Nonadditivity of mutational effects on the properties of catalase I and its application to efficient directed evolution

Catalase I of Bacillus stearothermophilus has high catalatic and low peroxidatic activities. The mutant from the first random mutant population, D130N, which has higher peroxidatic and lower catalatic activities than those exhibited by the wild-type enzyme, was subjected to second random mutagenesis in observance of the change in reaction specificity. From the second mutant population, the mutant I108T/D130N/I222T was selected and examined, The reaction specificity of the purified enzymes revealed that catalase I being originally 98% catalase and 2% peroxidase was brought to 58% specificity to peroxidase after two-step adaptive walks. From the statistical analysis of the two random mutant populations, the average degree of nonadditivity of the mutational effects was estimated to be 0.13 irrespective of the properties of the enzyme. It was demonstrated that the distribution pattern of a property of the second mutant population can be predicted well from the data of the first mutant population by taking into consideration the degree of nonadditivity. The strategy for an efficient adaptive walk in directed evolution of enzymes through the prediction of appropriate mutation rate and effective sample size for further mutation and selection was presented and discussed.