Expression, Purification, and Characterization of a Recombinant Methionine Adenosyltransferase pDS16 in Pichia pastoris

Methionine adenosyltransferase (MAT, EC2.5.1.6) catalyzes the synthesis of S-adenosylmethionine (SAM) using L-methionine and adenosine triphosphate (ATP) as substrates. The mutant MAT pDS16 was obtained through DNA shuffling previously in our lab. Overexpression of pDS16 in Pichia pastoris led to about 65% increase of MAT activity and SAM accumulation, compared with the strain overexpressing Saccharomyces cerevisiae MAT gene SAM2. Different strategies were tested to facilitate the expression and purification of pDS16. However, addition of the hexahistidine tag to pDS16 was shown to decrease the enzyme activity, and the yeast alpha-factor signal sequence could not effectivley direct the secretion of pDS16. The intracellular pDS16 was purified by a simple two-step procedure combining an ion exchange and hydrophobic interaction chromatography. Protein purity was verified by sodium dodecyl sulfate polyacrylamide gel electrophoresis to be 93 %, with the specific activity of 1.828 U/mg. Two-dimensional electrophoresis revealed pI of similar to 5.5. The purified enzyme followed Michaelis kinetics with a Km of 1.72 and 0.85 mM, and V-max of 1.54 and 1.15 mu mol/min/mg for ATP and L-methionine, respectively. pDS16 exhibited optimal activity at pH 8.5 and 45 degrees C with the requirement of divalent cation Mg2+ and was slightly stimulated by the monovalent cation K+. It showed an improved thermostability, about 50 % of the enzyme activity was retained even after preincubation at 50 degrees C for 2 h.