Enhanced dissolution of the substrate D,L-2-amino-delta~2-thiazoline-4-carboxylic acid and enzymatic production of L-cysteine at high concentrations

摘要

l-Cysteine is widely used as a precursor in the pharmaceutical, cosmetic, food, and feed additive industries. It has been industrially produced from hydrolysis of human and animal hairs, which is limited for industrial production. At the same time, chemical hydrolysis causes the formation of intractable waste material. Thus, environmentally friendly methods have been developed. A big obstacle of currently available methods is the low substrate solubility leading to poor l-cysteine yield. Here, a method for improving the low solubility of the substrate d,l-2-amino- delta~2-thiazoline-4-carboxylic acid (d,l-ATC) is presented and the enzymatic reaction at high concentration levels was optimized. The substrate was dissolved in large amounts in aqueous solutions by pH control using salts. d,l-ATC solubility increased with an increasing solution pH due to its enhanced hydrophilicity, which can be achieved by a shift to dissociated carboxylic group (-COO~). The highest d,l-ATC solubility of 610 mM was obtained at pH 10.5. The maximum l-cysteine yield of 250 mM was attained at pH 9.1, which lies between the optimum values for high substrate solubility and reaction rate. The product yield could be increased by more than 10 times compared to those in previous reports, which is industrially meaningful.