# Biological Production of 3-Hydroxpropionic Acid: An Update on the Current Status
## 3 Production of 3 HP from Sugars
### Use of S. cerevisiae as a Host Microorganism  
Regarding the use of glucose as carbon source, the yeast _S. cerevisiae_ is a very promising candidate, as it is very robust, is easy to handle, and described as GRAS. Another positive characteristic of this yeast is its high tolerance of low pH values, which can result from accumulation of 3 HP [58; 59]. To permit conversion of glucose to 3 HP, the malonyl-CoA pathway was also constructed in _S. cerevisiae_ cells [59]. This was done by expressing multiple copies of the _mcr_ gene from _C. aurantiacus_ and mutated _acc1_ genes. Moreover, in an attempt to improve the supply of acetyl-CoA, the native pyruvate decarboxylase _pdc1_ gene and the aldehyde dehydrogenase _ald6_ gene were overexpressed together with the expression of the acetyl-CoA synthase gene (_acs1641P_) from _Salmonella enterica_; these modifications increased the 3 HP titer by 80%. Finally, the intracellular NADPH, which is required for the action of the _mcr_, was increased by transferring the NADP-dependent glyceraldehyde-3-phosphate dehydrogenase gene (_gapdh_) from _Clostridium acetobutylicum_. The strain constructed produced 9.8 g/L 3 HP with a yield of 13% C-mol/C-mol (Table 5).  
In another study with _S. cerevisiae_ as host microorganism, the \(\beta\)-alanine pathway--which was identified through metabolic modeling to be the most attractive pathway from an economic point of view--was constructed [48]. De novo \(\beta\)-alanine synthesis was performed by the action of a \(\beta\)-alanine-pyruvate aminotransferase (bapat) from _Bacillus cereus_ (_ylhxA_ gene), which is a newly used bapat enzyme. The authors examined the effect on 3 HP production of different combinations of the _ylhxA_ gene (or another bapat gene from _Pseudomonas putida_) with various 3-hydroxypropionate dehydrogenase (hpdh) and 3-hydroxyisobutyrate dehydrogenase (hibadh), the most efficient combination being _ylhxA_ with the hpdh from _E. coli_ (_ydfG_ gene). Different aspartate-1-decarboxylases (_panD_) were also tested, the most promising being the one derived from _Tribolium castaneum_. Finally, improvement of the production yields was achieved by improving the supply of l-aspartate by overexpressing a native cytoplasmic aspartate aminotransferase (_aat2_) and two pyruvate carboxylases (_pyc1_, _pyc2_ genes). The strain that was constructed performed well in fed-batch bioreactors, producing 13.7 g/L 3 HP with a conversion yield of 0.14 C-mol/C-mol.  
Apart from glucose, xylose is also an important sugar that can be derived from lignocellulosic biomass, and it would be important to incorporate it in the 3 HP production process. One attempt to convert xylose to 3 HP in _S. cerevisiae_ was conducted by [PERSON] et al. [60] by incorporating either the malonyl-CoA pathway or the \(\beta\)-alanine pathway (using two approaches, one NADH-dependent route and one NADPH-dependent route) in a xylose-utilizing strain. The three different pathways were tested with the yeast growing on glucose or xylose, and the malonyl-CoA route gave the best results on glucose, and the NADPH-dependent \(\beta\)-alanine pathway resulted in the highest 3 HP yields from xylose. The second route was constructed by expressing two pyruvate carboxylases (_pyc1_ and _pyc2_), a bapat from _B. cereus_, a _panD_ gene from _T. castaneum_, and the NADPH-dependent hpdh from _E. coli_ (_ydfG_ gene). The strain performed well on xylose, producing 1.8 g/L during flask culture, which increased to 6.1 g/L when the culture was performed in batch bioreactors. Fed-batch cultivation further improved 3 HP production to 7.4 g/L, with an overall yield of 29% C-mol/C-mol. Although the concentration was relatively low, the results of this work are very promising, as glucose and xylose are two major sugars in lignocellulosic biomass, and further optimization of the strain would enable culture on lignocellulosic hydrolysates.