3.A.1.4.11
The phenylpropeneoid uptake porter, CouPSTW.  The purple photosynthetic bacterium Rhodopseudomonas palustris is able to grow photoheterotrophically under anaerobic conditions on a range of phenylpropeneoid lignin monomers, including coumarate, ferulate, caffeate, and cinnamate. RPA1789 (CouP) is the periplasmic binding-protein component of the ABC uptake system (CouPSTU).  CouP binds a range of phenylpropeneoid ligands with K_d values in the nanomolar range. The crystal structure of CouP with ferulate as the bound ligand shows H-bond interactions between the 4-OH group of the aromatic ring with His309 and Gln305. H-bonds are also made between the carboxyl group on the ferulate side chain and Arg197, Ser222, and Thr102 (Salmon et al. 2013). Within the same operon are a diguanylate cyclase (Q6N8W3) and a phenylacetate-CoA ligase (Q6N8W5).