**Conflict of interest**

*Saccharomyces*

by *S. cerevisiae* [59].

by CRISPRi [60].

strains will be a challenge [27].

by metabolic engineering. *E. coli* and *Saccharomyces cerevisiae* have been used for the production of biofuels. The classification of natural products is based on their structures. This classification contains alkaloids, terpenoids, phenylpropanoids, and polyketides. The source of natural products are natural sources. The cost of extracting natural products is high. The extraction of natural products leads to low yield. Natural products can be produced by chemical synthetic routes. However, production by chemical synthetic routes can generate multistep reactions and stereoisomers. Natural product production can be performed by metabolic engineering strategies. Selection of a host strain is based on the product. There are three ways to choose the host strain. First, the desired product can be overproduced by the host strain. Second, the desired product can be produced with low efficiency. Third, the target product is not produced by the host strain. The host strain will be generated using different metabolic engineering strategies. *E. coli* and *S. cerevisiae* are popular host strains to produce biodiesel hoewever, the efficiency is low. 'Generally recognized as safe' (GRAS) microorganisms should be used to produce food and pharmaceutical products for the safety issues. *Bacillus subtilis* and *S. cerevisiae* are well-known GRAS strains [57]. Artemisinic acid is known as an anti-malarial drug precursor and is produced in *S. cerevisiae* by introduction of heterologous pathways. Opioids were produced in *S. cerevisiae* [58]. Systems metabolic engineering can enhance the production of recombinant proteins such as artemisinic acid. Some of the target pathways are not available in microorganisms. Enzymes and metabolic pathways for the desired product should be designed by metabolic engineering. For example, lactams cannot be produced by natural pathways, and de novo pathways should be designed for lactams. Penicillin is a beta-lactam non-ribosomal peptide. Baker's yeast *Saccharomyces cerevisiae* can produce and secrete penicillin by metabolic engineering. Five genes in the benzylpenicillin pathway in *P. chrysogenum* were integrated into *S. cerevisiae*. Bioactive benzylpenicillin is then produced and secreted

If natural pathways of the production of the desired product is unknown, then GEM-Path, DESHARKY, RetroPath and RetroRules are used as prediction algorithm tools for metabolic pathway design. Mutations should be identified after these methods. Colorimetric assays, spectrophotometer fluorescence-activated cell sorting (FACS), or microfluidic sorting devices can be used to identify the beneficial mutation in the organism. The pathway should be optimized after the metabolic pathway is constructed in the host strain. Genome-scale metabolic simulation, plasmids, regulatory RNAs, and genome engineering are used to optimize the pathways of the host strain. Recombination-mediated genetic engineering is used to optimize pathways and produce the desired product efficiently. RecABCD system-based homologous recombination, the l Red recombination, site-specific recombination systems including Cre-lox and flippase-flippase recombinase target (Flp-FRT), zinc finger nuclease (ZFN) and CRISPR along with CRISPR/Cas are genome engineering tools [57]. The b-amyrin is a pentacyclic triterpenoid compound and was produced by *S. cerevisiae* strain engineered

Scale-up fermentation is an important step for biopharmaceuticals. The strain's growth performance and optimal fermentation conditions have been validated for lab-scale fermentation (0.5–30 L). After lab-scale fermentation is approved, pilot-scale fermentations (30–3000 L) and large scale production (3000–20,000 L) will be performed to see the conditions of the strain and the product of interest. Full-scale (20,000–2,000,000 L) production fermentation will be performed for the production of biopharmaceuticals. In scale-up fermentation, gradients of feed, oxygen concentrations, and maintaining the genomic stability of high-performing

**10**

The author declares no conflict of interest.
