## 3 Chassis Strains for 3-HP Production
### Klebsiella pneumoniae  
_K. pneumoniae_ is able to grow rapidly with glycerol as the only carbon source under both aerobic and aerobic conditions. Two groups' regulons involved in glycerol metabolism have been identified in _K. pneumoniae_. Under aerobic conditions, the glp reg  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Microorganisms** & **Carbon Source** & **Pathway** & **Engineering Strategy** & **Titer** & **Productivity** & **Yield** & **Ref. ** \\ \hline _S. cerevisiae_ & Glucose & Malonyl-CoA & Expression of _mcr_, _ALD6_, _pdtcl_, _acc1_, _acc1_, _acs_, _GAPDH_ & 9.8 & 0.09 & 0.06 & [44] \\  & & pathway & Expression of _pycl_, _pyc2_, _AT2_, _panD_, _Bapart_, and _sdf_G; optimization of _culture conditions & about 25 & - & 0.25 & [45] \\ _S. cerevisiae_ & Glucose & Oxaloacetate & Expression of _pyc_, _malC_, _pathway_ & _mmsB_, _glcT_, _ptcl_ & 18.1 & 0.17 & 0.125 & [26] \\  & Glucose or & Glycerol oxidation & Expression of _gpd_ and _via the coenzyme & _gpp_, _pdtcl_, _mcECH_, _anE_ and _sdf_AB; deletion of _ldhA_, _pdtacA_, _pdxB_, _gpk_ & 62.6 or 54.8 & 0.86 or 0.76 & 0.51 or 0.49 ** & [46] \\  & & xylose) & Pathway & _pdtacA_, _pdxB_, _gpk_ & & & \\ _C. glutamicum_ & Acetate & Malonyl-CoA & Optimization of _mc_ and _pathway_ & 17.1 & 0.14 & 0.10 & [47] \\ _H. bluzplugenesis_ & 1,3-Propenediol & 1,3-Propenediol & Expression of _dhnT_ & 154 & 2.4 & 0.93 ** & [48] \\  & Glucose and & 1,3-Propenediol & Expression of _ADH_ & Expression of _ADH_ & 45.8 & 1.86 & 1.14 * & [24] \\ _C. glutamineum_ & Glucose and & Glycerol oxidation & Glycerol via the coenzyme & Expression of _dhnB_ and _pdtacA_-independent _pdtch_, deletion of _glpK_ & 10 & - & 0.79 & [49] \\  & & & pathway & Expression of _dhnB_, _gdrAB_ and _ksgadii_; _gdchA_ of _gpdA_ & & & \\ _P. denitrificans_ & Glycerol or & via the coenzyme & deletion of _3 tphA_, _3 tphA_; _pdtacB_ glycerol & A-independent pathway & 102 or 65 & 2.5 or - & - & [50] \\  & & Glycerol oxidation & Expression of _dhnB_, _gdrAB_ and _ksgadii_; _gdchA_ of _g  
ulon (encodes a series of enzymes required for the conversion of glycerol to DHAP via sn-glycerol-3-phosphate) acts to oxidize and catabolize glycerol, mainly via the Embden-Meyerhof-Parnas (EMP) pathway. Under anaerobic conditions, instead of the glp regulon, the dha regulon (encodes a series of enzymes needed for the anaerobic growth on glycerol in the absence of exogenous electron acceptors) acts to oxidize glycerol via the EMP pathway, as well as to dehydrate glycerol to 3-HPA via GDH (encoded by _dhaB_), which can be converted to 3-HP [54]. Moreover, it possesses native genes associated with both the coenzyme A-dependent and coenzyme A-independent 3-HP biosynthetic pathways. However, the studies on 3-HP production in _K. pneumoniae_ were mainly focused on glycerol oxidation via the coenzyme A-independent pathway. Coenzyme B\({}_{12}\), which is an expensive but important co-factor for the dehydration of glycerol to produce 3-HPA, is naturally produced by _K. pneumoniae_ under anoxic conditions. Interestingly, the synthesis of coenzyme B\({}_{12}\) and the transcription of _dhaB_ showed a decreasing trend with increasing aeration, while the regeneration of NAD\({}^{+}\) (a co-factor of ALDH) through the electron transport chain required oxygen [55]. Therefore, the degree of aeration has a significant effect on the production of 3-HP in _K. pneumoniae_. [PERSON] et al. [56] investigated the production of 3-HP by engineered _K. pneumoniae_ at different aeration levels. Under high aeration and anaerobic conditions, only a small amount of 3-HP was produced from glycerol. High titers of 3-HP were only produced under microaereophilic conditions with constant dissolved oxygen (DO) at 5%. At present, the highest 3-HP titer in _K. pneumoniae_, which reached 102.61 g/L, was achieved by overexpressing PuuC through three tandem repeats of the tac promoter [27].