Engineering of LadA for enhanced hexadecane oxidation using random- and site-directed mutagenesis

LadA, a monooxygenase catalyzing the oxidation of n-alkanes to 1-alkanols, is the key enzyme for the degradation of long-chain alkanes (C(15)-C(36)) in Geobacillus thermodenitrificans NG80-2. In this study, random- and site-directed mutagenesis were performed to enhance the activity of the enzyme. By screening 7,500 clones from random-mutant libraries for enhanced hexadecane hydroxylation activity, three mutants were obtained: A102D, L320V, and F146C/N376I. By performing saturation site-directed mutagenesis at the 102, 320, 146, and 376 sites, six more mutants (A102E, L320A, F146Q/N376I, F146E/N376I, F146R/N376I, and F146N/N376I) were generated. Kinetic studies showed that the hydroxylation activity of purified LadA mutants on hexadecane was 2-3.4-fold higher than that of the wild-type enzyme, with the activity of F146N/N376I being the highest. Effects of the mutations on optimum temperature, pH, and heat stability of LadA were also investigated. A complementary study showed that Pseudomonas fluorescens KOB2Δ1 strains expressing the LadA mutants grew more rapidly with hexadecane than the strain expressing wild-type LadA, confirming the enhanced activity of LadA mutants in vivo. Structural changes resulting from the mutations were analyzed and the correlation between structural changes and enzyme activity was discussed. The mutants generated in this study are potentially useful for the treatment of environmental oil pollution and in other bioconversion processes.