5.5 Å crystallographic structure of penicillin β-lactamase and radius of gyration in solution

Abstract

An electron density map of crystalline R-TEM Escherichia coli β-lactamase (penicillinase) has been calculated from X-ray diffraction data at 5.5 Å resolution with protein phases based on Friedel mates from a high-quality samarium derivative. The mean figure of merit for 854 independent reflections is 0.75. The monomeric molecule is slightly ellipsoidal and contains one and possibly two regions of α-helix which are 25 Å long. The Crystallographic search for the substrate binding site has so far been inconclusive. The radius of gyration of the enzyme in solution at pH 7 is 17.1 ± 1.0 Å from small-angle X-ray scattering measurements. This compares with 18.6 å calculated from the low-resolution electron density map of the molecule in the crystal.