5bwd

Publication Abstract from PubMed

Various bacteria perform anaerobic degradation of small hydrocarbons as a source of energy and cellular carbon. In order to activate non-reactive hydrocarbons such as toluene, enzymes conjugate these molecules to fumarate in a radical-catalyzed, C-C bond-forming reaction. We have determined X-ray crystal structures of the glycyl radical enzyme that catalyzes the addition of toluene to fumarate, benzylsuccinate synthase (BSS), in two oligomeric states with fumarate alone or with both substrates. We find that fumarate is secured at the bottom of a long active site cavity with toluene bound directly above it. The two substrates adopt orientations that appear ideal for radical-mediated C-C bond formation: the methyl group of toluene is positioned between fumarate and a cysteine that forms a thiyl radical during catalysis, which is in turn adjacent to the glycine that serves as radical storage residue. Toluene is held in place by fumarate on one face and tight packing by hydrophobic residues on the other face and sides. These hydrophobic residues appear to become ordered, thus encapsulating toluene, only in the presence of BSSbeta, a small protein subunit that forms a tight complex with BSSalpha, the catalytic subunit. Enzymes related to BSS are able to metabolize a wide range of hydrocarbons through attachment to fumarate. Using our structures as a guide, we have constructed homology models of several of these 'X-succinate synthases' and determined conservation patterns that will be useful in understanding the basis for catalysis and specificity in this family of enzymes.

Substrate-bound Structures of Benzylsuccinate Synthase Reveal How Toluene is Activated in Anaerobic Hydrocarbon Degradation.,Funk MA, Marsh EN, Drennan CL J Biol Chem. 2015 Jul 29. pii: jbc.M115.670737. PMID:26224635^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.