Site-specific Mutation of Tyr240 → Phe in the Catalytic Chain of Escherichia coli Aspartate Transcarbamylase

Threonine 82 in the regulatory chain is important for nucleotide affinity and for the allosteric stabilization of Escherichia coli aspartate transcarbamoylase

The three-dimensional structure of Escherichia coli aspartate transcarbamoylase complexed with the allosteric effector CTP, shows an interaction between the hydroxyl of Thr-82 in the regulatory chain (Thr-82r) with the gamma-phosphate of CTP (R.P. Kosman, J.E. Gouaux, W.N. Lipscomb, Crystal structure of CTP-ligated T state aspartate transcarbamoylase at 2.5 A resolution: implications for aspartate transcarbamoylase mutants and the mechanism of negative cooperativity, Proteins Struct. Funct. Genet. 15 (1993) 147-176). In order to determine whether the Thr-82r interaction with the gamma-phosphate of CTP is important for either binding of the nucleotide effectors or their function, site-specific mutagenesis was employed. The mutant enzyme in which Thr-82r was replaced by Ala had almost the identical maximal observed specific activity as the wild-type enzyme; however, the mutant enzyme had a significantly increased [Asp]0.5, the aspartate concentration at one-half the maximal observed specific activity, as well as slightly increased homotropic cooperativity. The mutant enzyme was also activated more by ATP and inhibited less by CTP as compared to the wild-type enzyme. In addition, the nucleotide concentration required for one-half maximal effect was increased approx. 3-fold as compared to the corresponding values for the wild-type enzyme. The maximal inhibition of the mutant enzyme, in the presence of UTP and CTP was similar to that observed for the wild-type enzyme; however, higher concentrations of the nucleotides were required to achieve this level of inhibition. The reduced affinity of CTP, UTP and ATP induced by the mutation indicates that the hydrogen bonding interaction between the gamma-phosphate of the nucleotide effector and the side-chain hydroxyl of Thr-82r is important for the binding of the nucleotide effectors to the allosteric site. Furthermore, this interaction is important for the discrimination between CTP and CDP. Finally, the greater homotropic cooperativity, greater [Asp]0.5, diminished CTP inhibition and greater ATP activation of the mutant enzyme correlates with the X-ray structure of the mutant enzyme which shows that the unligated enzyme is in an 'extreme' T-state. These findings add support to the theory that the global stabilization of the enzyme is critical for both the homotropic and heterotropic properties of aspartate transcarbamoylase.

A continuous spectrophotometric assay for aspartate transcarbamylase and ATPases

A new continuous coupled uv-spectrophotometric assay is described for two phosphate-releasing enzymes, aspartate transcarbamylase and ATPase of herpes simplex virus (HSV). Phosphate release is coupled to the phosphorolysis of the nucleoside analog 7-methylinosine (m7Ino) catalyzed by purine nucleoside phosphorylase. When this reaction is monitored at 291 nm, the coupled assay can readily detect 10 nmol Pi released/min. Our method offers advantages over a recently reported continuous assay devised for measuring aspartate transcarbamylase activity using the nucleoside analog methylthioguanosine (MESG) as the linking substrate. In contrast to MESG, m7Ino is easily and inexpensively synthesized and is also commercially available. The spectrophotometric signal at 291 nm, produced by the difference in the extinction coefficients between nucleoside substrate and the base product, is significant over a much wider pH range than the signal difference between MESG and its phosphorolysis product at 360 nm. Saturation curves for aspartate and carbamyl phosphate and pH rate profiles have been reproduced using the purine nucleoside phosphorylase/m7Ino coupled assay. Initial velocity patterns constructed over micromolar to millimolar concentrations of aspartate and carbamyl phosphate yielded four kinetic parameters simultaneously. To further illustrate the application of this coupled assay, kinetic parameters were determined for the DNA-dependent ATPase reaction of HSV helicase-primase.

Domain closure in the catalytic chains of Escherichia coli aspartate transcarbamoylase influences the kinetic mechanism

