Microbial Production of L-Serine and Its Related Hydroxy Amino Acids

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L-Serine, which is a non-essential amino acid in the nutritional sense, is used primarily used as dietary supplement in parental nutrition and recently as a substrate for the enzymatic synthesis of other important amino acids such as L-tryptophan. Microbial serine production has so far been reported using heterotrophs and methyltrophs. Although extensive studies habe been done on the process by means of serine hydroxymethyltransferase (SHMT) reaction using as a raw material, the yields and produced amounts of L-serine are still so low. The present project aimed at establishing the fundamental methods for the production of L-serine by a methylotroph having the serine pathway, and also at developing this process to the production of serine related hydroxy amino acids.In the 1985 fiscal year, we studied the improvement of L-serine productivity of the serine-producing methylotroph, Hyphomicrobium methylotroph KM 146 by deriving a glycine-resistant mutant GM2. The mutant was found to produce 34 mg/ml L-serine (1.4-fold more than the parent) in a resting cell system. We also found that H. methylovorum and newly isolated methanol-utilizing bacterium, Pseudomonas sp. KM 193, produced L-threonine and allothreonine from ethanol and glycine by methanol-grown cells.In the 1986 fiscal year, we studied the purification and characterization of methanol dehydrogenase (MDH) and SHMT, enzymes involved in serine production, from H. methylovorum GM2. The enzymes were purified to homogeneity. Both showed a wide substrate specificity and high stability to long preservation. The optimum pH and temperature were 9 and 35゜C, and 8 and 37゜C for MDH and SHMT, respectively. Such basic information about the enzymatic properties of the two enzymes was considered to contribute to the further improvement of L-serine production by H. methylovorum.