title: Geometric restraint drives on- and off-pathway catalysis by the Escherichia coli menaquinol:fumarate reductase.
abstract: Complex II superfamily members catalyze the kinetically difficult interconversion of succinate and fumarate. Due to the relative simplicity of complex II substrates and their similarity to other biologically abundant small molecules, substrate specificity presents a challenge in this system. In order to identify determinants for on-pathway catalysis, off-pathway catalysis, and enzyme inhibition, crystal structures of Escherichia coli menaquinol:fumarate reductase (QFR), a complex II superfamily member, were determined bound to the substrate, fumarate, and the inhibitors oxaloacetate, glutarate, and 3-nitropropionate. Optical difference spectroscopy and computational modeling support a model where QFR twists the dicarboxylate, activating it for catalysis. Orientation of the C2-C3 double bond of activated fumarate parallel to the C(4a)-N5 bond of FAD allows orbital overlap between the substrate and the cofactor, priming the substrate for nucleophilic attack. Off-pathway catalysis, such as the conversion of malate to oxaloacetate or the activation of the toxin 3-nitropropionate may occur when inhibitors bind with a similarly activated bond in the same position. Conversely, inhibitors that do not orient an activatable bond in this manner, such as glutarate and citrate, are excluded from catalysis and act as inhibitors of substrate binding. These results support a model where electronic interactions via geometric constraint and orbital steering underlie catalysis by QFR.
authors.affiliation: Department of Pharmacology, Vanderbilt University Medical Center, Nashville, Tennessee 37232, USA.
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Tomasiak
authors.forename: Tara L
authors.initials: TL
authors.lastname: Archuleta
authors.forename: Juni
authors.initials: J
authors.lastname: Andréll
authors.forename: César
authors.initials: C
authors.lastname: Luna-Chávez
authors.forename: Tyler A
authors.initials: TA
authors.lastname: Davis
authors.forename: Maruf
authors.initials: M
authors.lastname: Sarwar
authors.forename: Amy J
authors.initials: AJ
authors.lastname: Ham
authors.forename: W Hayes
authors.initials: WH
authors.lastname: McDonald
authors.forename: Victoria
authors.initials: V
authors.lastname: Yankovskaya
authors.forename: Harry A
authors.initials: HA
authors.lastname: Stern
authors.forename: Jeffrey N
authors.initials: JN
authors.lastname: Johnston
authors.forename: Elena
authors.initials: E
authors.lastname: Maklashina
authors.forename: Gary
authors.initials: G
authors.lastname: Cecchini
authors.forename: Tina M
authors.initials: TM
authors.lastname: Iverson
mesh_terms: D002384:Catalysis; D042963:Electron Transport Complex II / Q000737:chemistry / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000378:metabolism; D005650:Fumarates / Q000737:chemistry / Q000378:metabolism; D008956:Models, Chemical; D008958:Models, Molecular; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism; D013379:Substrate Specificity / Q000502:physiology
chemical_list: D004791:Enzyme Inhibitors; D029968:Escherichia coli Proteins; D005650:Fumarates; D010088:Oxidoreductases; D042963:Electron Transport Complex II; C424907:quinol fumarate reductase