Asymmetric reduction of prochiral ketones by cell‐free systems from alcaligenes eutrophus

Abstract

AbstractA strain of Alcaligenes eutrophus has been isolated from the soil by enrichment culture technique with nerolidol (1), a sesquiterpene alcohol, as the sole source of carbon and energy. Fermentation of nerolidol (1) by this bacterium in a mineral salts medium resulted in the formation of two major metabolites, viz. geranylacetone (2) and an optically active alcohol, (S)‐(+)‐geranylacetol (3). Nerolidol (1)‐induced cells readily transformed 1,2‐epoxynerolidol (4) and 1,2‐dihydroxynerolidol (5) into geranylacetone (2). These cells also exhibited their ability to carry out stereospecific reduction of 2 into (S)‐(+)‐geranylacetol (3). Oxygen uptake studies clearly indicated that nerolidol‐induced cells oxidized compounds 2, 3, 4, 5 and ethyleneglycol (7). Based on the nature of the metabolites isolated, the ability of nerolidol‐induced cells to convert compounds 4 and 5 into geranylacetone (2), and oxygen uptake studies, a pathway for the microbial degradation of nerolidol (1) has been proposed. The proposed pathway envisages the epoxidation of the terminal double bond, opening of the epoxide and cleavage between C‐2 and C‐3 in a manner similar to the periodate oxidation of cis‐diol. The cell‐free extract prepared from nerolidol‐induced cells readily carried out the asymmetric reduction of compound 2 to an optically active alcohol (3) in the presence of NAD(P)H. The cell‐free extract carried out both oxidation and reduction reactions at two different pH values and exhibited wide substrate specificity towards various steroids besides terpenes.