The publication data currently available has been vetted by Vanderbilt faculty, staff, administrators and trainees. The data itself is retrieved directly from NCBI's PubMed and is automatically updated on a weekly basis to ensure accuracy and completeness.
If you have any questions or comments, please contact us.
Many oligomeric enzymes are functional only in the assembled form, and it is often difficult to determine unambiguously why monomers are inactive. In some cases individual monomers cannot fold into stable correct ("native") conformations without contributions from interchain interactions. For other oligomers, catalysis requires the contributions of amino acid residues at the interface between adjacent polypeptide chains, and monomers are inactive because they cannot form complete active sites. A test for the presence of shared sites was devised that is based on the formation of active hybrid oligomers from appropriate inactive parental mutants produced by site-directed mutagenesis. This approach was applied in a study of the catalytic trimer of aspartate transcarbamoylase (aspartate carbamoyltransferase, EC 18.104.22.168) from Escherichia coli, using three mutants, in which Ser-52 was replaced by His, Lys-84 by Gln, or His-134 by Ala. Hybrid trimers formed from the virtually inactive Ser and Lys mutants were 10(5) more active than the parental proteins, and the specific activities of each hybrid were about 33% that of the wild-type trimer, as expected for the scheme based on shared sites. Hybrids from the His and Lys mutants had comparable specific activities. Moreover, one hybrid with approximately 33% activity had one high-affinity binding site for a bisubstrate analog as compared to about three for wild-type trimer. As a further test, hybrids were also formed from wild-type and double-mutant (Lys-84----Gln and His-134----Ala) trimers. The hybrid containing two chains with the double mutation and one wild-type chain had very little activity, and that composed of one double mutant and two wild-type chains had 32% the specific activity of wild-type trimers. This negative complementation experiment is in quantitative accord with the scheme based on shared sites at or near the interfaces between adjacent chains. The techniques used to demonstrate shared active sites in the catalytic subunits of aspartate transcarbamoylase can be applied generally to various types of oligomers (dimers, tetramers, etc.) to determine whether the participation of amino acid residues from adjoining chains is essential for forming active sites in oligomeric enzymes.
Although intensive investigations have been conducted on the allosteric enzyme, aspartate transcarbamoylase, which catalyzes the first committed reaction in the biosynthesis of pyrimidines in Escherichia coli, little is known about the role of individual amino acid residues in catalysis or regulation. Two inactive enzymes produced by random mutagenesis have been characterized previously but the loss of activity is probably attributable to changes in the folding of the chains stemming from the introduction of charged and bulky residues (Asp for Gly-128 and Phe for Ser-52). Site-directed mutagenesis of pyrB, which encodes the catalytic chains of the enzyme, was used to probe the functional roles of several amino acids by making more conservative substitutions. Replacement of Lys-84 by either Gln or Arg leads to virtually inactive enzymes, confirming chemical studies indicating that Lys-84 is essential for catalysis. In contrast, substitution of Gln for Lys-83 has only a slight effect on enzyme activity, whereas chemical modification causes considerable inactivation. Gln-133, which has been shown by x-ray crystallography to reside near the contact region between the catalytic and regulatory chains, was replaced by Ala. This substitution has little effect on catalytic activity but leads to a marked increase in cooperativity. The Gln-83 mutant, in contrast, exhibits much less cooperativity. Since a histidine residue may be involved in catalysis and His-134 has been shown by x-ray diffraction studies to be in close proximity to the site of binding of a bisubstrate analog, His-134 was replaced by Ala, yielding a mutant with only 5% wild-type activity, considerable cooperativity, and lower affinity for aspartate and carbamoylphosphate. All of the mutants, unlike those in which charged or bulky residues replaced small side chains, bind the bisubstrate analog, which promotes the characteristic "swelling" of the enzymes indicative of the allosteric transition.
Site-directed mutagenesis was used to determine how the allosteric properties of aspartate transcarbamoylase (ATCase) are affected by amino acid replacements in the nucleotide binding region of the regulatory polypeptide chains. Amino acid substitutions were made for both Lys-60 and Lys-94 in the regulatory chain since those residues have been implicated by x-ray diffraction studies, chemical modification experiments, and site-directed mutagenesis as playing a role in binding CTP and ATP. Lys-60 was replaced by His, Arg, Gln, and Ala, and Lys-94 was changed to His. These mutant forms of ATCase exhibit bewildering changes in the allosteric properties compared to the wild-type enzyme as well as altered affinities for the nucleotide effectors. The enzyme containing His-60 lacks both homotropic and heterotropic effects and exhibits no detectable binding of nucleotides. In contrast, the holoenzymes containing either Gln-60 or Arg-60 retain both homotropic and heterotropic effects. Replacement of Lys-60 by Ala yields a derivative exhibiting altered heterotropic effects involving insensitivity to CTP and activation by ATP. The mutant enzyme containing His-94 in place of Lys exhibits cooperativity with reduced affinity for nucleotides. The multiple substitutions at Lys-60 in the nucleotide binding region of the regulatory chains of ATCase demonstrate that different amino acids in the same location can alter indirectly the delicate balance of interactions responsible for the allosteric properties of ATCase. The studies show that it is hazardous and frequently unwarranted from single amino acid replacements of a specific residue to attribute to that residue the properties observed for the wild-type enzyme.