## 4 Renewable Carbon Sources for the Biosynthesis of 3-HP
### Microbial Production of 3-HP from Glucose or Xylose
#### 4.1.2 Biosynthesis of 3-HP via the Malonyl-CoA Pathway  
The malonyl-CoA pathway is the most extensively studied biosynthetic pathway when using glucose as a substrate to produce 3-HP, and the highest titer of 3-HP obtained from glucose via the malonyl-CoA pathway (49.04 g/L) [9] was comparable to that obtained via the glycerol pathway (53.7 g/L) [70] in _E. coli._ By regulating the expression of key enzymes in the production pathway, enhancing the supply of acetyl-CoA, and inhibiting the synthesis of fatty acids, the researchers constructed a microbial cell factory that can efficiently produce 3-HP from glucose. The imbalance between the activity of the two functional domains of MCR, the key enzyme of the malonyl-CoA pathway, is an important factor that limits the production of 3-HP. [PERSON] et al. [72] overexpressed directionally evolved MCR-C and reduced the expression level of MCR-N to bring MCR-N and MCR-C into equilibrium, reaching a 3-HP titer of 40.6 g/L, corresponding to a yield of 0.19 g/g of glucose in fed-batch fermentation. Increasing the activity of ACC, another key enzyme of the malonyl-CoA pathway, is a common strategy to increase the 3-HP titer. Acc1 activity is tightly regulated at the transcriptional level and the protein level in _S. cerevisiae._ Manipulation of the phospholipid synthesis transcriptional regulators to control the transcription level of Acc1 [73] or the use of the phosphorylation-deficient mutant Acc1 [74] are effective strategies for increasing Acc1 activity. Recently, one strategy for the balanced expression of ACC subunits (AccBC and DtsR1) from _C. glutamicum_ was developed in _E. coli_, and the engineered strain achieved a maximal productivity of 1.03 g/(L-h) with a high yield (0.246 g/g glucose) and titer (up to 38.13 g/L) through the malonyl-CoA pathway using a cheap carbon source in fed-batch fermentation [75]. In order to further increase 3-HP titer, it is necessary to direct carbon flux towards the key precursor acetyl-CoA. Interestingly, an increase in acetyl-CoA flux in yeast was achieved by enhancing the pathway from pyruvate to acetyl-CoA via acetate, rather than by the direct production of acetyl-CoA from pyruvate. [PERSON] et al. [44] integrated multiple copies of _mcr_ from _C. aurantiacus_ and a phosphorylation-deficient _acc1_ variant into the genome of _S. cerevisiae_, after which the supply of acetyl-CoA was optimized by overexpressing key enzymes (acetyl-CoA synthase (ACS), aldehyde dehydrogenase (ALD6), and pyruvate decarboxylase (Pdc1)) associated with the transformation of acetate into acetyl-CoA. As a result, the strain produced 9.8 g/L of 3-HP from glucose in fed-batch fermentation. Cellobiose was used as a substrate for 3-HP biosynthesis by displaying \(\beta\)-glucosidase (BGL) on the surface of _S. pombe_, which hydrolyzes cellobiose to glucose. In addition, the acetyl-CoA supply was enhanced by the overexpression of endogenous ATD1 (an _S. pombe_ homolog of the _S. cerevisiae_ ALD6) and ACS from _E. coli_ in the engineered strain with an introduced malonyl-CoA pathway, and the engineered strain produced 11.4 g/L of 3-HP in fed-batch fermentation [42]. It is also possible to enhance the supply of acetyl-CoA for 3-HP biosynthesis from glucose by the exogenous addition of acetate [61]. Malonyl-CoA is also a precursor of fatty acid synthesis, but preventing malonyl-CoA spillover by knocking out the fatty acid synthesis pathway is not feasible, as it results in lethality [76]. Researchers have attempted to inhibit fatty acid synthesis by the exogenous addition of cerulenin [77; 78], but the high cost of cerulenin makes it unsuitable for the mass production of 3-HP. The genes involved in fatty acid synthesis can be downregulated by antisense RNA technology [76] or the overexpression of the FadR (fatty acid degradation inhibitor) strategy [34], resulting in an enhanced supply of malonyl-CoA in _E.