Engineering thermal stability of l-asparaginase by in vitro directed evolution

Abstract

l-Asparaginase (EC 3.5.1.1, l-ASNase) catalyses the hydrolysis of l-Asn, producing l-Asp and ammonia. This enzyme is an anti-neoplastic agent; it is used extensively in the chemotherapy of acute lymphoblastic leukaemia. In this study, we describe the use of in vitro directed evolution to create a new enzyme variant with improved thermal stability. A library of enzyme variants was created by a staggered extension process using the genes that code for the l-ASNases from Erwinia chrysanthemi and Erwinia carotovora. The amino acid sequences of the parental l-ASNases show 77% identity, but their half-inactivation temperature (Tm) differs by 10 °C. A thermostable variant of the E. chrysamthemi enzyme was identified that contained a single point mutation (Asp133Val). The Tm of this variant was 55.8 °C, whereas the wild-type enzyme has a Tm of 46.4 °C. At 50 °C, the half-life values for the wild-type and mutant enzymes were 2.7 and 159.7 h, respectively. Analysis of the electrostatic potential of the wild-type enzyme showed that Asp133 is located at a neutral region on the enzyme surface and makes a significant and unfavourable electrostatic contribution to overall stability. Site-saturation mutagenesis at position 133 was used to further analyse the contribution of this position on thermostability. Screening of a library of random Asp133 mutants confirmed that this position is indeed involved in thermostability and showed that the Asp133Leu mutation confers optimal thermostability. © 2009 FEBS.