New roles for CO 2 in the microbial metabolism of aliphatic epoxides and ketones

Abstract

Short-chain aliphatic epoxides and ketones are two classes of toxic organic compounds formed biogenically and anthropogenically. In spite of their toxicity, these compounds are utilized as primary carbon and energy sources or are generated as intermediate metabolites in the metabolism of other compounds (e.g., alkenes, alkanes, and secondary alcohols) by a number of diverse bacteria. One bacterium capable of using both classes of compounds is the gram-negative aerobe Xanthobacter strain Py2. Studies of epoxide and ketone (acetone) metabolism by Xanthobacter strain Py2 have revealed a central role for CO2 in these processes. Both classes of compounds are metabolized by carboxylation reactions that produce beta-keto acids as products. The epoxide- and ketone-converting enzymes are distinct carboxylases with molecular properties and cofactor requirements unprecedented for other carboxylases. Epoxide carboxylase is a four-component multienzyme complex that requires NADPH and NAD+ as cofactors. In the course of epoxide carboxylation, a transhydrogenation reaction occurs wherein NADPH undergoes oxidation and NAD+ undergoes reduction. Acetone carboxylase is a multimeric (three-subunit) ATP-dependent enzyme that forms AMP and inorganic phosphate as ATP hydrolysis products in the course of acetone carboxylation. Recent studies have demonstrated that acetone metabolism in diverse anaerobic bacteria (sulfate reducers, denitrifiers, phototrophs, and fermenters) also proceeds by carboxylation reactions. ATP-dependent acetone carboxylase activity has been demonstrated in cell-free extracts of the anaerobic acetone-utilizers Rhodobacter capsulatus, Rhodomicrobium vannielii, and Thiosphaera pantotropha. These studies have identified new roles for CO2 as a cosubstrate in the metabolism of two classes of important xenobiotic compounds. In addition, two new classes of carboxylases have been identified, the investigation of which promises to reveal new insights into biological strategies for the fixation of CO2 to organic substrates.

Carboxylases in Natural and Synthetic Microbial Pathways

“…Functionalization of inert substrates. The introduction of a carboxylic group is a common strategy in the assimilation of substrates that lack any functionalizable terminal carbon group, such as phenol, acetophenone, epoxypropane, or acetone (39,55,70,106) (Fig. 3).…”

“…Anaerobic metabolism of acetophenone in Aromatoleum aromaticum requires acetophenone carboxylase, which yields benzoylacetate (70). Epoxypropane and acetone are transformed into acetoacetate by NADPH:2-ketopropyl-CoM carboxylase/oxidoreductase (Xanthobacter autotrophicus Py2) and acetone carboxylase (Rhodobacter capsulatus), respectively (38,39). Subsequently, all these carboxylic acids are activated to their corresponding CoA-esters and further converted to central metabolites.…”

Butane metabolism by butane-grown ‘Pseudomonas butanovora’

“…When acetone consumption was examined in the absence of 2-propanol, a slow rate of consumption [9 nmol min-' (mg protein)-'] was observed. Recently, Ensign and coworkers (Ensign et al, 1998) demonstrated that for some bacteria the first step in the metabolism of ketones involves a carboxylation reaction. For example, the consumption of acetone by resting cells of Xanthobacter Py2 was stimulated by addition of CO, (Sluis et al, 1996).…”

Engineered and Native Coenzyme B12-dependent Isovaleryl-CoA/Pivalyl-CoA Mutase

“…The PCM described in this study is derived from X. autotrophicus strain Py2, a facultative aerobe that was first isolated from black pool sludge, has versatile metabolic capabilities, and is able to utilize 2-hydroxyisobutyric acid, propene, and trichloroethylene, 1-butene among others, as carbon sources (31). The large subunit of PCM has Leu and Asn residues that are also seen in other genes annotated as being MCM-like in the database (Fig.…”