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 Rg with concentration, after accounting for the effects of nonideality. Apparent Rg 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 Rg 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 Mr PEGs to induce crystallization of these enzymes.