Structural and functional characterization of S-adenosylmethionine (SAM) synthetase from Pichia ciferrii

Heterologous expression and characterization of a thermoalkaliphilic SAM-synthetase from giant leucaena (Leucaena leucocephala subsp glabrata)

The cDNA encoding S-adenosylmethionine (SAM) synthetase was isolated from giant leucaena (Leucaena leucocephala subsp. glabrata) root tissue mRNA. Transcriptome data and 5'-RLM-RACE were used to obtain the transcript sequence and clone into the T₇-expression vector pEt14b. N-terminal Histidine-tagged recombinant protein was expressed highly in Escherichia coli, purified and characterized by activity assays. A straightforward method using isocratic reverse-phase HPLC analysis (mobile phase: 0.02M o-phosphoric acid) of enzyme assays determined optimal enzyme activity at pH 10.0, 55 °C and 200 mM KCl. In addition to thermophilic activity, giant leucaena SAM-synthetase remains highly active in solutions containing up to 4 M KCl and accepts Na⁺ to some extent as a substitute for K⁺, a known required cofactor for SAM-synthetases. The enzyme followed Michaelis-Menten kinetics (K_m = 1.82 mM, K_cat = 1.17 s^-1, V_max 243.9 μM. min^-1) and was not inhibited by spermidine, spermine or nicotianamine. Giant leucaena SAM-synthetase is a highly tolerant enzyme to extreme conditions, suggesting further studies on plant SAM-synthetases.

Identification of methionine adenosyltransferase with high diastereoselectivity for biocatalytic synthesis of (S)-S-adenosyl-l-methionine and exploring its relationship with fluorinated biosynthetic pathway

Natural fluorinated products are rare and attract great attention. The de novo fluorometabolites biosynthetic pathway in microbes has been studied. It is revealed that the carbon-fluorine (C-F) bond is formed by an exotic enzyme called fluorinase (FLA) when using fluorine ions and S-adenosyl-l-methionine (SAM) as substrates. However, the resource of the precursor SAM is still elusive. To solve this, a novel methionine adenosyltransferase from Streptomyces xinghaiensis (SxMAT) was identified and characterized. We proved that SAM was enzymatically synthesized by SxMAT, an enzyme that mediated the reaction between adenosine triphosphate (ATP) and l-methionine (l-Met) with 99% diastereoisomeric excess (d.e.) and 80% yield. Such high diastereoselectivity had never been reported before. SxMAT was a Co²⁺-dependent metalloenzyme. The results showed that the metal cobalt ion contributes to the activity and selectivity of SxMAT. Molecular docking was performed to reveal its catalytic mechanism. The optimal temperature and pH were 55 °C and 8.5, respectively. Lastly, a two-step tandem enzymatic reaction using SxMAT and FLA both from S. xinghaiensis to generate 5'-fluoro-deoxyadenosine (5'-FDA) was performed. This implied that SxMAT may be present in this fluorometabolites biosynthetic route. These results suggested that SxMAT could be a useful biocatalyst for the synthesis of optically pure (S)-S-adenosyl-l-methionine, an important nutraceutical. In addition, SxMAT will probably play an important role in the biosynthetic pathway of fluorinated natural products in bacteria.

Improving the production of S-adenosyl-L-methionine in Escherichia coli by overexpressing metk

S-adenosyl-L-methionine (SAM) has important applications in many fields including chemical therapy and pharmaceutical industry. In this study, the recombinant Escherichia coli strain was constructed for effective production of SAM by introducing the SAM synthase gene (metK). This strain produced 34.5 mg/L of SAM in basic medium in shake flask. Yeast extract, pH, and loaded volume had a significant positive effect on the yield of SAM. Their optimal values were 35 g/L, 7.5, and 30 mL, respectively. The final conditions optimized were as follows: glucose 20, g/L; peptone, 40 g/L; yeast extract, 35 g/L; NaCl, 10 g/L; MgSO₄, 1.2 g/L; L-methionine, 1 g/L; rotate speed, 220 rpm; loaded volume, 30 mL; inoculation, 1%; temperature, 37°C; and initial medium, pH 7.5. The recombinant strain produced 128.2 mg/L of SAM under the above conditions in shake flask. The production of SAM in a 5 L fermentor was also investigated. The maximal biomass of the recombinant strain was 60.4 g/L after the cells were cultured for 20 hr, and the highest yield of SAM was 300.9 mg/L after induction for 8 hr in a 5 L fermentor. This study provides a good foundation for the future production and use of SAM.

Proteomic Analysis of Silk Viability in Maize Inbred Lines and Their Corresponding Hybrids

A long period of silk viability is critical for a good seed setting rate in maize (Zea mays L.), especially for inbred lines and hybrids with a long interval between anthesis and silking. To explore the molecular mechanism of silk viability and its heterosis, three inbred lines with different silk viability characteristics (Xun928, Lx9801, and Zong3) and their two hybrids (Xun928×Zong3 and Lx9801×Zong3) were analyzed at different developmental stages by a proteomic method. The differentially accumulated proteins were identified by mass spectrometry and classified into metabolism, protein biosynthesis and folding, signal transduction and hormone homeostasis, stress and defense responses, and cellular processes. Proteins involved in nutrient (methionine) and energy (ATP) supply, which support the pollen tube growth in the silk, were important for silk viability and its heterosis. The additive and dominant effects at a single locus, as well as complex epistatic interactions at two or more loci in metabolic pathways, were the primary contributors for mid-parent heterosis of silk viability. Additionally, the proteins involved in the metabolism of anthocyanins, which indirectly negatively regulate local hormone accumulation, were also important for the mid-parent heterosis of silk viability. These results also might imply the developmental dependence of heterosis, because many of the differentially accumulated proteins made distinct contributions to the heterosis of silk viability at specific developmental stages.

Molecular cloning and characterization of S-adenosylmethionine synthetase gene from Lycoris radiata

S-adenosylmethionine (SAM) synthetase catalyzes the synthesis of SAM, a molecule important for all cellular organisms. It is also considered to play an important role in salt tolerance of plants. Here, we cloned a Lycoris radiata (L. radiata) SAM synthetase gene LrSAMS to determine its biological function. The gene encodes a protein of 401 amino acids with a calculated molecular weight of 43.9 kDa. Amino acid sequence analysis of the deduced protein LrSAMS reveals high sequence identity to SAM synthetases from other organisms, such as Arabidopsis thaliana and Oryza sativa. The deduced LrSAMS protein contains conserved amino acids residues and sequences motifs that closely related to the function of SAM synthetase. Otherwise, the transcript levels of LrSAMS were significantly induced by NaCl treatment in L. radiata leaves, which implied that LrSAMS might play an important role in tolerance to salt stress in L.radiata. Complete ORF of LrSAMS was inserted into expression vector pET-29a(+) and transformed into Escherichia coli BL21 (DE3). The difference between the growth curve of the transgenic strain and control strain with blank vector showed that over-expressing LrSAMS could provide growth advantage to the engineered strain in high salt concentration.