC). The results demonstrated that the soluble phaC_A–04 from pColdTF-phaCAB_A–04 was expressed at a level of as high as 47.4 ± 2.4% of total protein and pColdTF-phaCAB_A–04 enhanced soluble protein formation to approximately 3.09−4.1 times higher than that from pColdI-phaCAB_A–04 by both conventional method and short induction method developed in this study. Cultivation in a 5-L fermenter led to PHB production of 89.8 ± 2.3% PHB content, a Y_P/S value of 0.38 g PHB/g glucose and a productivity of 0.43 g PHB/(L.h) using pColdTF-phaCAB_A–04. The PHB film exhibited high optical transparency and possessed M_w 5.79 × 10⁵ Da, M_n 1.86 × 10⁵ Da, and PDI 3.11 with normal melting temperature and mechanical properties.

The influence of medium composition on the microbial secretory production of hydroxyalkanoate oligomers

With the aid of a chain transfer (CT) reaction, hydroxyalkanoate (HA) oligomers can be secreted by recombinant Escherichia coli carrying the gene encoding a lactate-polymerizing enzyme (PhaC1_PsSTQK) using Luria-Bertani (LB) medium supplemented with a carbon source and CT agent. In this study, HA oligomers were produced through microbial secretion using a mineral-based medium instead of LB medium, and the impact of medium composition on HA oligomer secretion was investigated. The focused targets were medium composition and NaCl concentration related to osmotic conditions. It was observed that 4.21 g/L HA oligomer was secreted by recombinant E. coli in LB medium, but the amount secreted in the mineral-based modified R (MR) medium was negligible. However, when the MR medium was supplemented with 5 g/L yeast extract, 3.75 g/L HA oligomer was secreted. This can be accounted for by the enhanced expression and activity of PhaC1_PsSTQK upon supplementation with growth-activated nutrients as supplementation with yeast extract also promoted cell growth and intracellular growth-associated polymer accumulation. Furthermore, upon adding 10 g/L NaCl to the yeast extract-supplemented MR medium, HA oligomer secretion increased to 6.86 g/L, implying that NaCl-induced osmotic pressure promotes HA oligomer secretion. These findings may facilitate the secretory production of HA oligomers using an inexpensive medium.

Engineering of Aeromonas caviae Polyhydroxyalkanoate Synthase Through Site-Directed Mutagenesis for Enhanced Polymerization of the 3-Hydroxyhexanoate Unit

Polyhydroxyalkanoate (PHA) synthase is an enzyme that polymerizes the acyl group of hydroxyacyl-coenzyme A (CoA) substrates. Aeromonas caviae PHA synthase (PhaC_Ac) is an important biocatalyst for the synthesis of a useful PHA copolymer, poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] [P(3HB-co-3HHx)]. Previously, a PhaC_Ac mutant with double mutations in asparagine 149 (replaced by serine [N149S]) and aspartate 171 (replaced by glycine [D171G]) was generated to synthesize a 3HHx-rich P(3HB-co-3HHx) and was named PhaC_Ac NSDG. In this study, to further increase the 3HHx fraction in biosynthesized PHA, PhaC_Ac was engineered based on the three-dimensional structural information of PHA synthases. First, a homology model of PhaC_Ac was built to target the residues for site-directed mutagenesis. Three residues, namely tyrosine 318 (Y318), serine 389 (S389), and leucine 436 (L436), were predicted to be involved in substrate recognition by PhaC_Ac. These PhaC_Ac NSDG residues were replaced with other amino acids, and the resulting triple mutants were expressed in the engineered strain of Ralstonia eutropha for application in PHA biosynthesis from palm kernel oil. The S389T mutation allowed the synthesis of P(3HB-co-3HHx) with an increased 3HHx fraction without a significant reduction in PHA yield. Thus, a new workhorse enzyme was successfully engineered for the biosynthesis of a higher 3HHx-fraction polymer.

Expanded amino acid sequence of the PhaC box in the active center of polyhydroxyalkanoate synthases

Polyhydroxyalkanoate (PHA) synthases catalyze the polymerization reaction of the acyl moiety of hydroxyacyl-coenzyme A into polyester. The catalytic subunit PhaC of PHA synthase has the PhaC box sequence at the active site that is typically described as G-X-C-X-G-G (X is an arbitrary amino acid), and cysteine is an active center. In this study, an amino acid replacement was introduced into the PhaC box of the PHA synthase derived from Ralstonia eutropha (PhaC_Re ) to investigate the importance of highly conserved residues in polymerizing activity. Point mutagenesis revealed that PhaC_Re mutants with the expanded PhaC box sequence ([GAST]-X-C-X-[GASV]-[GA]) are functional PHA synthases. These findings highlight the low mutational robustness of the last glycine residue in the PhaC box as well as that of the active center cysteine.