Purification and characterization of a mutant aspartate transcarbamoylase lacking enzyme activity

Importance of domain closure for the catalysis and regulation of Escherichia coli aspartate transcarbamoylase

Two hybrid versions of Escherichia coli aspartate transcarbamoylase were studied to determine the influence of domain closure on the homotropic and heterotropic properties of the enzyme. Each hybrid holoenzyme had one wild-type and one inactive catalytic subunit. In the first case the inactive catalytic subunit had Arg-54 replaced by alanine. The holoenzyme with this mutation in all six catalytic chains exhibits a 17,000-fold reduction in activity, no loss in substrate affinity, and an R state structurally identical to that of the wild-type enzyme. In the second case, the inactive catalytic subunit had Arg-105 replaced by alanine. The holoenzyme with this mutation in all six catalytic chains exhibits a 1,100-fold reduction in activity, substantial loss in substrate affinity, and loss of the ability to be converted to the R state. Thus, the R54A substitution results in a holoenzyme that can undergo closure of the catalytic chain domains to form the high activity, high affinity active site and to undergo the allosteric transition, whereas the R105A substitution results in a holoenzyme that can neither undergo domain closure nor the allosteric transition. The hybrid holoenzyme with one wild-type and one R54A catalytic subunit exhibited the same maximal velocity per active site as the wild-type holoenzyme, reduced cooperativity, and normal heterotropic interactions. The hybrid with one wild-type and one R105A catalytic subunit exhibited significantly reduced maximal velocity per active site as compared with the wild-type holoenzyme, reduced cooperativity, and substantially reduced heterotropic interactions. Small angle x-ray scattered was used to verify that the R105A-containing hybrid could attain an R state structure. These results indicate the global nature of the conformational changes associated with the allosteric transition in the enzyme. If one catalytic subunit cannot undergo domain closure to create the active sites, then the entire molecule cannot attain the high activity, high activity R state.

Association of the catalytic subunit of aspartate transcarbamoylase with a zinc-containing polypeptide fragment of the regulatory chain leads to increases in thermal stability

The regulatory enzyme aspartate transcarbamoylase (ATCase), comprising 2 catalytic (C) trimers and 3 regulatory (R) dimers, owes its stability to the manifold interchain interactions among the 12 polypeptide chains. With the availability of a recombinant 70-amino acid zinc-containing polypeptide fragment of the regulatory chain of ATCase, it has become possible to analyze directly the interaction between catalytic and regulatory chains in a complex of simpler structure independent of other interactions such as those between the 2 C trimers, which also contribute to the stability of the holoenzyme. Also, the effect of the interaction between the polypeptide, termed the zinc domain, and the C trimer on the thermal stability and other properties can be measured directly. Differential scanning microcalorimetry experiments demonstrated that the binding of the zinc domain to the C trimer leads to a complex of markedly increased thermal stability. This was shown with a series of mutant forms of the C trimer, which themselves varied greatly in their temperature of denaturation due to single amino acid replacements. With some C trimers, for which tm varied over a range of 30 degrees C due to diverse amino acid substitutions, the elevation of tm resulting from the interaction with the zinc domain was as large as 18 degrees C. The values of tm for a variety of complexes of mutant C trimers and the wild-type zinc domain were similar to those observed when the holoenzymes containing the mutant C trimers were subjected to heat denaturation.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptide-protein interaction markedly alters the functional properties of the catalytic subunit of aspartate transcarbamoylase

Interaction of a 70-amino acid zinc-binding polypeptide from the regulatory chain of aspartate transcarbamoylase (ATCase) with the catalytic (C) subunit leads to dramatic changes in enzyme activity and affinity for ligand binding at the active sites. The complex between the polypeptide (zinc domain) and wild-type C trimer exhibits hyperbolic kinetics in contrast to the sigmoidal kinetics observed with the intact holoenzyme. Moreover, the Scatchard plot for binding N-(phosphonacetyl)-L-aspartate (PALA) to the complex is linear with a Kd corresponding to that evaluated for the holoenzyme converted to the relaxed (R) state. Additional evidence that the binding of the zinc domain to the C trimer converts it to the R state was attained with a mutant form of ATCase in which Lys 164 in the catalytic chain is replaced by Glu. As shown previously (Newell, J.O. & Schachman, H.K., 1990, Biophys. Chem. 37, 183-196), this mutant holoenzyme, which exists in the R conformation even in the absence of active site ligands, has a 50-fold greater affinity for PALA than the free C subunit. Adding the zinc domain to the C trimer containing the Lys 164-->Glu substitution leads to a 50-fold enhancement in the affinity for the bisubstrate analog yielding a value of Kd equal to that for the holoenzyme. A different mutant ATCase containing the Gln 231 to Ile replacement was shown (Peterson, C.B., Burman, D.L., & Schachman, H.K., 1992, Biochemistry 31, 8508-8515) to be much less active as a holoenzyme than as the free C trimer. For this mutant holoenzyme, the addition of substrates does not cause its conversion to the R state. However, the addition of the zinc domain to the Gln 231-->Ile C trimer leads to a marked increase in enzyme activity, and PALA binding data indicate that the complex resembles the R state of the holoenzyme. This interaction leading to a more active conformation serves as a model of intergenic complementation in which peptide binding to a protein causes a conformational correction at a site remote from the interacting surfaces resulting in activation of the protein. This linkage was also demonstrated by difference spectroscopy using a chromophore covalently bound at the active site, which served as a spectral probe for a local conformational change. The binding of ligands at the active sites was shown also to lead to a strengthening of the interaction between the zinc domain and the C trimer.

