L-Serine

BACKGROUND: ;L-tryptophan is an important ingredient in medicines, especially in neuromedicines such as antidepressants. Many commercial processes employ various microorganisms with high tryptophan synthase activity to produce L-tryptophan from indole and L-serine, but these processes are very costly due to the costs of precursors, especially L-serine.;OBJECTIVES: ;For this reason, we studied the ability to use processed Iranian cane and beet molasses as L-serine sources for L-tryptophan production, which enables us to reach a cost-effective process.;MATERIALS AND METHODS: ;Whole cells of Escherichia coli ATCC 11303 were induced for L-tryptophan synthase by addition of indole to the growth medium and bacterial cells harvested from the growth medium were used as biocatalysts in the production medium. Conditions of the production medium were optimized and Iranian cane and beet molasses were processed by solvent extraction with ethanol and n-butanol and used as L-serine sources of the production medium. Amount of L-tryptophan and theoretical yield of L-tryptophan production were determined by High Performance Liquid Chromatography and by a colorimetrical method on the basis of the remaining indole assay, respectively.

Enzymes within a family often catalyze different reactions. In some cases, this variety stems from different catalytic machinery, but in other cases the machinery is identical; nevertheless, the enzymes catalyze different reactions. In this review, we examine the subset of α/β-hydrolase fold enzymes that contain the serine-histidine-aspartate catalytic triad. In spite of having the same protein fold and the same core catalytic machinery, these enzymes catalyze seventeen different reaction mechanisms. The most common reactions are hydrolysis of C-O, C-N and C-C bonds (Enzyme Classification (EC) group 3), but other enzymes are oxidoreductases (EC group 1), acyl transferases (EC group 2), lyases (EC group 4) or isomerases (EC group 5). Hydrolysis reactions often follow the canonical esterase mechanism, but eight variations occur where either the formation or cleavage of the acyl enzyme intermediate differs. The remaining eight mechanisms are lyase-type elimination reactions, which do not have an acyl enzyme intermediate and, in four cases, do not even require the catalytic serine. This diversity of mechanisms from the same catalytic triad stems from the ability of the enzymes to bind different substrates, from the requirements for different chemical steps imposed by these new substrates and, only in about half of the cases, from additional hydrogen bond partners or additional general acids/bases in the active site.

Reversible phosphorylation of bacterial transcriptional regulators (TRs) belonging to the family of two-component systems (TCSs) is a well-established mechanism for regulating gene expression. Recent evidence points to the fact that reversible phosphorylation of bacterial TRs on other types of residue, i.e. serine, threonine, tyrosine and cysteine, is also quite common. The phosphorylation of the ester type (phospho-serine/threonine/tyrosine) is more stable than the aspartate phosphorylation of TCSs. The kinases which catalyse these phosphorylation events (Hanks-type serine/threonine protein kinases and bacterial protein tyrosine kinases) are also much more promiscuous than the TCS kinases, i.e. each of them can phosphorylate several substrate proteins. As a consequence, the dynamics and topology of the signal transduction networks depending on these kinases differ significantly from the TCSs. Here, we present an overview of different classes of bacterial TR phosphorylated and regulated by serine/threonine and tyrosine kinases. Particular attention is given to examples when serine/threonine and tyrosine kinases interact with TCSs, phosphorylating either the histidine kinases or the response regulators.