Pseudomonas aeruginosa is one of only a few Pseudomonas species that are able to use acyclic monoterpenoids, such as citronellol and citronellate, as carbon and energy sources. This is achieved by the acyclic terpene utilization pathway (Atu), which includes at least six enzymes (AtuA, AtuB, AtuCF, AtuD, AtuE, AtuG) and is coupled to a functional leucine-isovalerate utilization (Liu) pathway. Here, quantitative proteome analysis was performed to elucidate the terpene metabolism of P. aeruginosa. The proteomics survey identified 187 proteins, including AtuA to AtuG and LiuA to LiuE, which were increased in abundance in the presence of citronellate. In particular, two hydratases, AtuE and the PA4330 gene product, out of more than a dozen predicted in the P. aeruginosa proteome showed an increased abundance in the presence of citronellate. AtuE (isohexenyl-glutaconyl coenzyme A [CoA] hydratase; EC 4.2.1.57) most likely catalyzes the hydration of the unsaturated distal double bond in the isohexenyl-glutaconyl-CoA thioester to yield 3-hydroxy-3-isohexenyl-glutaryl-CoA. Determination of the crystal structure of AtuE at a 2.13-Å resolution revealed a fold similar to that found in the hydratase (crotonase) superfamily and provided insights into the nature of the active site. The AtuE active-site architecture showed a significantly broader cavity than other crotonase superfamily members, in agreement with the need to accommodate the branched isoprenoid unit of terpenes. Glu139 was identified to be a potential catalytic residue, while the backbone NH groups of Gly116 and Gly68 likely form an oxyanion hole. The present work deepens the understanding of terpene metabolism in Pseudomonas and may serve as a basis to develop new strategies for the biotechnological production of terpenoids.
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