Insights into the Biosynthesis of the Vibrio cholerae Major Autoinducer CAI-1 from the Crystal Structure of the PLP-Dependent Enzyme CqsA

CqsA is an enzyme involved in the biosynthesis of cholerae autoinducer-1 (CAI-1), the major Vibrio cholerae autoinducer engaged in quorum sensing. The amino acid sequence of CqsA suggests that it belongs to the family of alpha-oxoamine synthases that catalyse the condensation of an amino acid to an acyl-CoA substrate. Here we present the apo- and PLP-bound crystal structures of CqsA and confirm that it shares structural homology with the dimeric alpha-oxoamine synthases, including a conserved PLP-binding site. The chemical structure of CAM suggests that decanoyl-CoA may be one substrate of CqsA and that another substrate may be L-threonine or L-2-aminobutyric acid. A crystal structure of CqsA at 1.9-angstrom resolution obtained in the presence of PLP and L-threonine reveals an external aldimine that has lost the L-threonine side chain. Similarly, a 1.9-angstrom-resolution crystal structure of CqsA in the presence of PLP, L-threonine, and decanoyl-CoA shows a trapped external aldimine intermediate, suggesting that the condensation and decarboxylation steps have occurred, again with loss of the L-threonine side chain. It is suggested that this side-chain loss, an observation supported by mass spectrornetry is due to a retro-aldol reaction. Although no structural data have been obtained on CqsA using L-2-aminobutyric acid and decanoyl-CoA as substrates, mass spectrometry confirms the expected product of the enzyme reaction. It is proposed that a region of structure that is disordered in the apo structure is involved in the release of product. While not confirming if CqsA alone is able to synthesize CAM, these results suggest possible synthetic routes. (C) 2009 Elsevier Ltd. All rights reserved.