## 3 Pathway Metabolic Engineering Strategies to Improve Glycerol-Based Bioprocesses
### Polyols  
Sugar alcohols or polyols (e.g., mannitol, erythiritol, xylitol, sorbitol, maltitol, lactitol and isomalt) are a group of noncyclic hydrogenated carbohydrates finding applications in the food and pharmaceutical industries, and as building blocks for the synthesis of value-added compounds, such as polymers (e.g., polyurethane foam). The wide variety of applications renders them valuable chemicals with constantly increasing demand. Polyols, apart from being a carbon source for many microorganisms, also take part in the regulation of coenzymes and in osmoregulation. Owing to these metabolic functions, many bacteria, fungi and yeasts naturally produce polyols. For readers further interested in the conversion of renewable feedstocks to sugar alcohols by yeasts, the recent review of [PERSON] and Sauer [96] is highly recommended. _Y. lipolytica_ has been successfully used for the production of erythroid (1,2,3,4-butanetetrol) (Table 3), i.e., a sugar alcohol with sweetening properties used as a food additive (E968), and various metabolic engineering strategies have been proposed. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Strain**} & \multirow{2}{*}{
\begin{tabular}{c} **Carbon** \\ **Source**1 \\ \end{tabular} } & \multirow{2}{*}{**Product**} & \multirow{2}{*}{**Engineering 2**} & \multirow{2}{*}{**Strategy**} & \multirow{2}{*}{**Set-Up 3**} & **Data from** \\  & & & & & & **[Ref.]** \\ \hline \multirow{4}{*}{Wratislavia K1} & \multirow{4}{*}{CG} & 40.7 g L\({}^{-1}\) & & Acetate negative & & \\  & & erythiritol; 65 g L\({}^{-1}\) & SE; ME & mutants; pH 3.0 for & & \\  & & citric acid & & erythiritol, pH 5.5/6.5 & & \\  & & & for citric acid; NaCl & & & \\ \hline \multirow{4}{*}{_Y. divulgata_} & \multirow{4}{*}{CG} & 59.8–62.7 g L\({}^{-1}\) & & & & \\  & & polyols from & & & & \\  & & soap-derived; & & & & \\  & & 76.8–79.5 g L\({}^{-1}\) & & & & \\  & & from & & & & \\  & & biodiesel-derived & & & & \\  & & glycerol & & & & \\ \hline \multirow{4}{*}{Wratislavia K1} & \multirow{4}{*}{CG} & 100.65 g L\({}^{-1}\) & & & Overexpression of & \\  & & & heterologous _SUC2_ & & & \\  & & polyols & & and native _GUTT;_ & & \\  & & & polyols & & step-wise fed-batch & \\  & & & culture with molasses & & \\  & & & & and CG; pH 3.0 & & \\ \hline \multirow{4}{*}{Wratislavia K1} & \multirow{4}{*}{CG} & 110 g L\({}^{-1}\) citric & & Acetate negative & & \\  & & acid; 81 g L\({}^{-1}\) & SE; PE & mutant; fed-batch; & & \\  & & erythiritol & & pH 5.5 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Engineering strategies (strain, medium, process) and cultivation systems using pure or crude glycerol as carbon source for the production of polyols by _Y. lipolytica_. Erythirtl synthesis from glycerol is not redox-balanced due to the net amount of the required oxidized cofactors. However, it is more advantageous than using glucose since erythroid synthesis from glucose consumes redox cofactors that must be replenished by glucose oxidation. When glycerol is used, DHAP is converted by a triosephosphate isomerase (TIM) into glyceraldehyde-3-phosphate (GA3P), which enters into the PPP, where a transketolase (TK) converts it into erythrose-4-phosphate (E-4-P). E-4-P is then dephosphorylated by an erythrose-4-P phosphatase (E4 PP) and reduced to erythroid by an erythrose reductase (ER) with NAD(P)H oxidation (Figure 2) [104]. Specific ER(s) catalyze the last step of erythroid synthesis. In the work of [PERSON] et al., a native ER gene (YALIOF18590g) from erythritol-producing yeast _Y. lipolytica_ MK1 was identified, overexpressed and characterized. Gene overexpression increased erythroid titer up to 44.44 g L\({}^{-1}\) (20% over the control), yield on glycerol to 0.44 g g\({}^{-1}\), and productivity to 0.77 g L\({}^{-1}\) h\({}^{-1}\). However, low erythroid production was still determined after gene deletion, indicating that other ER(s) might exist in the genome