# Biological Production of 3-Hydroxpropionic Acid: An Update on the Current Status
## 3 Production of 3 HP from Sugars
### Use of E. coli as a Host Microorganism  
As discussed previously, _E. coli_ is a very commonly used microorganism when it comes to metabolic engineering for the construction of a microbial cell factory, and it has been used extensively for metabolic engineering aimed at 3 HP production. On the other hand, the lack of the ability to naturally produce coenzyme B12, which is required for the action of some enzymes involved in the pathways of glycerol conversion to 3 HP, is an important drawback. On the other hand, production of 3 HP using the malonyl-CoA pathway is coenzyme B12-independent, and presents a promising alternative when using _E. coli_ cell factories. As discussed previously, the malonyl-CoA pathway starts with the conversion of acetyl-CoA to malonyl-CoA by the enzyme acetyl-CoA carboxylase, and optimal expression and activity of this enzyme is important. [PERSON] et al. [54] constructed this route in _E. coli_ by expressing the acetyl-CoA carboxylase from _Corynebacterium glutamicum_, and the malonyl-CoA reductase from _Chloroflexus aurantiacus_. To improve the function of the acetyl-CoA carboxylase, the authors added biotin and NaHCO\({}_{3}\) (to provide CO\({}_{2}\) for the carboxylation of acetyl-CoA). Under optimal conditions, 1.8 g/L 3 HP was produced in flasks with a yield of 0.18 g/g. Scale-up of the process in a fed-batch bioreactor improved the titers obtained to 10.08 g/L (Table 4). It was proposed that low yields of 3 HP through the malonyl-CoA pathway can be caused by functional imbalance of the two fragments of the malonyl-CoA reductase (_mcr_), namely the _mcr-c_ and _mcr-n_ fragments [55]. The _mcr_ catalyzes the conversion of malonyl-CoA to 3 HP through malonate semialdehyde, with the _mcr-c_ fragment catalyzing the first step and the _mcr-n_ fragment catalyzing the second. [PERSON] et al. [55] tried to minimize the functional imbalances between the two fragments by employing directed evolution tools, aiming to enhance the action of the _mcr-c_ fragment and then fine-tune the levels of expression of the _mcr-n_ fragment. The newly engineered strain performed well during fed-batch cultivation on glucose, reaching a 3 HP production of to 40.6 g/L, with a conversion yield of 0.19 g/g. In another study, the \(\beta\)-alanine pathway was constructed in _E. coli_ cells [49]. The authors used a previously developed strain, which was capable of producing \(\beta\)-alanine [56], to construct the pathway towards 3 HP. Based on this strain, the authors tested several downstream enzymes for their efficiency regarding 3 HP production [49]. Among the different combinations, the most promising genes were the _ydf_G from _E. coli_ (encoding a malonic semialdehyde reductase) and the _pa0132_ from _Pseudomonas aeruginosa_ (encoding a \(\beta\)-alanine pyruvate transaminase). Fine-tuning of other genes related to the pathway (such as overexpression of _ppc_ gene, encoding phosphoenolpyruvate carboxylase, and replacement of the native promoter with the strong _lac_ promoter for the _sdhC_ gene, encoding a succinate dehydrogenase) resulted in the production of 31.1 g/L 3 HP, with a yield of 0.423 g/g. A different approach to conversion of glucose to 3 HP would be to take advantage of the naturally produced glycerol in the central metabolism of the host and construct a pathway for the conversion of glycerol to 3 HP. In this way, both glucose and xylose (two of the main sugars of lignocellulosic  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Genes Transfered** & **Culture Conditions** & **Concentration** & **Productivity** & **Ref. ** \\ \hline _acc_ from _C. glutamicum_ and _mcr_ from _C. aurantiacus_ & Fed-batch bioreactor 5 L, 2.5 L, pH 7, 37 \({}^{\circ}\)C, 1 vvm & 10.1 & 0.28 & [54] \\ \hline _AccADBC_, _mcr_ with enhanced activity of the _mc\(\sim\)_ fragment and tuning of the _mcr-c_ fragment levels of _mc\(\sim\)_ fragment & Fed-batch bioreactor 5 L, 2 L, pH 7, 37 \({}^{\circ}\)C & 40.6 & 0.56 & [55] \\ \hline _pa0132_ from _P. aeruginosa_, _ydf_G from _E.