Fig. 9

The putative chart for erythritol biosynthesis from the non-oxidative pathway. In this pathway, the oxidative phosphate pentose pathway (in the green box) was blocked by disrupted zwf gene or downregulated by inhibition of zwf gene expression by replacing its native promoter with a weak or inducible promoter. TKL1 gene was upregulated by overexpression and fructose 6-P and glyceraldehyde 3-P were converted to xylulose 5-P and erythrose 4-P, which is the intermediate of erythritol. Xylulose 5-P can be reversibly converted into ribulose 5-P by RPE, then ribulose 5-P can be converted to ribose 5-P by RPI. Ribose 5-P and xylulose 5-P were converted to fructose 6-P and erythrose 4-P by TKL1 and TAL. The non-oxidative pathway (in pink box) process involves no carbon loss and the carbon atom economy can be improved. NADPH-dependent ERs can be modified by protein engineering to accept NADH as a cofactor, which is generated from the TCA cycle. Since no carbon atom or reduced carbon atom was lost via the oxidative pathway, erythritol production or yield could be further improved