Diacetyl reductases

Diacetyl, acetoin, and diketones form during fermentation. Diacetyl has a pronounced effect on flavor, with a threshold of perception of 0.1—0.2 ppm at 0.45 ppm it produces a cheesy flavor. U.S. lager beer has a very mild flavor and generally has lower concentrations of diacetyl than ale. Diacetyl probably forms from the decarboxylation of a-ethyl acetolactate to acetoin and consequent oxidation of acetoin to diacetyl. The yeast enzyme diacetyl reductase can kreversibly reduce diacetyl to acetoin. Aldehyde concentrations are usually 10—20 ppm. Thek effects on flavor must be minor, since the perception threshold is about 25 ppm. 

Figure 13.8 Pathway for metabolism of citrate by Leuconostoc spp. and S. lactis subsp. diacetylactis. (1) Citrate permease, (2) citrate lyase, (3) oxaloacetic acid decarboxylase, (4) pyruvate decarboxylase, (5) a-acetolactate synthetase, (6) a-acetolactate carboxylase, (7) diacetyl synthetase, (8) diacetyl reductase, and (9) acetoin reductase.

Diacetyl reductase (acetoin dehydrogenase) isolated from Lactobacillus kefir converts the prochiral diacetyl into optically pure (+)-acetoin (ee>94%) [145]. 

Diacetyl reductase (acetoin dehydrogenase, 1.1.1.5) is widespread in bacteria (207, 208, 219, 220, 221), including the species (Streptococcus diacetilactis, Lactobacillus casei) used to prepare cultured dairy products. Mutants lacking diacetyl reductase also fail to synthesize diacetyl (222). The enzyme has been purified 30-fold from L. casei (223). The activity was not fully separable from an NADH oxidase activity, and the enzyme appeared to be a flavoprotein. Maximum activity was at pH 4.5. The NADH oxidase activity is associated with diacetyl reductase in other sources. 

Figure 15-40. Reduction of a-diketones by diacetyl reductase from Bacillus stearothermophilus 233).

Figure 16.2-22. Kinetic resolution of racemic syn-diols by Bacillus stearothermophilus diacetyl reductase (BSDR). A reaction with LDH-catalyzed regeneration of NAD+ B selection of syn-diols applied.

Recently, diacetyl reductase (Acetoin reductase, E.C. 1.1.1.5) from Bacillus stearo-thermophilus (BSDR) was reported to be a powerful catalyst in the oxidative kinetic resolution of vic-diols (Fig. 16.2-22)1901. All syn-diols tested yielded the enantiopure (R,R) diols in almost maximum theoretical yields, a-hydroxy ketones were largely further oxidized to the corresponding diketones. Oxidation of vic-anti diols only gave ee values in the range of 62-76%. 

Recently isolated from baker s yeast are two diacetyl reductases. One enzyme shows a high S and the other a high R selectivity for the reductions of 2,3-pentanedione, 2,3-hexanedione and other mono- and dikctoncs218. 

Unfortunately diacetyl is not stable in most cultured food products. The microorganisms that synthesize diacetyl also contain diacetyl reductase that reduce diacetyl to acetoin and 2,3-butanediol (Figure 5.9). Thus fermented products such as buttermilk, which depend on diacetyl for flavor have an optimum or peak flavor. As the product is stored, the diacetyl is reduced and flavor strength and quality decrease. 

Fig. 10.33. Formation of diacetyl and butanediol from citrate by Streptococci. 1 citratase, 2 oxaloac-etate decarboxylase, 3 pyruvate decarboxylase, 4 a-acetolactate synthase, 5 diacetyl reductase, 6 a-acetolactate decarboxylase, 7 2,3-butanediol dehydrogenase...

A novel mechanism for stereoisomer formation was described for Bacillus polymyxa. The RR-acetoin formed from pyruvate is converted into RR-butanediol by diacetyl (acetoin) reductase. The same enzyme reduces diacetyl to RR-acetoin. An S-acetoin-forming diacetyl reductase converts diacetyl to SS-acetoin. The racemic acetoin molecules are acted upon by a butanediol dehydrogenase, which generates either RR-butanediol or meso-butanediol (Ui et al. 1986). 

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