Resolving the Fluorescence Response of Escherichia coli Carbamoyl Phosphate Synthetase:  Mapping Intra- and Intersubunit Conformational Changes^†

Carbamoyl phosphate synthetase (CPS) from Escherichia coli is potentially overlaid with a network of allosterism, interconnecting active sites, effector binding sites, and aggregate interfaces to control its mechanisms of catalytic synchronization, regulation, and oligomerization, respectively. To characterize these conformational changes, a tryptophan-free variant of CPS was genetically engineered by substituting six native tryptophans with tyrosines. Each tryptophan was then reinserted, singly, as a specific fluorescence probe of its corresponding microenvironment. The amino acid substitutions themselves result in little apparent disruption of the protein; variants maintain catalytic and allosteric functionality, and the fluorescence properties of each tryptophan, while unique, are additive to wild-type CPS. Whereas the collective, intrinsic fluorescence response of E. coli CPS is largely insensitive to ligand binding, changes of the individual probes in intensity, lifetime, anisotropy, and accessibility to acrylamide quenching highlight the dynamic interplay between several protein domains, as well as between subunits. W213 within the carboxy phosphate domain, for example, exhibits an almost 40% increase in intensity upon saturation with ATP; W437 of the oligomerization domain, in contrast, is essentially silent in its fluorescence to the binding of ligands. Nucleotide and bicarbonate association within the large subunit induces fluorescence changes in both W170 and W175 of the small subunit, indicative of the type of long-range interactions purportedly synchronizing the carboxy phosphate and amidotransferase domains of the enzyme to initiate catalysis. ATP and ADP engender different fluorescence responses in most tryptophans, perhaps reflecting coordinating, conformational changes accompanying the cycling of reactants and products during catalysis.