# Biological Production of 3-Hydroxpropionic Acid: An Update on the Current Status
## 4 Production of 3 HP from Other Sources  
One of the alternative carbon sources that have been used for the production of 3 HP is propionic acid. For this reason, the propionyl-CoA pathway (which has already been described) was constructed in _E. coli_ by transferring a propionyl-CoA dehydrogenase gene (_pacd_) from _Candida rugora_, a propionate-CoA transferase gene (_pct_) from _Megasphaera elsdenii_, and a 3-hydroxypropionyl-CoA dehydatase gene (_lpcd_) from _C. aurantiacus_[51]. In addition, deletion of the _ygfH_ and _ppPC_ genes led to the production of 3 HP at 2.2 g/L (Table 7). The main drawback of using propionate is that it is not a low-cost carbon source, which affects the production cost of 3 HP. Moreover, the conversion yield obtained in this work was relatively low (35.4%). On the other hand, as the authors also suggested, more work should be conducted to fully understand and control the pathway, and propionyl-CoA could be derived from glucose through succinyl-CoA, thus avoiding the use of propionic acid. Recent research has shown that 3 HP can also be produced from hydrolysis of the toxic compound 3-hydroxypropionitrile, which is a platform chemical widely used in many organic syntheses for the production of medicines, pesticides, and polymeric compounds [61]. Hydrolysis of 3-hydroxypropionitrile can yield 3 HP; however, the chemical hydrolysis is not an environmentally friendly process, as it requires the use of strong basic solutions and high temperatures, and generates hypersaline waste water [62]. Green methods for the hydrolysis of 3-hydroxypropionitrile involve the use of biocatalysts, such as enzymes or whole cells. [PERSON] et al. [63] tried to isolate microorganisms that had nitrile-hydrolyzing activity from environmental samples. During their work, they identified a yeast strain, _Meyerozyma guillermondii_ CGMCC12935, with nitrile-hydrolyzing activity. The catalytic activity of the resting yeast cells was found to be optimal at 55 \({}^{\circ}\)C and at pH 7.5, with the enzyme showing a broad activity against various nitriles (with 3-hydroxypropionitrile being among them). The authors also evaluated the effects of several metal ions and alcohols on the nitrilase activity, and the presence of Ag\({}^{+}\) and Pb\({}^{2+}\) turned out to be inhibitory. During the bioconversion of 3-hydroxypropionitrile to 3 HP from the resting yeast cells, addition of glucose was found to be beneficial for the conversion yield, which reached a production of 19.5 g/L 3 HP from 35.5 g/L 3-hydroxypropionitrile. In another study, whole-cell biocatalysis was achieved by using recombinant _E. coli_ cells harboring a particular nitrilase gene, which was selected among 15 different nitrilase genes tested [62]. The authors tested both free and immobilized cells, optimizing the incubation parameters for the highest bioconversion yields. The newly developed biocatalyst was very robust and capable of hydrolyzing up to approximately 320 g/L 3-hydroxypropionitrile in 24 h when free cells were used. Immobilization of the cells allowed the use of a concentration of 3-hydroxypropionitrile as high as 497 g/L, which gave complete hydrolysis within 24 h. The immobilized cells showed remarkable stability during the reusability trials, retaining their activity for 30 batches, with a production of 184.7 g/L 3 HP during these 30 batches, and a volumetric productivity of 36.9 g/L-h. Despite the obvious focus on heterotrophic 3 HP production from renewable resources, the interest of some research groups lies in studying autotrophic producers, such as photosynthetic cyanobacteria, which have the ability to transform sunlight and CO\({}_{2}\) into a variety of compounds. After modifications by genetic engineering, the recombinant strains are also capable of producing 3 HP through CO\({}_{2}\) fixation by the Calvin cycle, and further transformation to phosphoenolpyruvate, which can be converted to 3 HP by two newly developed pathways, namely the malonyl-CoA intermediate pathway and the \(\beta\)-alanine intermediate pathway [64; 65].