Isolation and properties of a species produced by the partial dissociation of aspartate transcarbamylase from Escherichia coli.

A species produced by the reaction of aspartate transcarbamylase (C6R6) with 6 to 12 eq of p-hydroxymercuribenzoate was isolated by DEAE-Sephadex chromatography. Purified material was completely dissociated with mercurials and the relative amounts of catalytic (C) and regulatory (R) subunits were determined by three methods: (a) quantitative cellulose acetate electrophoresis; (b) Lowry analysis after separating the catalytic and regulatory subunits by sucrose gradient centrifugation; (c) dissociation of the species with sodium dodecyl sulfate and determination of the relative amounts of catalytic and regulatory chain by sodium dodecyl sulfate gel electrophoresis. All three methods gave consistent results, indicating that the molecule consists of 75% (by weight) catalytic chain and 25% regulatory chain. The molecular weight determined by gel filtration, sedimentation velocity, and sedimentation equilibrium experiments was found to be approximately 270,000. These observations establish that this species has the structure C6R4, and is produced by the release of a single regulatory dimer R2 from the intact aspartate transcarbamylase complex. This protein (C6R4) contains 20 cysteines and four zinc ions, consistent with the proposed subunit structure. The purified intermediate C6R4 contains no mercury. The parent molecule C6R6 can be reconstituted from C6R4 by incubation with isolated regulatory subunit (R2) in the presence of zinc and beta-mercaptoethanol. Titration of C6R4 yields an end point which corresponds to the addition of 1 mol of regulatory subunit (R2) per mol of C6R4. The intermediate is quite stable at neutral pH but tends to disproportionate into aspartate transcarbamylase and catalytic subunit after prolonged storage or at elevated pH. The kinetic properties of this species have been investigated. The specific activity of C6R4 is virtually identical with that of the native enzyme but the regulatory properties are substantially reduced. Both homotropic and heterotropic interactions are reduced but not abolished, indicating that the intact structure C6R6 is not required for the allosteric transitions involved in regulation.