Communication Between Catalytic Subunits in Hybrid Aspartate Transcarbamoylase Molecules: Effect of Ligand Binding to Active Chains on the Conformation of Unliganded, Inactive Chains

In the regulatory enzyme aspartate transcarbamoylase (aspartate carbamoyltransferase; carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) of Escherichia coli, the six catalytic polypeptide chains are arranged as two distinct catalytic trimers "crosslinked" by three regulatory dimers. Because in allosteric proteins it is assumed that the binding of a ligand to one site promotes a conformational change that affects the subsequent binding to other sites in the oligomeric protein, it was of interest to determine directly whether the effects of ligand binding to chains in one catalytic subunit are "communicated" to unliganded chains in the other subunit. Accordingly, hybrid enzyme molecules were constructed containing sensitive chromophores on the three inactive catalytic chains in one subunit along with an active catalytic subunit and three native regulatory subunits. The derivative exhibited the allosteric properties characteristic of the native enzyme. Communication between the two catalytic subunits was demonstrated by spectral measurements showing that the effects of ligand binding to the three active chains are propagated to the chromophores on the unliganded, inactive chains in the other subunit. Moreover, the change in the tertiary structure of the unliganded catalytic chains is tightly linked to the alteration in the quaternary structure.