The closure of the two domains of the catalytic chains of Escherichia coli aspartate transcarbamoylase, which is critical for completion of the T-->R transition, is stabilized by salt-bridges between Glu-50 and both Arg-167 and Arg-234. Mutation of Glu-50 to Ala shifts the enzyme toward a low activity, low affinity state (Newton, C. J., and Kantrowitz, E. R. (1990) Biochemistry, 29, 1444-1451). Kinetic isotope effects (KIE) and equilibrium isotope exchange kinetics (EIEK) have been used to probe the dynamic properties of the Glu-50-->Ala enzyme. Unlike the behavior of the wild-type enzyme, the observed kinetic isotope effect for 13C versus 12C at the carbonyl group of carbamoyl phosphate (CP) increased upon the binding of ligands which promote the formation of the R-state (Asp, N-phosphonacetyl-L-aspartate (PALA), or ATP). The maximum rate for the [14C]Asp<-->Carbamoyl aspartate (CAsp) exchange with the Glu-50-->Ala enzyme was 500-fold slower than for the wild-type enzyme; however, the rate for the [14C]CP<-->CAsp exchange was only 50-fold slower, reversing the relative rates observed with the wild-type enzyme. In addition, upon variation of substrate pairs involving Asp or CAsp, loss of inhibition effects in the CP<-->CAsp exchange indicated that the Glu-50-->Ala substitution caused the kinetic mechanism for the mutant enzyme to shift from ordered to random. Computer simulations of the EIEK data indicate that the Glu-50-->Ala mutation specifically causes strong decreases in the rates of catalysis and association-dissociation for Asp and CAsp, with minimal effects on the CP and Pi on-off rates. With substrates bound, the Glu-50-->Ala enzyme apparently does not attain a full R-state conformation. The PALA-activated Glu-50-->Ala enzyme, however, exhibits substrate affinities comparable to those for the wild-type enzyme, but fails to restore the preferred order substrate binding. Unlike the wild-type enzyme, both the T and R-states of the Glu-50-->Ala enzyme contribute to catalysis. A third state, I, is proposed for the Glu-50-->Ala enzyme, in which random order substrate binding is exhibited, and the catalytic step contributes significantly to overall rate limitation.

L-aspartate association contributes to rate limitation and induction of the T-->R transition in Escherichia coli aspartate transcarbamoylase. Equilibrium exchanges and kinetic isotope effects with a Vmax-enhanced mutant, Asp-236-->Ala

Equilibrium isotope exchange kinetics (EIEK) and kinetic isotope effects have been used to determine the mechanistic basis for the altered kinetic characteristics of a mutant version of Escherichia coli aspartate transcarbamylase in which Asp-236 of the catalytic chain is replaced by alanine (Asp-236-->Ala). The [14C]Asp<--> N-carbamyl-L-aspartate (CAsp) and [14C]CP<-->CAsp exchange rates, observed as a function of various reactant-product pairs, exhibited dramatic increases in maximal rates, along with decreases in substrate half-saturation values and cooperativity. The carbon kinetic isotope effect, 13C versus 12C at the carbonyl group of carbamoyl phosphate, for the Asp-236-->Ala enzyme decreased toward unity as [Asp] increased, as observed for the wild-type enzyme. Both the kinetic isotope effects and EIEK results indicate that the Asp-236-->Ala enzyme operates by the same ordered kinetic mechanism as the wild-type enzyme. Although activation effects by ATP and N-phosphonacetyl-L-aspartate are lost, inhibition by CTP was apparent in equilibrium exchanges. Simulation of the EIEK data indicated that the best fit to the observed changes in saturation curves was obtained by preferentially increasing the rate of the T-->R transition, kappa T-->R, thereby destabilizing the T-state and increasing the equilbrium constant for the T<-->R transition. A multistep model for Asp bindng to aspartate transcarbamoylase is proposed, in which Asp induces the initial conformational changes that in turn trigger the T-->R transition, followed by stepwise filling of the remaining active sites.

Preferred order random kinetic mechanism for homoserine dehydrogenase of Escherichia coli (Thr-sensitive) aspartokinase/homoserine dehydrogenase-I: equilibrium isotope exchange kinetics

Isotope exchange kinetics at chemical equilibrium have been used to investigate the kinetic mechanism of homoserine dehydrogenase (EC 1.1.1.3) of the (Thr-sensitive) aspartokinase/homoserine dehydrogenase-I multifunctional enzyme from E. coli. For the reaction (L-ASA + NADPH + H+ = L-Hse + NADP+), at pH 9.0, 37 degrees C, Keq = 100 (+/- 20). Under these conditions, the rate for exchange of [14C]-L-homoserine (Hse) in equilibrium L-aspartate-beta-semialdehyde (ASA) is nearly twice that for the [3H]-NADP+ in equilibrium NADPH exchange. This indicates that covalent interconversion between reactants and products bound in the active site cannot be rate-limiting. Upon variation of the concentrations of all four substrates in constant ratio at equilibrium (to minimize dead-end complex formation), the Hse in equilibrium ASA exchange increased smoothly toward a maximum. In contrast, the NADP+ in equilibrium NADPH exchange rate increased to a maximum value at partial saturation, then decreased to approximately half the maximum rate. These data are consistent with a preferred-order random kinetic mechanism in which the dominant pathway involves association of NADPH prior to L-ASA and dissociation of L-Hse prior to NADP+.