The structure of LL-diaminopimelate aminotransferase from Chlamydia trachomatis: implications for its broad substrate specificity.

We have previously reported the structures of the native holo and substrate-bound forms of LL-diaminopimelate aminotransferase from Arabidopsis thaliana (AtDAP-AT). Here, we report the crystal and molecular structures of the LL-diaminopimelate aminotransferase from Chlamydia trachomatis (CtDAP-AT) in the apo-form and the pyridoxal-5'-phosphate-bound form. The molecular structure of CtDAP-AT shows that its overall fold is essentially identical with that of AtDAP-AT except that CtDAP-AT adopts an "open" conformation as opposed to the "closed" conformation of AtDAP-AT. Although AtDAP-AT and CtDAP-AT are approximately 40% identical in their primary sequence, they have major differences in their substrate specificities; AtDAP-AT is highly specific for LL-DAP, whereas CtDAP-AT accepts a wider range of substrates. Since all of the residues involved in substrate recognition are highly conserved between AtDAP-AT and CtDAP-AT, we propose that differences in flexibility of the loops lining the active-site region between the two enzymes likely account for the differences in substrate specificity.

Exploration of inhibitors for diaminopimelate aminotransferase.

Bacteria and higher plants make l-lysine from diaminopimelic acid (DAP). In mammals l-lysine is an essential amino acid that must be acquired from the diet as the biosynthetic pathway is absent for this key constituent of proteins. Recently, ll-diaminopimelate aminotransferase (ll-DAP-AT), a pyridoxal-5'-phosphate (PLP)-dependent enzyme, was reported to catalyze a key step in the route to l-lysine in plants and Chlamydia. Specific inhibitors of this enzyme could thus potentially serve as herbicides or antibiotics that are non-toxic to mammals. In this work, 29,201 inhibitors were screened against ll-DAP-AT and the IC(50) values were determined for the top 46 compounds. An aryl hydrazide and rhodanine derivatives were further modified to generate 20 analogues that were also tested against ll-DAP-AT. These analogues provide additional structure-activity relationships (SAR) that are useful in guiding further design of inhibitors.

Mechanism of substrate recognition and PLP-induced conformational changes in LL-diaminopimelate aminotransferase from Arabidopsis thaliana.

LL-Diaminopimelate aminotransferase (LL-DAP-AT), a pyridoxal phosphate (PLP)-dependent enzyme in the lysine biosynthetic pathways of plants and Chlamydia, is a potential target for the development of herbicides or antibiotics. This homodimeric enzyme converts L-tetrahydrodipicolinic acid (THDP) directly to LL-DAP using L-glutamate as the source of the amino group. Earlier, we described the 3D structures of native and malate-bound LL-DAP-AT from Arabidopsis thaliana (AtDAP-AT). Seven additional crystal structures of AtDAP-AT and its variants are reported here as part of an investigation into the mechanism of substrate recognition and catalysis. Two structures are of AtDAP-AT with reduced external aldimine analogues: N-(5'-phosphopyridoxyl)-L-glutamate (PLP-Glu) and N-(5'-phosphopyridoxyl)- LL-Diaminopimelate (PLP-DAP) bound in the active site. Surprisingly, they reveal that both L-glutamate and LL-DAP are recognized in a very similar fashion by the same sets of amino acid residues; both molecules adopt twisted V-shaped conformations. With both substrates, the alpha-carboxylates are bound in a salt bridge with Arg404, whereas the distal carboxylates are recognized via hydrogen bonds to the well-conserved side chains of Tyr37, Tyr125 and Lys129. The distal C(epsilon) amino group of LL-DAP is specifically recognized by several non-covalent interactions with residues from the other subunit (Asn309*, Tyr94*, Gly95*, and Glu97* (Amino acid designators followed by an asterisk (*) indicate that the residues originate in the other subunit of the dimer)) and by three bound water molecules. Two catalytically inactive variants of AtDAP-AT were created via site-directed mutagenesis of the active site lysine (K270N and K270Q). The structures of these variants permitted the observation of the unreduced external aldimines of PLP with L-glutamate and with LL-DAP in the active site, and revealed differences in the torsion angle about the PLP-substrate bond. Lastly, an apo-AtDAP-AT structure missing PLP revealed details of conformational changes induced by PLP binding and substrate entry into the active site.

Crystal structure of LL-diaminopimelate aminotransferase from Arabidopsis thaliana: a recently discovered enzyme in the biosynthesis of L-lysine by plants and Chlamydia.

The essential biosynthetic pathway to l-Lysine in bacteria and plants is an attractive target for the development of new antibiotics or herbicides because it is absent in humans, who must acquire this amino acid in their diet. Plants use a shortcut of a bacterial pathway to l-Lysine in which the pyridoxal-5'-phosphate (PLP)-dependent enzyme ll-diaminopimelate aminotransferase (LL-DAP-AT) transforms l-tetrahydrodipicolinic acid (L-THDP) directly to LL-DAP. In addition, LL-DAP-AT was recently found in Chlamydia sp., suggesting that inhibitors of this enzyme may also be effective against such organisms. In order to understand the mechanism of this enzyme and to assist in the design of inhibitors, the three-dimensional crystal structure of LL-DAP-AT was determined at 1.95 A resolution. The cDNA sequence of LL-DAP-AT from Arabidopsis thaliana (AtDAP-AT) was optimized for expression in bacteria and cloned in Escherichia coli without its leader sequence but with a C-terminal hexahistidine affinity tag to aid protein purification. The structure of AtDAP-AT was determined using the multiple-wavelength anomalous dispersion (MAD) method with a seleno-methionine derivative. AtDAP-AT is active as a homodimer with each subunit having PLP in the active site. It belongs to the family of type I fold PLP-dependent enzymes. Comparison of the active site residues of AtDAP-AT and aspartate aminotransferases revealed that the PLP binding residues in AtDAP-AT are well conserved in both enzymes. However, Glu97* and Asn309* in the active site of AtDAP-AT are not found at similar positions in aspartate aminotransferases, suggesting that specific substrate recognition may require these residues from the other monomer. A malate-bound structure of AtDAP-AT allowed LL-DAP and L-glutamate to be modelled into the active site. These initial three-dimensional structures of LL-DAP-AT provide insight into its substrate specificity and catalytic mechanism.

Aryl annulation of cyclic ketones via a magnesium carbometalation-6-pi- electrocyclization protocol.

A new strategy for the aryl annulation of cyclic ketones is described. Palladium(0) coupling of a propargyl alcohol with the enol triflate of a ketone and addition of vinylmagnesium chloride generates a triene as a magnesium chelate that may be quenched with an electrophile. In some cases, the triene cyclizes under the reaction conditions. Aromatization is accomplished by exposure to manganese dioxide or dichlorodicyanoquinone (DDQ). [reaction: see text]