New acetoin reductases from Bacillus stearothermophilus: meso and 2R,3R-butanediol as fermentation products

The fermentation of sucrose with Bacillus stearothermophilus ATCC 2027 afforded 2R,3R- and meso-2,3-butanediol together with R-acetoin following the bacterial catabolic pathway. This metabolic route was confirmed by the conversion of pyruvate to R-acetoin with the cell free extract of B. stearothermophilus. On the other hand the reduction of 3R-acetoin to 2R,3R- and meso-butanediol with the same cell free extract allowed at least the presence of two NADH-dependent reductases. Together with the S-stereospecific diacetyl reductase (BSDR), purified in a previous work, a new S-stereospecific acetoin reductase (S-ACR) was partially purified and a fraction containing R-stereospecific acetoin reductase (R-ACR) was obtained. On the other hand in B. stearothermophilus fermentation of sucrose the “butanediol cycle” is negligible because the acetylacetoin synthase, responsible of the first step of this cycle, is an inducible enzyme that needs low concentration of sugar and high concentration of acetoin to be expressed.