Electron density in the catalytic cleft of BlaC. (a) Refined model of the entire tetramer (σ = 1.1) in the asymmetric unit after mixing. The electron density (2Fo − Fc) is shown in blue in the binding pockets. Subunits A and C contain phosphate; subunits B and D have a bound ceftriaxone, with that in subunit D being bound slightly more strongly. (b) Enlarged section of the apo (red electron density) subunit D binding pocket showing electron density for phosphate. (c) Enlarged section of the mixed (blue electron density) subunit D binding pocket showing electron density for ceftriaxone. (b) and (c) show slightly different views of the same subunit-binding pocket; however, there are minimal changes to the ligand-binding sphere. From Kupitz et al. (2016).