# Biological Production of 3-Hydroxpropionic Acid: An Update on the Current Status
## 2 Production of 3 HP from Glycerol
### Engineering of K. pneumoniae Cells for the Conversion of Glycerol to 3 HP
\begin{table}
\begin{tabular}{l l l l l} \hline \hline \multicolumn{1}{c}{**Genes Transfered**} & \multicolumn{1}{c}{**Culture Conditions**} & \multicolumn{1}{c}{**Concentration**} & \multicolumn{1}{c}{**Productivity**} & \multicolumn{1}{c}{**Ref.**} \\ \hline _duaS_ gene from _B. subtilis_ & Fed-batch bioreactor 5 L, 3 L, pH 7, 37 \({}^{\circ}\)C, & 18.0 & 0.77 & [27] \\  & 1.5 vvm, 400 rpm & & & \\ \hline _aldH_ gene from _E. coli_ & Fed-batch bioreactor, 5 L, 2.8 L, pH 6.8–7.0, & & & \\  & 37 \({}^{\circ}\)C, microaeration 1.5 vvm, 400 rpm & & & \\ \hline \begin{tabular}{l} Overexpression of _kgaults_, \\ _duHaS_, _aldH_, _aldH_, _aldH_ \\ _luL_, _fuL_, _and_ \\ \end{tabular} & \begin{tabular}{l} Fed-batch bioreactor 1.5 L, 1 L, pH 7.5, 37 \({}^{\circ}\)C, \\ 1 vvm, 400 rpm \\ \end{tabular} & 43.0 & 0.90 & [12] \\ \hline \
\begin{tabular}{l} _PnuC_ overexpression, deletion \\ of _ldH_1, _ldH_2, and _pta_ \\ \end{tabular} &
\begin{tabular}{l} Fed-batch bioreactor 5 L, 3 L, pH 7, 37 \({}^{\circ}\)C, \\ 1.5 vvm, 400 rpm \\ \end{tabular} & 83.8 & 1.16 & [29] \\ \hline _aldH_ gene from _E. coli_ & Flasks 250 mL, 100 mL, 37 \({}^{\circ}\)C, microaeration, & & & \\  & 150 rpm & & & \\ \hline \multirow{4}{*}{-} & Co-cultivation with _Glucobacter oxidants_ in & & & \\  & fed-batch bioreactor, 7 L, 40 L, list step: pH 7, & & & \\  & 37 \({}^{\circ}\)C, 0.2 vvm, 150 rpm 2 nd step: pH 5.5, & & & \\  & 28 \({}^{\circ}\)C, 0.5 vvm, 600 rpm & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of 3 HP production from glycerol using _K. pneumoniae_. production of either 3 HP or 1,3-propanediol. The highest 3 HP concentration (48.9 g/L) was reached under microaerobic conditions (1.5 vvm aeration, which resulted in decrease of the dissolved oxygen from 100% to 0% in 2.7 h of culture), with the simultaneous production of 25.3 g/L 1,3-propanediol after 28 h of cultivation in a fed-batch bioreactor. The overall yield of 1,3-propanediol and 3 HP was 0.66 mol/mol. The authors also demonstrated that the formation of other by-products was also affected by the level of aeration under micro-anaerobic conditions, with the production of lactic acid and acetic acid being enhanced, whereas ethanol and succinic acid production was reduced by increasing the aeration level. Formate production initially increased with increasing levels of aeration, followed by a sharp decrease when aeration was higher than 0.6 vvm. [PERSON] et al. [12] also attempted to control the production of other metabolic by-products, especially of acetic acid, with the aim of improving co-production of 3 HP and 1,3-propanediol from the recombinant strain of _K. pneumoniae_ (J2B), which overexpressed the ald gene (encoding alpha-ketoglutaric semialdehyde dehydrogenase--_kgsadh_). The methods that they evaluated for acetate reduction were reduction of the glycerol assimilation through the glycolytic pathway, increasing the glycerol flow towards 3 HP and 1,3-propanediol formation, and finally controlling the aeration levels. To improve the co-production, the authors evaluated the deletion and overexpression of several genes. The best results were obtained when the genes encoding lactate dehydrogenase (_ldhA_), succinate dehydrogenase (_frdA_), and alcohol dehydrogenase (_adhE_) were deleted, which--together with the overexpression of _dhaB_ and _gdrAB_--resulted in 3 HP at 43 g/L and 1,3-propanediol at 21 g/L during fed-batch bioreactor cultivation, with an overall yield of 0.49 mol/mol. Although these genetic manipulations reduced the amount of acetate, a considerable amount of acetate (>150 mM) had accumulated in the bioreactor by the end of the cultivation. In another study conducted by [PERSON] et al. [29], a systematic optimization of glycerol metabolism took place in order to improve the 3 HP production titers. During this work, different promoters for the overexpression of _PuuC_ (a native ald of _K. pneumoniae_), which is a key enzyme for 3 HP formation, were investigated.