Catalytic-regulatory subunit interactions and allosteric effects in aspartate transcarbamylase

The conserved residues glutamate-37, aspartate-100, and arginine-269 are important for the structural stabilization of Escherichia coli aspartate transcarbamoylase

Aspartate transcarbamoylase from Escherichia coli is a dodecameric enzyme consisting of two trimeric catalytic subunits and three dimeric regulatory subunits. The X-ray structure of this enzyme indicates that the side chains of His-41, Asp-100, and Asp-90 from one catalytic chain form interactions with the side chains of Glu-37, Arg-65, and Arg-269, respectively, from an adjacent catalytic chain. In order to determine whether these interactions are important for the structural stabilization of the enzyme and/or homotropic and heterotropic effects, four mutant versions of aspartate transcarbamoylase, Glu-37-->Ala, Asp-100-->Asn, Asp-100-->Ala, and Arg-269-->Ala, were created by site-specific mutagenesis. The Glu-37-->Ala holoenzyme exhibits essentially wild-type behavior with respect to homotropic cooperativity and heterotropic regulation by ATP and CTP. The Glu-37-->Ala catalytic subunit exhibits a half-life of inactivation at 69 +/- 0.5 degrees C of 4.9 min, as compared to 5.8 min for the wild-type catalytic subunit. The Asp-100-->Asn and Asp-100-->Ala holoenzymes are slightly more active than the wild-type holoenzyme, exhibit 1.4-fold and 1.8-fold reductions in the aspartate concentration at half the maximal specific activity, respectively, and show increased affinities for ATP and CTP. Both the Asp-100-->Asn and Asp-100--> Ala catalytic subunits exhibit a 2-fold reduction in the half-life of inactivation at 69 +/- 0.5 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Function of serine-52 and serine-80 in the catalytic mechanism of Escherichia coli aspartate transcarbamoylase

Carbamoyl phosphate is held in the active site of Escherichia coli aspartate transcarbamoylase by a variety of interactions with specific side chains of the enzyme. In particular, oxygens of the phosphate of carbamoyl phosphate interact with Ser-52, Thr-53 (backbone), Arg-54, Thr-55, and Arg-105 from one catalytic chain, as well as Ser-80 and Lys-84 from an adjacent chain in the same catalytic subunit. In order to define the role of Ser-52 and Ser-80 in the catalytic mechanism, two mutant versions of the enzyme were created with Ser-52 or Ser-80 replaced by alanine. The Ser-52----Ala holoenzyme exhibits a 670-fold reduction in maximal observed specific activity, and a loss of both aspartate and carbamoyl phosphate cooperativity. This mutation also causes 23-fold and 5.6-fold increases in the carbamoyl phosphate and aspartate concentrations required for half the maximal observed specific activity, respectively. Circular dichroism spectroscopy indicates that saturating carbamoyl phosphate does not induce the same conformational change in the Ser-52----Ala holoenzyme as it does for the wild-type holoenzyme. The kinetic properties of the Ser-52----Ala catalytic subunit are altered to a lesser extent than the mutant holoenzyme. The maximal observed specific activity is reduced by 89-fold, and the carbamoyl phosphate concentration at half the maximal observed velocity increases by 53-fold while the aspartate concentration at half the maximal observed velocity increases 6-fold.(ABSTRACT TRUNCATED AT 250 WORDS)

Co-operative interactions between the catalytic sites in Escherichia coli aspartate transcarbamylase. Role of the C-terminal region of the regulatory chains

In aspartate transcarbamylase (ATCase) each regulatory chain interacts with two catalytic chains each one belonging to a different trimeric catalytic subunit (R1-C1 and R1-C4 types of interactions as defined in Fig. 1). In order to investigate the interchain contacts that are involved in the co-operative interactions between the catalytic sites, a series of modified forms of the enzyme was prepared by site-directed mutagenesis. The amino acid replacements were devised on the basis of the previously described properties of an altered form of ATCase (pAR5-ATCase) which lacks the homotropic co-operative interactions between the catalytic sites. The results obtained (enzyme kinetics, bisubstrate analog influence and pH studies) show that the R1-C4 interaction is essential for the establishment of the enzyme conformation that has a low affinity for aspartate (T state), and consequently for the existence of co-operativity between the catalytic sites. This interaction involves the 236-250 region of the aspartate binding domain of the catalytic chain (240s loop) and the 143-149 region of the regulatory chain which comprises helix H3'.

The catalytic site of Escherichia coli aspartate transcarbamylase: interaction between histidine 134 and the carbonyl group of the substrate carbamyl phosphate

Previous pKa determinations indicated that histidine 134, present in the catalytic site of aspartate transcarbamylase, might be the group involved in the binding of the substrate carbamyl phosphate and, possibly, in the catalytic efficiency of this enzyme. In the present work, this residue was replaced by an asparagine through site-directed mutagenesis. The results obtained show that histidine 134 is indeed the group of the enzyme whose deprotonation increases the affinity of the catalytic site for carbamyl phosphate. In the wild-type enzyme this group can be titrated only by those carbamyl phosphate analogues that bear the carbonyl group. In the modified enzyme the group whose deprotonation increases the catalytic efficiency is still present, indicating that this group is not the imidazole ring of histidine 134 (pKa = 6.3). In addition, the pKa of the still unknown group involved in aspartate binding is shifted by one unit in the mutant as compared to the wild type.

