X-ray structural analysis of $\beta$-lactamase of {\it Enterobacter cloacae\/} P99 and penicillin binding to a target enzyme DD-peptidase of {\it Streptomyces\/} R61

The crystal structure of Enterobacter cloacae P99 $\beta$-lactamase (commonly cephalosporinase) has been analyzed to 3.0A using X-ray diffraction techniques. The enzyme has molecular weight of 39,000 Dalton and crystallizes in orthorhombic space group P2$\sb1$2$\sb1$2. The unit cell dimensions are a = 77.6A, b = 70.1A, c = 63.3A, with 4 molecules per unit cell.^ The data were collected using high intensity synchrotron radiation and solution of the structure was attempted using both heavy atom and molecular replacement methods. An electron density map is presented based on the molecular replacement method using Streptomyces R61 DD-peptidase as model molecule and refined to 3.0A resolution.^ Four heavy atom derivatives were examined. The most successful heavy atom complex is that of TbCl$\sb3$ which showed weak binding sites and consequently was studied using single isomorphous replacement (SIR) phase enhancement method.^ Three $\beta$-lactams, a monobactam, $\alpha$, and $\beta$-cyclopropyl phenyl penicillins, were used for finding studies with Streptomyces R61 DD-peptidase. The difference Fourier maps of these complexes at 2.2A show they all bind at a common site near the active site serine 62.