EXPERIMENTAL SKETCH OF LANDSCAPES IN PROTEIN-SEQUENCE SPACE

A landscape in protein sequence space shows the relationship between the primary structure and the level of a property of each protein. We developed methods for observing local landscapes experimentally using catalase I from Bacillus stearothermophilus with respect to its catalatic activity, peroxidatic activity, and thermostability. The enzyme gene was randomly mutated and a mutant library composed of 2648 transformants was obtained. Based on the activity and productivity of these transformants, 82 were selected as a sample group for measuring the altitude of catalase I. The altitude of the wild-type enzyme is close to the highest level in the mutant population for the thermostability landscape, but is at the average level for the peroxidatic activity. As for the catalatic activity, its altitude lies in between the two positions. A positive correlation was found between the altitudes of the catalatic and the peroxidatic activities, indicating that the locations of the hills and valleys in the landscapes of the two activities roughly correspond with each other. In contrast, the thermostability landscape appeared quite differently. The smoothness of the landscape was examined via the number of mutations in the structural genes of the mutant enzymes of different properties. The correlation between the number of mutations and the level of each property showed that the thermostability landscape is smooth, but not the two activity landscapes. Thus, the results show that even from a rough sketch of the landscapes based on the experimental data, the characteristic features of catalase I can be elucidated. The sketch of a landscape, therefore, provides a new view in understanding enzymes.