# Biological Production of 3-Hydroxpropionic Acid: An Update on the Current Status
## 3 Production of 3 HP from Sugars
### Use of Other Host Microorganisms  
Apart from work with the two commonly used microorganisms (_E. coli_ and _S. cerevisiae_) for the construction of a strain capable of transforming glucose (and in some cases xylose) to 3 HP, a few other host microorganisms have also been used. The yeast _Schizosaccharomyces pombe_ is one example; this microorganism showed good tolerance of low pH values, which can result the accumulation of 3 HP [58]. The malonyl-CoA pathway was constructed in this yeast by overexpression of an endogenous acetyl-CoA carboxylase (_cut6p_ gene) and heterologous expression of the _mcr_ gene from _C. aurantiacus_ in a protease-deficient strain (aimed at protecting the secreted proteins from degradation). Supplementation of the culture broth with acetate was also beneficial for 3 HP production; the 3 HP concentration reached 3.5 g/L (with a productivity of 0.03 g/L-h) during culture in flasks, which was further increased to 7.6 g/L (with a productivity of 0.25 g/L-h) when high-density culture was used (Table 6). Another host microorganism that was used is the bacterium _Corynebacterium glutamicum_. [PERSON] et al. [53] engineered a strain that could convert glucose and xylose to 3 HP through glycerol as an intermediate. The pathway was constructed by using the _pdu_ operon from _K. pneumoniae_, fusion of the _gpd1_ and _gpp2_ genes (which are involved in glycerol synthesis), and expression of the _gabD_ aldehyde dehydrogenase (which was found to be the most efficient of the aldehyde dehydrogenases tested). Furthermore, genes involved in the formation of by-products were deleted in order to improve the carbon flux towards 3 HP. The _ptsH_ gene (encoding the phosphoenolpyruvate dependent phosphotransferase system, which is the main route for glucose uptake) was also replaced with the inositol permeases gene (_io1T1_) and glucokinase gene (_glk_); finally, to allow pentose utilization, the pentose transport genes _araE_ and the xylose catabolite gene _xylAB_ were also expressed. The resulting strain produced 3 HP at 37.4 g/L and 35.4 g/L when grown on glucose and xylose, respectively, in batch \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Genes Transfered** & **Microorganism** & **Culture Conditions** & \begin{tabular}{c} **Concentration** \\ **(g/L)** \\ \end{tabular} & \begin{tabular}{c} **Productivity** \\ **(g/L-h)** \\ \end{tabular} & **Ref.** \\ \hline _mcr_ from _C. aurantiacus_ and & _S. pombe_ & \begin{tabular}{c} flasks 100 mL with \\ high-density cultures, \\ 10 mL, 30 °C, 250 rpm \\ \end{tabular} & 7.6 & 0.25 & [58] \\ \hline _plu_ from _K. pneumoniae, spdl_ and & & & & & \\ _gpp2_ from _S. cerevisiae, spdl_ from _C. reactor, spdl_ from _E. coli, anE_ from _C. glutamicum_, deletion of _lMaA_, & \begin{tabular}{c} _C. glutamicum_ \\ 1 vvm \\ \end{tabular} & \
\begin{tabular}{c} Fed-batch bioreactor \\ 62.6 \\ \end{tabular} & 0.87 & [53] \\ \hline _plu_ from _K. pneumoniae, spdl_ and & & & & & \\ _gpp2_ from _S. cerevisiae, spdl_ from _C. reactor, spdl_ from _E. coli, anE_ from _C. glutamicum_ &
\begin{tabular}{c} Fed-batch bioreactor \\ 5 L with glucose and \\ xylose, 2 L, pH 7.2, \\ 30 °C, 1 vvm \\ \end{tabular} & 54.8 & n.a. & [53] \\ \hline \multicolumn{4}{l}{n.a.: not available.} \\ \end{tabular}
\end{table} 
Table 6: Results of 3 HP production from glucose using other host microorganisms. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Genes Transfered** & **Culture Conditions** & \ \begin{tabular}{c} **Concentration** \\ **(g/L)** \\ \end{tabular}  & \begin{tabular}{c} **Productivity** \\ **(g/L-h)** \\ \end{tabular}  & **Ref. ** \\ \hline Multiple copies of _mcr_ from _C. aurantiacus_ and _m mutant acc1_, _acs1641_ from _S. enterica_, & & & & \\ overexpression of _mcr_ and _alde_, _gpdh_ from _C. acetubulfbioreactors. The amount of 3 HP produced for the co-consumption of the two sugars was 36.2 g/L, with a conversion yield of 0.45 g/g.