Surface charge-based rational design of aspartase modifies the optimal pH for efficient β-aminobutyric acid production

β-Aminobutyric acid (BABA) can be widely used in the preparation of anti-tumor drugs, AIDS drugs, penicillin antibiotics, and plant initiators. However, the efficient, economical, and environmentally friendly production of BABA still faces challenges. Its important production enzyme, aspartase, catalyzes the substrate crotonic acid, and depends on harsh conditions, such as high temperatures and the presence of strong alkali. Here, we modified the surface charge of the enzyme to enable it to become more negatively charged (K19E, N87E, N125D, S133D, Q262E, and N451E; from -60 to -80), reducing its optimal pH from 9.0 to 8.0. The M20 enzyme showed improved specific activity (400.21 mU/mg at pH 8.0; 2.47-fold that of aspartase), and at pH 7.0, its activity increased 3-fold. The thermal stability of the enzyme was also improved. For the production of BABA, a 500 g/L whole-cell transformation was obtained with a 1.41-fold increase in yield, and the final production of BABA reached 556.1 g/L within 12 h. Our method provides a new strategy for modifying the characteristics of the enzyme through the modification of its surface charge, which also represents the first modification of the optimal pH for aspartase.

Keywords: Aspartase; Beta-aminobutyric acid; Optical pH; Rational design; Surface charge.