Crystal structure of DltA. Implications for the reaction mechanism of non ribosomal peptide synthetase adenylation domains

DltA, the d alanine d alanyl carrier protein ligase responsible for the initial step of lipoteichoic acid d alanylation in Gram positive bacteria, belongs to the adenylation domain superfamily, which also includes acetyl CoA synthetase and the adenylation domains of non ribosomal synthetases. The two step reaction catalyzed by these enzymes substrate adenylation followed by transfer to the reactive thiol group of CoA or the phosphopantheinyl prosthetic group of peptidyl carrier proteins has been suggested to proceed via large scale rearrangements of structural domains within the enzyme. The structures of DltA reported here reveal the determinants for d Ala substrate specificity and confirm that the peptidyl carrier protein activating domains are able to adopt multiple conformational states, in this case corresponding to the thiolation reaction. Comparisons of available structures allow us to propose a mechanism whereby small perturbations of finely balanced metastable structural states would be able to direct an ordered formation of non ribosomal synthetase product

High resolution structures of the D alanyl carrier protein Dcp DltC from Bacillus subtilis reveal equivalent conformations of apo and holo forms

Abstractd-Alanylation of lipoteichoic acids plays an important role in modulating the properties of Gram-positive bacteria cell walls. The d-alanyl carrier protein DltC from Bacillus subtilis has been solved in apo- and two cofactor-modified holo-forms, whereby the entire phosphopantetheine moiety is defined in one. The atomic resolution of the apo-structure allows delineation of alternative conformations within the hydrophobic core of the 78 residue four helix bundle. In contrast to previous reports for a peptidyl carrier protein from a non-ribosomal peptide synthetase, no obvious structural differences between apo- and holo-DltC forms are observed. Solution NMR spectroscopy confirms these findings and demonstrates in addition that the two forms exhibit similar backbone dynamics on the ps–ns and ms timescales

Structural basis for acyl acceptor specificity in the achromobactin biosynthetic enzyme AcsD

Siderophores are known virulence factors, and their biosynthesis is a target for new antibacterial agents. A non-ribosomal peptide synthetase-independent siderophore biosynthetic pathway in Dickeya dadantii is responsible for production of the siderophore achromobactin. The D. dadantii achromobactin biosynthesis protein D (AcsD) enzyme has been shown to enantioselectively esterify citric acid with l-serine in the first committed step of achromobactin biosynthesis. The reaction occurs in two steps: stereospecific activation of citric acid by adenylation, followed by attack of the enzyme-bound citryl adenylate by l-serine to produce the homochiral ester. We now report a detailed characterization of the substrate profile and mechanism of the second (acyl transfer) step of AcsD enzyme. We demonstrate that the enzyme catalyzes formation of not only esters but also amides from the citryl-adenylate intermediate. We have rationalized the substrate utilization profile for the acylation reaction by determining the first X-ray crystal structure of a product complex for this enzyme class. We have identified the residues that are important for both recognition of l-serine and catalysis of ester formation. Our hypotheses were tested by biochemical analysis of various mutants, one of which shows a reversal of specificity from the wild type with respect to non-natural substrates. This change can be rationalized on the basis of our structural data. That this change in specificity is accompanied by no loss in activity suggests that AcsD and other members of the non-ribosomal peptide synthetase-independent siderophore superfamily may have biotransformation potential