The purification, crystallization and preliminary X-ray diffraction analysis of two isoforms of meso-diaminopimelate decarboxylase from Arabidopsis thaliana

Thermostable D-amino acid decarboxylases derived from Thermotoga maritima diaminopimelate decarboxylase

Diaminopimelate decarboxylases (DAPDCs) are highly selective enzymes that catalyze the common final step in different lysine biosynthetic pathways, i.e. the conversion of meso-diaminopimelate (DAP) to L-lysine. We examined the modification of the substrate specificity of the thermostable decarboxylase from Thermotoga maritima with the aim to introduce activity with 2-aminopimelic acid (2-APA) since its decarboxylation leads to 6-aminocaproic acid (6-ACA), a building block for the synthesis of nylon-6. Structure-based mutagenesis of the distal carboxylate binding site resulted in a set of enzyme variants with new activities toward different D-amino acids. One of the mutants (E315T) had lost most of its activity toward DAP and primarily acted as a 2-APA decarboxylase. We next used computational modeling to explain the observed shift in catalytic activities of the mutants. The results suggest that predictive computational protocols can support the redesign of the catalytic properties of this class of decarboxylating PLP-dependent enzymes.

Structure-function analyses of two plant meso-diaminopimelate decarboxylase isoforms reveal that active-site gating provides stereochemical control

meso-Diaminopimelate decarboxylase catalyzes the decarboxylation of meso-diaminopimelate, the final reaction in the diaminopimelate l-lysine biosynthetic pathway. It is the only known pyridoxal-5-phosphate-dependent decarboxylase that catalyzes the removal of a carboxyl group from a d-stereocenter. Currently, only prokaryotic orthologs have been kinetically and structurally characterized. Here, using complementation and kinetic analyses of enzymes recombinantly expressed in Escherichia coli, we have functionally tested two putative eukaryotic meso-diaminopimelate decarboxylase isoforms from the plant species Arabidopsis thaliana We confirm they are both functional meso-diaminopimelate decarboxylases, although with lower activities than those previously reported for bacterial orthologs. We also report in-depth X-ray crystallographic structural analyses of each isoform at 1.9 and 2.4 Å resolution. We have captured the enzyme structure of one isoform in an asymmetric configuration, with one ligand-bound monomer and the other in an apo-form. Analytical ultracentrifugation and small-angle X-ray scattering solution studies reveal that A. thaliana meso-diaminopimelate decarboxylase adopts a homodimeric assembly. On the basis of our structural analyses, we suggest a mechanism whereby molecular interactions within the active site transduce conformational changes to the active-site loop. These conformational differences are likely to influence catalytic activity in a way that could allow for d-stereocenter selectivity of the substrate meso-diaminopimelate to facilitate the synthesis of l-lysine. In summary, the A. thaliana gene loci At3g14390 and At5g11880 encode functional. meso-diaminopimelate decarboxylase enzymes whose structures provide clues to the stereochemical control of the decarboxylation reaction catalyzed by these eukaryotic proteins.