FIG. 4.

An unorthodox A. aceti CAC oxidizes acetate. The gray arrows indicate CAC genes that are not found in a draft A. aceti 1023 genome sequence (Kappock et al., unpublished). Each has a functional replacement. Using periplasmic dehydrogenases, A. aceti produces large quantities of acetic acid from ethanol and must contend with a constant influx of the former. Together with a complete oxidative phosphorylation pathway, this variant CAC functions in the eight-electron oxidation of acetic acid: CH₃COOH + 2 O₂ → 2 CO₂ + 2 H₂O. This pathway skips AckA, Acs, and SCS (sucCD), the only steps that would be directly influenced by cytoplasmic nucleotide pools, and reduces the number of enzymes that have free CoA substrates from two to one. The initial electron acceptors, two NAD⁺, one FAD, and one ubiquinone (presumed to be Q9 in A. aceti), would give a lower energy yield than a canonical CAC but have an additional irreversible step, the Mqo-mediated quinone reduction. Genes for the glyoxylate shunt enzymes isocitrate lyase and malate synthase are not found in A. aceti 1023.