Hybridization as a technique for studying interchain interactions in the catalytic trimers of aspartate transcarbamoylase

Since subunit interactions in regulatory enzymes mediate the ligand-promoted conformational changes responsible for their allosteric properties, it is necessary to have techniques for determining the effects of ligands and mutational alterations on the strength of the interchain interactions. In aspartate transcarbamoylase from Escherichia coli, the multiple interchain interactions are so linked that it is difficult to study them separately. Therefore, we have focused on the nonallosteric catalytic trimers isolated from the holoenzyme and have used the rate of hybrid formation between native and succinylated protein as a measure of the dissociation of the trimers into single polypeptide chains. Although catalytic trimers exhibit no evident dissociation in sedimentation studies at 10(-8) M, incubation of mixtures of native and succinylated trimers for long periods of time (days) yielded hybrids which are readily detected by polyacrylamide gel electrophoresis. This sensitive technique was used to demonstrate that the substrate, carbamoylphosphate, and the bisubstrate analog, N-(phosphonacetyl)-L-aspartate, cause a marked strengthening of the interchain interactions, whereas the inhibitor, sodium pyrophosphate, at concentrations as low as 10 mM, promotes dissociation of the trimers. This weakening of the interchain interactions by pyrophosphate facilitated the isolation and purification of functionally competent hybrid trimers by a technique which was much more convenient and provided higher yields than previous, more drastic methods which employed urea or guanidine hydrochloride to cause dissociation of the trimers. The hybridization technique was useful in studying the effects of mutational alterations on the strength of the interchain interactions and the ability of active and inactive mutants to bind pyrophosphate.

2.5 A structure of aspartate carbamoyltransferase complexed with the bisubstrate analog N-(phosphonacetyl)-L-aspartate

In an X-ray diffraction study using the method of multiple isomorphous replacement, the structure of aspartate carbamoyltransferase (EC 2.1.3.2) complexed with the bisubstrate analog N-(phosphonacetyl)-L-aspartate (PALA) has been solved to 2.5 A. Ten rounds of model building and 123 cycles of restrained reciprocal space refinement have resulted in a model containing 94.4% of the theoretical atoms of the protein-inhibitor complex with an R-factor of 0.231. The fit of the model to the density is excellent, except for occasional side-chains and two sections of the regulatory chains that may be disordered. The electron density for the PALA molecule is readily identifiable for both catalytic (c) chains of the asymmetric unit and bonding interactions with several important residues including Ser52, Arg54, Thr55, Ser80, Lys84, Arg105, His134, Arg165, Arg229 and Gln231 are apparent. The carboxylate groups of the PALA molecule are in a nearly cis conformation. Gross quaternary changes between the T and R forms are noted and in agreement with earlier work from this laboratory. Namely, in the new structure the catalytic trimers move apart by 12 A along the 3-fold axis of the enzyme and relocate by 10 degrees relative to each other, adopting a more eclipsed position. The regulatory (r) chains in the new structure reorient about their 2-fold axis by 15 degrees. Large tertiary changes that include domain migration and rearrangement are also present between these two forms. In the R form both domains of the catalytic chain relocate closer to each other in order to bind to the inhibitor. The polar domain seems to bind primarily to the carbamoyl phosphate moiety of PALA, and the equatorial domain binds primarily to the L-aspartate moiety. Other changes in tertiary structure bring the 80s loop (from an adjacent catalytic chain) and the 240s loop into a position to interact with the PALA molecule. Changes have been searched for in all interface regions of the enzyme. While the C1-C4 and C1-R4 regions have been completely altered, most of the other interchain interfaces are similar in the T and R forms. The intrachain interfaces, between domains of the same catalytic chains, have undergone some reorganization as these domains move closer to each other when the inhibitor is bound. This new structure allows a reinterpretation of genetic and chemical modification studies done to date.(ABSTRACT TRUNCATED AT 400 WORDS)