, 2010), although physiological roles of SHMT in different organisms are not well characterized, except for the photorespiratory role in the mitochondria (Voll et al., click here 2005; Jamai et al., 2009). In cyanobacteria, only a single gene encoding SHMT could be found, suggesting that the cyanobacterial SHMT may have multiple functions in cells. SHMT in A. halophytica should play a unique role because its cells accumulate a large amount of glycine betaine under high salinity conditions.
Our present data clearly indicate that the expression of ApSHMT is up-regulated by NaCl (Fig. 1a), and in vitro experiments demonstrate that the overexpression of ApSHMT increased the accumulation levels of serine, choline, and glycine betaine and caused the increased salinity tolerance of E. coli. It should be mentioned that A. halophytica uses another pathway for glycine betaine synthesis than E. coli and plants. In this pathway, C1-units (i.e. methyl groups) are directly used to methylate the precursor glycine instead of synthesizing choline. Therefore, SHMT in A. halophytica AZD2014 research buy would play important role in glycine betaine synthesis. Regardless of these facts, the enhanced salt tolerance by SHMT was observed for E. coli only. Therefore,
this result cannot be generalized to other organisms, especially cyanobacteria that do not synthesize glycine betaine. Biochemical analysis of the recombinant ApSHMT showed that the apparent Km value of ApSHMT for dl-threo-3-phenylserine was 0.183 mM with Vmax 3522 nmol min−1 mg−1 (Fig. 2b and Table 1). This Km value is significantly small compared Nutlin-3 with those from other organisms such as Plasmodium vivax (8.6 mM) and sheep (84 mM; Ulevitch & Kallen, 1977; Sopitthummakhun et al., 2009). The apparent Km values of ApSHMT for l-serine and THF were 0.379 mM (Vmax, 1104 nmol
min−1 mg−1) and 0.243 mM (Vmax, 814 nmol min−1 mg−1), respectively, which were similar [0.1–1 mM range (for l-serine) and 0.02–0.8 mM range (for THF)] to those of other organisms such as P. vivax, E. coli, B. stearothermophilus, sheep, rabbit, and human (Ulevitch & Kallen, 1977; Schirch et al., 1985; Di Salvo et al., 1998; Jala et al., 2002; Sopitthummakhun et al., 2009). Higher affinity of ApSHMT to dl-threo-3-phenylserine would suggest some physiological function of ApSHMT, but that remains to be clarified. Figures 4a and b showed that expression of ApSHMT in E. coli resulted in the increase in amino acids glycine/serine. This is interesting because the amino acid l-serine is required for pharmaceutical purposes (Stolz et al., 2007). The total annual demand for l-serine is estimated to be 300 tons (Stolz et al., 2007). The production processes currently utilized still rely on the extraction of l-serine. The present data suggest the possibility to exploit ApSHMT for the production of serine.