Self-association of α-chymotrypsin at low ionic strength in the vicinity of its pH optimum

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Abstract
The self-association of [bovine pancreatic] .alpha.-chymotrypsin and its di-isopropyl phosphoryl derivative in I [ionic strength] 0.03 sodium phosphate buffer, pH7.9, was investigated by velocity sedimentation, equilibrium sedimentation and difference gel chromatography. No differences between the native and chemically modified enzyme were observed in the ultracentrifuge studies, and only a marginal (0.6%) difference in weight-average elution volume was detected by difference gel chromatography of 5 g/li solutions on Sephadex G-75. From quantitative analyses of sedimentation velocity and sedimentation-equilibrium distributions obtained with (di-isopropylphosphoryl)-chymotrypsin, the polymerizing system is postulated to involve an indefinite association of dimer (with an isodesmic association constant of 0.68 l/g) that is formed by a discrete dimerization step with equilibrium constant 0.25 l/g. In addition to providing the best fit of the experimental results, this model of chymotrypsin polymerization at low ionic strength is also consistent with an earlier observation that dimer formation is a symmetrical head-to-head phenomenon under conditions of higher ionic strength (IO. 29, pH 7.9) where association is restricted to a monomer-dimer equilibrium. It is proposed that the dimerization process is essentially unchanged by variation in ionic strength at pH7.9, and that higher polymers are formed by an entirely different mechanism involving largely electrostatic interactions between dimeric species.