Heterologous gene expression and characterization of two serine hydroxymethyltransferases from Thermoplasma acidophilum

Characterization of thermostable serine hydroxymethyltransferase for β-hydroxy amino acids synthesis

β-hydroxy amino acids, such as serine, threonine, and phenylserine, are important compounds for medical purposes. To date, there has been only limited exploration of thermostable serine hydroxylmethyltransferase (SHMT) for the synthesis of these amino acids, despite the great potential that thermostable enzymes may offer for commercial use due to their high stability and catalytic efficiencies. ITBSHMT_1 (ITB serine hydroxylmethyltransferase clone number 1) from thermophilic and methanol-tolerant bacteria Pseudoxanthomonas taiwanensis AL17 was successfully cloned. Biocomputational analysis revealed that ITBSHMT_1 contains Pyridoxal-3'-phosphate and tetrahydrofolatebinding residues. Structural comparisons show that ITBSHMT_1 has 5 additional residues VSRQG on loop near PLP-binding site as novel structural feature which distinguish this enzyme with other characterized SHMTs. In silico mutation revealed that the fragment might have very essential role in maintaining of PLP binding on structure of ITBSHMT_1. Recombinant protein was produced in Escherichia coli Rosetta 2(DE3) in soluble form and purified using NiNTA affinity chromatography. The purified protein demonstrated the best activity at 80 °C and pH 7.5 based on the retro aldol cleavage of phenylserine. Activity decreased significantly in the presence of 3 mM transition metal ions but increased in the presence of 30 mM β-mercaptoethanol. ITBSHMT_1 demonstrated Vmax, Km, Kcat, and Kcat/Km at 242 U/mg, 23.26 mM, 186/s, and 8/(mM.s), respectively. The aldol condensation reaction showed the enzyme's best activity at 80 °C for serine, threonine, or phenylserine, with serine synthesis showing the highest specific activity. Biocomputational analysis revealed that high intramolecular interaction within the 3D structure of ITBSHMT_1 might be correlated with the enzyme's high thermal stability. The above data suggest that ITBSHMT_1 is a potential and novel enzyme for the production of various β-hydroxy amino acids.

Molecular cloning, characterization, and homology modeling of serine hydroxymethyltransferase from psychrophilic bacterium Psychrobacter sp

Serine hydroxymethyltransferase (SHMT) plays a significant role in the synthesis of l-serine, purine, and thymidylate, which could be extensively applied in the treatment of cancers and the development of antibiotics. In this study, cloned from Psychrobacter sp. ANT206, a novel cold-adapted SHMT gene (psshmt, 1257 bp) encoding a protein of 418 amino acids was expressed in Escherichia coli. The homology modeling result revealed that PsSHMT owned fewer Proline (Pro) residues and hydrogen bonds compared with its homologs from mesophilic E. coli and thermophilic Geobacillus stearothermophilus. In addition, the molecular weight of the purified recombinant PsSHMT (rPsSHMT) was identified to be 45 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis, approximately. The enzymatic characteristics of the cold-adapted rPsSHMT displayed that its optimum temperature and pH were 30°C and 7.5, respectively, and its enzymatic activity could be inhibited by Cu²⁺ , significantly. rPsSHMT also showed a high k_cat value and low ΔG at low temperatures. Furthermore, arginine (Arg) could affect the activity of rPsSHMT and be vital to its active sites. The results of this study reflected that these characteristics of the cold-adapted rPsSHMT made it a remarkable candidate that could be utilized in multiple industrial fields under low temperatures.

Screening, cloning and functional characterization of key methyltransferase genes involved in the methylation step of 1-deoxynojirimycin alkaloids biosynthesis in mulberry leaves

The novel C-methyltransferase, MaMT1, could catalyze the conversion of piperidine to 2-methylpiperidine, which may be involved in the methylation step of DNJ biosynthesis in mulberry leaves. Mulberry (Morus alba L.) is a worldwide crop with medicinal, feeding and nutritional value, and 1-deoxynojirimycin ((2R, 3R, 4R, 5S)-2-hydroxymethyl-3, 4, 5-trihydroxypiperidine, DNJ) alkaloid, a potent α-glucosidase inhibitor, is its main active ingredient. Our previous researches clarified the biosynthetic pathway of DNJ from lysine to Δ¹-piperideine, but its downstream pathway is unclear. Herein, eight differential methyltransferases (MTs) genes were screened from transcriptome profiles of mulberry leaves with significant differences in DNJ content (P < 0.01). Subsequently, MaMT1 (OM140666) and MaMT2 (OM140667) were hypothesized as candidate genes related to DNJ biosynthesis by correlation analysis of genes expression levels and DNJ content of mulberry leaves at different dates. Functional characterization of MaMT1 and MaMT2 were performed by cloning, prokaryotic expression and enzymatic reaction in vitro, and it showed that MaMT1 protein could catalyze the conversion of piperidine to 2-methylpiperidine. Moreover, molecular docking confirmed the interaction of MaMT1 protein with piperidine and S-adenosyl-L-methionine (SAM), indicating that MaMT1 had C-methyltransferase activity, while MaMT2 did not. The above results suggested that MaMT1 may be involved in the methylation step of DNJ alkaloid biosynthesis in mulberry leaves, which is a breakthrough in the analysis of DNJ alkaloid biosynthetic pathway. It is worth mentioning that the novel MaMT1, annotated as serine hydroxymethyltransferase, could rely on SAM to perform C-methyltransferase function. Therefore, our findings contribute new insights into the research of DNJ alkaloid biosynthesis and C-methyltransferase family.