A cooperative Escherichia coli aspartate transcarbamoylase without regulatory subunits

New regulatory mechanism-based inhibitors of aspartate transcarbamoylase for potential anticancer drug development

Aspartate transcarbamoylase (ATCase) is a key enzyme which regulates and catalyzes the second step of de novo pyrimidine synthesis in all organisms. Escherichia coli ATCase is a prototypic enzyme regulated by both product feedback and substrate cooperativity, whereas human ATCase is a potential anticancer target. Through structural and biochemical analyses, we revealed that R167/130's loop region in ATCase serves as a gatekeeper for the active site, playing a new and unappreciated regulatory role in the catalytic cycle of ATCase. Based on virtual compound screening simultaneously targeting the new regulatory region and active site of human ATCase, two compounds were identified to exhibit strong inhibition of ATCase activity, proliferation of multiple cancer cell lines, and growth of xenograft tumors. Our work has not only revealed a previously unknown regulatory region of ATCase that helps uncover the catalytic and regulatory mechanism of ATCase, but also successfully guided the identification of new ATCase inhibitors for anticancer drug development using a dual-targeting strategy. DATABASE: Structure data are available in Protein Data Bank under the accession numbers: 6KJ7 (G166P ecATCase), 6KJ8 (G166P ecATCase-holo), 6KJ9 (G128/130A ecATCase), and 6KJA (G128/130A ecATCase-holo).

Solution NMR Spectroscopy for the Study of Enzyme Allostery

Allostery is a ubiquitous biological regulatory process in which distant binding sites within a protein or enzyme are functionally and thermodynamically coupled. Allosteric interactions play essential roles in many enzymological mechanisms, often facilitating formation of enzyme-substrate complexes and/or product release. Thus, elucidating the forces that drive allostery is critical to understanding the complex transformations of biomolecules. Currently, a number of models exist to describe allosteric behavior, taking into account energetics as well as conformational rearrangements and fluctuations. In the following Review, we discuss the use of solution NMR techniques designed to probe allosteric mechanisms in enzymes. NMR spectroscopy is unequaled in its ability to detect structural and dynamical changes in biomolecules, and the case studies presented herein demonstrate the range of insights to be gained from this valuable method. We also provide a detailed technical discussion of several specialized NMR experiments that are ideally suited for the study of enzymatic allostery.

From Genome to Structure and Back Again: A Family Portrait of the Transcarbamylases

Enzymes in the transcarbamylase family catalyze the transfer of a carbamyl group from carbamyl phosphate (CP) to an amino group of a second substrate. The two best-characterized members, aspartate transcarbamylase (ATCase) and ornithine transcarbamylase (OTCase), are present in most organisms from bacteria to humans. Recently, structures of four new transcarbamylase members, N-acetyl-l-ornithine transcarbamylase (AOTCase), N-succinyl-l-ornithine transcarbamylase (SOTCase), ygeW encoded transcarbamylase (YTCase) and putrescine transcarbamylase (PTCase) have also been determined. Crystal structures of these enzymes have shown that they have a common overall fold with a trimer as their basic biological unit. The monomer structures share a common CP binding site in their N-terminal domain, but have different second substrate binding sites in their C-terminal domain. The discovery of three new transcarbamylases, l-2,3-diaminopropionate transcarbamylase (DPTCase), l-2,4-diaminobutyrate transcarbamylase (DBTCase) and ureidoglycine transcarbamylase (UGTCase), demonstrates that our knowledge and understanding of the spectrum of the transcarbamylase family is still incomplete. In this review, we summarize studies on the structures and function of transcarbamylases demonstrating how structural information helps to define biological function and how small structural differences govern enzyme specificity. Such information is important for correctly annotating transcarbamylase sequences in the genome databases and for identifying new members of the transcarbamylase family.

Crystal structure and biochemical properties of putrescine carbamoyltransferase from Enterococcus faecalis: Assembly, active site, and allosteric regulation

Putrescine carbamoyltransferase (PTCase) catalyzes the conversion of carbamoylputrescine to putrescine and carbamoyl phosphate (CP), a substrate of carbamate kinase (CK). The crystal structure of PTCase has been determined and refined at 3.2 Å resolution. The trimeric molecular structure of PTCase is similar to other carbamoyltransferases, including the catalytic subunit of aspartate carbamoyltransferase (ATCase) and ornithine carbamoyltransferase (OTCase). However, in contrast to other trimeric carbamoyltransferases, PTCase binds both CP and putrescine with Hill coefficients at saturating concentrations of the other substrate of 1.53 ± 0.03 and 1.80 ± 0.06, respectively. PTCase also has a unique structural feature: a long C-terminal helix that interacts with the adjacent subunit to enhance intersubunit interactions in the molecular trimer. The C-terminal helix appears to be essential for both formation of the functional trimer and catalytic activity, since truncated PTCase without the C-terminal helix aggregates and has only 3% of native catalytic activity. The active sites of PTCase and OTCase are similar, with the exception of the 240's loop. PTCase lacks the proline-rich sequence found in knotted carbamoyltransferases and is unknotted. A Blast search of all available genomes indicates that 35 bacteria, most of which are Gram-positive, have an agcB gene encoding PTCase located near the genes that encode agmatine deiminase and CK, consistent with the catabolic role of PTCase in the agmatine degradation pathway. Sequence comparisons indicate that the C-terminal helix identified in this PTCase structure will be found in all other PTCases identified, suggesting that it is the signature feature of the PTCase family of enzymes.

Allostery and cooperativity in Escherichia coli aspartate transcarbamoylase

The allosteric enzyme aspartate transcarbamoylase (ATCase) from Escherichia coli has been the subject of investigations for approximately 50 years. This enzyme controls the rate of pyrimidine nucleotide biosynthesis by feedback inhibition, and helps to balance the pyrimidine and purine pools by competitive allosteric activation by ATP. The catalytic and regulatory components of the dodecameric enzyme can be separated and studied independently. Many of the properties of the enzyme follow the Monod, Wyman Changeux model of allosteric control thus E. coli ATCase has become the textbook example. This review will highlight kinetic, biophysical, and structural studies which have provided a molecular level understanding of how the allosteric nature of this enzyme regulates pyrimidine nucleotide biosynthesis.