Diacetyl synthase

Figure 13.5 Diacetyl production from pyruvate. The chemical oxidative decarboxylation of a-acetolactate into diacetyl is shown by a dotted arrow. ALS, a-acetolactate synthase PDHC, pyruvate dehydrogenase complex DS, diacetyl synthase.

Diacetyl synthase 2 acetoin dehydrogenase 3 d-(—)-butanediol dehydrogenase 4 acetolactate dehydrogenase 5 acetolactate synthase... 

Hydroxy-2-butanone (acetoin) is a characteristic constituent of butter flavour used for flavouring margarine and can be obtained as a by-product of molasses-based and lactic acid fermentations [49, 71]. The closely related 2,3-butanedione (diacetyl) has a much lower organoleptic threshold than acetoin and is an important strongly butter-like flavour compound in butter and other dairy products [72] in buttermilk, for instance, the diacetyl concentration is only about 2-4 mg [73]. a-Acetolactate (a-AL) is an intermediate of lactic acid bacteria mainly produced from pyruvate by a-acetolactate synthase. In most lactic acid bacteria, a-AL is decarboxylated to the metabolic end product acetoin by a-AL decarboxylase (ALDB) [71] (Scheme 23.5). 

Scheme 23.5 Metabolic pathways of lactic acid bacteria leading from pyruvate to a-acetolactate and acetoin and chemical diacetyl formation. ALS a-acetolactate synthase, ALDB a-acetolactate decarboxylase, DDH diacetyl dehydrogenase. (Adapted from [72])...

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...

Three enzymes are involved in the synthesis of 2,3-BD a-acetolactate synthase (EC 4.1.3.18), a-acetolactate decarboxylase (EC 4.1.1.5), and butanediol dehydrogenase (also known as diacetyl [acetoin] reductase Larsen and Stormer 1973 Johansen et al. 1975 Stormer 1975). Two different enzymes form acetolactate from pyruvate. The first, termed catabolic a-acetolactate synthase, has a pH optimum of 5.8 in acetate and is part of the butanediol pathway. The other enzyme, termed anabolic a-acetolactate synthase or acetohydroxyacid synthetase, has been well studied and characterized and will not be discussed here. This enzyme is part of the biosynthetic pathway for isoleucine, leucine, and valine and is coded for by the ilvBN, ilvGM, and ilvH genes in E. colt and Salmonella typhimurium (Bryn and Stormer 1976). 

Figure 1.2. Citrate metabolism in Lactococcus, Leuconostoc, and Weissella species. Key for the enzymes CL, citrate lyase OAD, oxaloacetate decarboxylase LDH, lactate dehydrogenase PDC, pyruvate decarboxylase ALS, a-acetolactate synthase ADC, a-acetolactate decarboxylase DAR, diacetyl acetoln reductase BDH, 2,3-butanediol dehydrogenase Tppi, thiamine pyrophosphate.

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