Importance of domain closure for homotropic cooperativity in Escherichia coli aspartate transcarbamylase

The importance of the interdomain bridging interactions observed only in the R-state structure of Escherichia coli aspartate transcarbamylase between Glu-50 of the carbamoyl phosphate domain with both Arg-167 and Arg-234 of the aspartate domain has been investigated by using site-specific mutagenesis. Two mutant versions of aspartate transcarbamylase were constructed, one with alanine at position 50 (Glu-50----Ala) and the other with aspartic acid at position 50 (Glu-50----Asp). The alanine substitution totally prevents the interdomain bridging interactions, while the aspartic acid substitution was expected to weaken these interactions. The Glu-50----Ala holoenzyme exhibits a 15-fold loss of activity, no substrate cooperativity, and a more than 6-fold increase in the aspartate concentration at half the maximal observed specific activity. The Glu-50----Asp holoenzyme exhibits a less than 3-fold loss of activity, reduced cooperativity for substrates, and a 2-fold increase in the aspartate concentration at half the maximal observed specific activity. Although the Glu-50----Ala enzyme exhibits no homotropic cooperativity, it is activated by N-(phosphonoacetyl)-L-aspartate (PALA). As opposed to the wild-type enzyme, the Glu-50----Ala enzyme is activated by PALA at saturating concentrations of aspartate. At subsaturating concentrations of aspartate, both mutant enzymes are activated by ATP, but are inhibited less by CTP than is the wild-type enzyme. At saturating concentrations of aspartate, the Glu-50----Ala enzyme is activated by ATP and inhibited by CTP to an even greater extent than at subsaturating concentrations of aspartate.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular characterization of a Dictyostelium discoideum gene encoding a multifunctional enzyme of the pyrimidine pathway

We have isolated and characterized a Dictyostelium discoideum gene (PYR1-3) encoding a multifunctional protein that carries the three first enzymatic activities of the de novo pyrimidine biosynthetic pathway. The PYR1-3 gene is adjacent to another gene of the pyrimidine biosynthetic pathway (PYR4); the two genes are separated by a 1.5-kb non-coding sequence and transcribed divergently. The PYR1-3 gene is transcribed to form a 7.5-kb polyadenylated mRNA. As with the other genes of the pyrimidine biosynthetic pathway, the PYR1-3 mRNA level is high during growth and decreases sharply during development. We have determined the nucleotide sequence of 63% of the coding region of the PYR1-3 gene. We have identified the activities of the protein encoded by the D. discoideum PYR1-3 gene by comparison of amino acid sequences with the products of genes of known function. The PYR1-3 gene contains four distinct regions that probably correspond to four domains in the protein. From the NH2 extremity to the COOH extremity, these domains are: glutamine amidotransferase, carbamoylphosphate synthetase, dihydroorotase and aspartate transcarbamylase. This organization is identical to the one found in the rudimentary gene of Drosophila. The evolutionary implications of this finding are discussed.

Propagation of allosteric changes through the catalytic-regulatory interface of Escherichia coli aspartate transcarbamylase

Each of two previously isolated strains of Escherichia coli containing a single nonsense codon within the pyrB gene was suppressed with four different nonsense suppressors. The kinetic analysis using crude extracts of these nonsense-suppressed strains indicated that the mutant aspartate transcarbamylases had altered cooperativity and affinity for aspartate as judged by the substrate concentration at half of the maximal velocity. Both pyrB genes were cloned and then sequenced. In both cases, a single base change was identified which converted a glutamine GAC codon into a TAC nonsense codon. Both mutations occurred in the catalytic chain of aspartate transcarbamylase and were identified at positions 108 and 246. The glutamine at position 108 in the wild-type structure is located at the interface between the catalytic and regulatory chains and is involved in a number of interactions with backbone and side chains of the regulatory chain. The glutamine at position 246 in the wild-type structure is located in the 240s loop of the enzyme. Two additional mutant versions of aspartate transcarbamylase were created by site-directed mutagenesis to further investigate the 108-position in the structure, a glutamine to tyrosine substitution at position 108 of the catalytic chain, and an asparagine to glycine change at position 113 of the regulatory chain, a residue which interacts directly with glutamine-108 in the wild-type structure. Both mutant enzymes have reduced affinity for aspartate. However, the Tyr-108 mutant enzyme exhibits a reduced Hill coefficient while the Gly-113 enzyme exhibits an increased Hill coefficient. The response to the allosteric effectors ATP and CTP is also changed for both the mutant enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)