Abstract

Micrococcus sp.S-11 is capable of converting racemic 2-amino-Δ²-thiazoline-4-carboxylic acid (DL-ATC) to L-cysteine. The enzyme-producing conditions were optimized by using response surface methodology to maximize enzyme activity. Glucose, DL-ATC·3H₂O and rotational speed have significant effects on enzyme activity. Their optimal values were 21.7 g/l glucose, 7.3 g/l DL-ATC·3H₂O and 141 rpm rotational speed. Then validation experiments were conducted under optimal conditions. The enzyme activity was up to 2291 U/g, which was close to thepredicted maximum enzyme activity (2284 U/g) and 31.4% higher than the initial activity (1743 U/g). A two-step culture was performed with the optimal conditions for cell growth in the first step and enzyme activity in the second step. The results suggested that high carbon/nitrogen (C/N) mass ratio was favorable to enzyme activity and the two-step culture strategy was proposed to be adopted for achieving both high biomass and enzyme production simultaneously.

Key words: L-Cysteine, enzyme activity, Plackett-Burman design, response surface methodology, two-step culture.