Improvement of crystal quality by surface mutations of β-lactamase Toho-1

The β-lactamase Toho-1 exhibits a strong tendency to form merohedrally twinned crystals. Here, the crystal quality of Toho-1 was improved by using surface modification to remove a sulfate ion involved in crystal packing. The surface-modified Toho-1 variant (R274N/R276N) was crystallized under similar conditions to those used for wild-type Toho-1. R274N/R276N did not form merohedrally twinned crystals. The crystals diffracted to a significantly higher resolution (∼0.97 Å) than the wild-type crystals (1.65 Å); they belonged to the same space group and had almost identical unit-cell parameters to those of wild-type Toho-­1.