Enzymes of the benzoyl‐coenzyme A degradation pathway in the hyperthermophilic archaeon F erroglobus placidus

The F e( III )‐respiring F erroglobus placidus is the only known archaeon and hyperthermophile for which a complete degradation of aromatic substrates to CO 2 has been reported. Recent genome and transcriptome analyses proposed a benzoyl‐coenzyme A ( CoA ) degradation pathway similar to that found in the phototrophic R hodopseudomonas palustris , which involves a cyclohex‐1‐ene‐1‐carboxyl‐ CoA (1‐enoyl‐ CoA ) forming, ATP ‐dependent key enzyme benzoyl‐ CoA reductase ( BCR ). In this work, we demonstrate, by first in vitro studies, that benzoyl‐ CoA is ATP ‐dependently reduced by two electrons to cyclohexa‐1,5‐dienoyl‐ CoA (1,5‐dienoyl‐ CoA ), which is further degraded by hydration to 6‐hydroxycyclohex‐1‐ene‐1‐carboxyl‐ CoA (6‐ OH ‐1‐enoyl‐ CoA ); upon addition of NAD + , the latter was subsequently converted to β‐oxidation intermediates. The four candidate genes of BCR were heterologously expressed, and the enriched, oxygen‐sensitive enzyme catalysed the two‐electron reduction of benzoyl‐ CoA to 1,5‐dienoyl‐ CoA . A gene previously assigned to a 2,3‐didehydropimeloyl‐ CoA hydratase was heterologously expressed and shown to act as a typical 1,5‐dienoyl‐ CoA hydratase that does not accept 1‐enoyl‐ CoA . A gene previously assigned to a 1‐enoyl‐ CoA hydratase was heterologously expressed and identified to code for a bifunctional crotonase/3‐ OH ‐butyryl‐ CoA dehydrogenase. I n summary, the results consistently provide biochemical evidence that F . placidus and probably other archaea predominantly degrade aromatics via the T hauera/ A zoarcus type and not or only to a minor extent via the predicted R . palustris‐ type benzoyl‐ CoA degradation pathway.