Allosteric signal transmission involves synergy between discrete structural units of the regulatory subunit of aspartate transcarbamoylase

Structural Investigation of Cold Activity and Regulation of Aspartate Carbamoyltransferase from the Extreme Psychrophilic Bacterium Moritella profunda

Aspartate carbamoyltransferase (EC 2.1.3.2) is extensively studied as a model for cooperativity and allosteric regulation. The structure of the Escherichia coli enzyme has been thoroughly analyzed by X-ray crystallography, and recently the crystal structures of two hyperthermophilic ATCases of the same structural class have been characterized. We here report the detailed functional and structural investigation of the ATCase from the psychrophilic deep sea bacterium Moritella profunda. Our analysis indicates that the enzyme conforms to the E. coli model in that two allosteric states exist that are influenced by similar homotropic interactions. The heterotropic properties differ in that CTP and UTP inhibit the holoenzyme, but ATP seems to exhibit a dual regulatory pattern, activating the enzyme at low concentrations and inhibiting it in the mM range. The crystal structure of the unliganded M. profunda ATCase shows resemblance to a more extreme T state reported previously for an E. coli ATCase mutant. A detailed molecular analysis reveals potential features of adaptation to cold activity and cold regulation. Moreover, M. profunda ATCase presents similarities with certain mutants of E. coli ATCase altered in their kinetic properties or temperature relationships. Finally, structural and functional comparison of ATCases across the full physiological temperature range agrees with an important, but fundamentally different role for electrostatics in protein adaptation at both extremes, i.e. an increased stability through the formation of ion pairs and ion pair networks at high physiological temperatures, and an increased flexibility through enhanced protein solvation at low temperatures.

Dissection of the conduit for allosteric control of carbamoyl phosphate synthetase by ornithine

Ornithine is an allosteric activator of carbamoyl phosphate synthetase (CPS) from Escherichia coli. Nine amino acids in the vicinity of the binding sites for ornithine and potassium were mutated to alanine, glutamine, or lysine. The residues E783, T1042, and T1043 were found to be primarily responsible for the binding of ornithine to CPS, while E783 and E892, located within the carbamate domain of the large subunit, were necessary for the transmission of the allosteric signals to the active site. In the K loop for the binding of the monovalent cation potassium, only E761 was crucial for the exhibition of the allosteric effects of ornithine, UMP, and IMP. The mutations H781K and S792K altered significantly the allosteric properties of ornithine, UMP, and IMP, possibly by modifying the conformation of the K-loop structure. Overall, these mutations affected the allosteric properties of ornithine and IMP more than those of UMP. The mutants S792K and D1041A altered the allosteric regulation by ornithine and IMP in a similar way, suggesting common features in the activation mechanism exhibited by these two effectors.

Allosteric regulation of catalytic activity: Escherichia coli aspartate transcarbamoylase versus yeast chorismate mutase

Allosteric regulation of key metabolic enzymes is a fascinating field to study the structure-function relationship of induced conformational changes of proteins. In this review we compare the principles of allosteric transitions of the complex classical model aspartate transcarbamoylase (ATCase) from Escherichia coli, consisting of 12 polypeptides, and the less complicated chorismate mutase derived from baker's yeast, which functions as a homodimer. Chorismate mutase presumably represents the minimal oligomerization state of a cooperative enzyme which still can be either activated or inhibited by different heterotropic effectors. Detailed knowledge of the number of possible quaternary states and a description of molecular triggers for conformational changes of model enzymes such as ATCase and chorismate mutase shed more and more light on allostery as an important regulatory mechanism of any living cell. The comparison of wild-type and engineered mutant enzymes reveals that current textbook models for regulation do not cover the entire picture needed to describe the function of these enzymes in detail.