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. 2011 Apr;77(8):2582–2588. doi: 10.1128/AEM.01616-10

Fig. 3.

Fig. 3.

Proposed 2,3-BD biosynthesis pathway in the ACR-expressing C. acetobutylicum transformants of this study. The boxed area indicates the intracellular space. The size of the arrows indicates the relative flux toward d- and l-acetoin from pyruvic acid. The gray arrows illustrate the impact of supplementing the medium with 20 mM racemic d/l-acetoin. The C. beijerinckii acetoin reductase (Cb-ACR) enzyme is indicated in bold type on a gray background. The proposed conversion of d-acetoin to meso-2,3-BD by an endogenous alcohol dehydrogenase (C. acetobutylicum alcohol dehydrogenase [Ca-ADH]) is indicated. In the control strain fermentation, the ratio of the two acetoin enantiomers is 12:1 (d:l).