Acryloyl‐CoA reductase from Clostridium propionicum: An enzyme complex of propionyl‐CoA dehydrogenase and electron‐transferring flavoprotein

Abstract

Acryloyl‐CoA reductase from Clostridium propionicum catalyses the irreversible NADH‐dependent formation of propionyl‐CoA from acryloyl‐CoA. Purification yielded a heterohexadecameric yellow–greenish enzyme complex [(α 2 βγ)4; molecular mass 600 ± 50 kDa] composed of a propionyl‐CoA dehydrogenase (α 2, 2 × 40 kDa) and an electron‐transferring flavoprotein (ETF; β, 38 kDa; γ, 29 kDa). A flavin content (90% FAD and 10% FMN) of 2.4 mol per α 2 βγ subcomplex (149 kDa) was determined. A substrate alternative to acryloyl‐CoA (Km = 2 ± 1 µ m; kcat = 4.5 s−1 at 100 µ m NADH) is 3‐buten‐2‐one (methyl vinyl ketone; Km = 1800 µ m; kcat = 29 s−1 at 300 µ m NADH). The enzyme complex exhibits acyl‐CoA dehydrogenase activity with propionyl‐CoA (Km = 50 µ m; kcat = 2.0 s−1) or butyryl‐CoA (Km = 100 µ m; kcat = 3.5 s−1) as electron donor and 200 µ m ferricenium hexafluorophosphate as acceptor. The enzyme also catalysed the oxidation of NADH by iodonitrosotetrazolium chloride (diaphorase activity) or by air, which led to the formation of H 2 O 2 (NADH oxidase activity). The N‐terminus of the dimeric propionyl‐CoA dehydrogenase subunit is similar to those of butyryl‐CoA dehydrogenases from several clostridia and related anaerobes (up to 55% sequence identity). The N‐termini of the β and γ subunits share 40% and 35% sequence identities with those of the A and B subunits of the ETF from Megasphaera elsdenii, respectively, and up to 60% with those of putative ETFs from other anaerobes. Acryloyl‐CoA reductase from C. propionicum has been characterized as a soluble enzyme, with kinetic properties perfectly adapted to the requirements of the organism. The enzyme appears not to be involved in anaerobic respiration with NADH or reduced ferredoxin as electron donors. There is no relationship to the trans‐2‐enoyl‐CoA reductases from various organisms or the recently described acryloyl‐CoA reductase activity of propionyl‐CoA synthase from Chloroflexus aurantiacus.