# Biological Production of 3-Hydroxpropionic Acid: An Update on the Current Status
## 2 Production of 3 HP from Glycerol
### Engineering of E. coli Cells for the Conversion of Glycerol to 3 HP
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Genes Transfered** & **Culture Conditions** & \
\begin{tabular}{l} **Concentration** \\ **(g/L)** \\ \end{tabular} &
\begin{tabular}{l} **Productivity** \\ **(g/L-h)** \\ \end{tabular} & **Ref.** \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_ & Flasks 250 mL, 100 mL, 37 °C, & 5.1 & n.a. & [31] \\ and _gkgall_ from _A. brasilense_ & 200 rpm & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_, _draft from _E. coli_, overexpression of & Fed-batch bioreactor 5 L, 2 L, & 42.1 & 1.32 & [4] \\ _glyF_, deletion of _ackA-pta_, _yubD_, & pH 7.35 °C, 1 vvm, 500 rpm & & & \\ and _glyR_ & & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_, _gsubD_ from _C. nectar_ with & Fed-batch bioreactor 5 L, 2 L, & & & \\ side-directed mutagenesis, deletion of & pH 7.35 °C, 1 vvm, 500 rpm & & & \\ _yghD_ and _ackA-pta_ & & & & \\ \hline UTR-engineered _dha_, _gdrAB_ from _K. pneumoniae_ and _kgall_ from & Fed-batch bioreactor 5 L with & & & \\ _A. brasilense_, deletion of _yghD_ and _ackA-pta_ & & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_ and _kgall_ from _A. brasilense_ & Fed-batch bioreactor 15 L with & & & \\ and _kgall_ from _A. brasilense_ & pH 7.37 °C, 1 vvm, 500 rpm & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_ & Fed-batch bioreactor 5 L with & & & \\ and _kgall_ from _A. brasilense_ & pH 7.37 °C, 1 vvm, 500 rpm & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_ & Fed-batch bioreactor 5 L, with & & & \\ and _kgall_ from _A. brasilense_ & pH 6.5, 37 °C, 450 rpm & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_ & Fed-batch bioreactor 15 L, 1 L, & & & \\ and _kgall_ from _A. brasilense_ & pH 7.37 °C, 0.5 vvm, 650 rpm & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_ & & & & \\ and _kgall_ from _A. brasilense_ & pH 5.37 °C, 450 rpm & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_ & Fed-batch bioreactor 15 L, 1 L, & & & \\ and _kgall_ from _A. brasilense_ & pH 7.37 °C, 0.5 vvm, 650 rpm & & & \\ \hline _dha_B and _gdrAB_ from _K. pneumoniae_, _draft from _A. brasilense_, _gth_ from _K. pneumoniae_ & Flasks 250 mL, 60 mL, pH 6–7, & & & \\ \hline n.a.: not available. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of 3 HP production from glycerol using _E. coli_. tested, two showed a high degree of growth in the presence of 25 g/L 3 HP, and one of them, _E. coli W_, outperformed the rest, and produced 41.5 g/L 3 HP with a yield of 0.31 g/g. During this work, it was found that there are significant differences in 3 HP tolerance among _E. coli_ strains, and this should be taken into account when selecting an appropriate host microorganism. To improve the metabolic flow of glycerol towards 3 HP, other genes of glycerol catabolism are often deleted, which might result in strains with poor growth [38]. To avoid this and at the same time improve the metabolic flow towards 3 HP, [PERSON] et al. [38] suggested the use of a metabolic toggle switch (MTS), and tested the conditional repression of the following genes: _glpk_ (encoding glycerol kinase), _tpiA_ (encoding triosephosphate isomerase), and _gapA_ (encoding glyceraldehyde-3-phosphate dehydrogenase). After testing the three different strains for their efficiency in 3 HP production, it was found that only the strain with the MTS for _gapA_ improved the production of 3 HP to 4.88 g/L. Deletion of the gene _yqhD_ (whose protein product is responsible for the conversion of 3-hydroxypropionaldehyde to 1,3-propanediol) further improved the concentration of 3 HP to 6.06 g/L, with a yield of 0.515 mol/mol. Finally, another strategy for genetically modifying _E. coli_ was proposed by [PERSON] et al. [39], who used a dual synthetic pathway for the construction of _E. coli_ strains. More specifically, they transferred the genes from both the _dhu_ and the _pdu_ operons for the construction of one strain, and compared with the results obtained using a strain with only the genes from the _dhu_ operon.