The association behavior of β‐lactamases in polyethylene glycol solution

The β‐lactamases (EC 3.5.2.6) from Bacillus licheniformis 749/C, Enterobacter cloacae P99, and TEM plasmid RP4 are studied in 10–14% (w/v) polyethyleneglycol (PEG) 8000 solutions at pH 6.5 by x‐ray scattering and in 18% PEG by equilibrium sedimentation. Although all three enzymes crystallize with twofold crystal symmetry from PEG 8000, it is not possible in this study to prove that dimerization occurs; however, both techniques give evidence for association above 1% (w/v) protein concentration. For the B. lichen. , P99, and TEM enzymes, a dimerization of at most 0, 5, and 10% (v/v), respectively, account for the variation of radii of gyration R g with concentration, after accounting for the effects of nonideality. Apparent R g were 3–5% smaller in PEG solution than in PEG‐free solution. Enhanced ordering of the molecules in PEG solution or the presence of a PEG‐depleted hydration shell around the enzymes can account for the observation of reduced R g values. Accordingly, values of the partial specific volume (defined at constant chemical potential of PEG) indicate considerable PEG exclusion and are consistent with the ability of high M r PEGs to induce crystallization of these enzymes.