Diacetyl biosynthesis

As a side activity, many decarboxylases catalyze the formation of C-C bonds. In the reaction of two pyruvate molecules, catalyzed by pyruvate decarboxylase (PDC, E.C. 4.1.1.1), a-acetolactate is formed, an important intermediate of valine biosynthesis. In turn, a-acetolactale can be oxidatively decarboxylated by oxygen to diacetyl or enzymatically decarboxylated by acetolactate decarboxylase (ADC, E.C. 4.1.1.5) to (] )-acetoin (Figure 7.29). 

Unfortunately diacetyl formation is still not well understood. Acetoin formation occurs either by nonspecific interaction of acetaldehyde with the a-hydroxyethyl thiamine pyrophosphate intermediate in pyruvate decarboxylation (209) or by decarboxylation of a-acetolactate (210), which in turn arises either from interaction of pyruvate with a-hydroxyethyl thiamine pyrophosphate (211) or as a specific intermediate in valine biosynthesis (212, 213). Diacetyl does not appear to be formed directly from acetoin (208, 214). It is formed from a-acetolactate, in absence of cells, by O2 oxidation (215), and even under N2 (216), although an oxidation must occur. It is also formed from acetyl CoA (217, 218), probably by interaction with a-hydroxyethyl thiamine pyrophosphate [cf. stimulation by acetyl CoA addition to a solution of pyruvate and pyruvate decarboxylase (2i5)]. It is not known whether this involves a specific enzyme or is a mere side reaction. 

Low Molecular Weight Carbonyl Compounds. In the dairy field, a major product made this way is starter distillate. The main component is diaceyl which is a very important aroma compound responsible for the characteristic buttery flavor of fermented dairy products such as sour cream or buttermilk. The dairy industry relies upon fermentation by lactic streptococci for the production of diacetyl in cultured products. Starter distillate is a natural product rich in diacetyl which is produced by distilling such lactic cultures. The key intermediate in the biosynthesis of diacetyl is aL-acetolactic acid which is decarboxylated to form diacetyl (Figure 3). The starting material of the biosynthetic pathway is citrate which is a natural component of milk. 

Figure 3. Biosynthesis of diacetyl from citric acid.

Mearsine (336) was isolated as a minor alkaloid of Peripentadenia mearsii [703]. Its structure was determined with elemental analysis of the crystalline picrate, MS, IR, UV, and 13C NMR, diacetylation of the NaBH4 reduction product, and X-ray crystallography [703,704]. The structure was confirmed by synthesis of (+)-336 from (-)-5-methyl-2-cyclohexenone via a Mannich reaction [705]. The structure of 336 suggested its biosynthesis was from acetate via a polyketide pathway [703]. 

Usnic Acid. 2I6 Diacetyl 7t9-dihydraxy 8i9b-di-methyl-It3(2H,9bH)-dibenzofurandione usninic acid usnein Usniacin. C H,407 mol wt 344.31. C 62.79%. H 4.68%, O 32.53%. Antibacterial substance found in lichens. Isolation from varieties of Usnea barbota (L.) Wigg., Usneaceae Rochleder, Heidi> Ann. 48, If (1843) Widman Ann. 310, 230 (1900) 324, 139 (1902). Isoin from Ramalina reticulata Marshak, Public Health Reports 62, 3 (1947) Stark et of., J. Am. Chem. Soc. 72, 1819 (1950). Occurs in nature in both the d- and /-forms as well as a racemic mixture. Structure Curd, Robertson, J. Chem. Soc. 1937, 894 Schopf, Ross, Ann. 546, (1941) Barton, Brunn, /. Chem. Soc. 1953, 603. Resolution of ( )-usnic acid Dean et ai, ibid. 1250. Synthesis Barton et a/., ibid. 1956, 530. Biosynthesis in vitro Penttila, FaJes, Chem. Commun 1966, 656. Abs config of (+)-fomv S. Huneck et aL, Tetrahedron Letters 22, 351 (1981). 

Figure 7.1. Overview of the biochemical routes leading to glycerol, pyruvate, and ethanol. Furthermore, valine biosynthesis and diacetyl formation are shown, which may be bypassed by introduction of a heterologous a-acetolactate decarboxylase that directly converts a-acetolactate to acetoin. GPDl and GPD2, glycerol dehydrogenases 1 and 2 ADHl, alcohol dehydrogenase 1 ILV2, acetolactate synthetase ID/5, acetolactate reductoisomerase [Refs in 502].

Biosynthesis of riboflavin from guanine, ribitol and diacetyl. 

Native sophorolipids are understood to be produced by the wild-type organism and contain a C16-C18 hydroxy fatty acid. Two types of variations occur the nature of the lipophilic moiety and the amount of esterification. For the lipophilic moiety, predominantly hydroxy stearic acid is observed in the case of de novo synthesis [23] and when feeding a fatty acid of a suitable length, the concurrent hydroxy fatty acid is found. The extent of the esterification, either acetylation or lactonization, determines the solubility of the final product and is influenced by the pH. A value of 3-3.5 predominantly yields the poorly soluble diacetyl lactone and biosynthesis using a pH of 2 results in a more soluble and thus less esterified product. It is this last parameter that most influences the emulsifying properties of the sophorolipids. Adapting these properties to specific applications is further explored in Section 11.2. 

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