Squalene

Squalene is a linear triterpene formed via the MVA or MEP biosynthetic pathway and is widely distributed in bacteria, fungi, algae, plants, and animals. Metabolically, squalene is not only used as a precursor in the synthesis of complex secondary metabolites such as sterols, hormones, and vitamins, but also as a carbon source in aerobic and anaerobic fermentation in micro-organisms. Owing to the increasing roles of squalene as an antioxidant, anticancer, and anti-inflammatory agent, the demand for this chemical is highly urgent. As a result, with the exception of traditional methods of the isolation of squalene from animals (shark liver oil) and plants, biotechnological methods using microorganisms as producers have afforded increased yield
This enzyme is so-called acetyl-CoA transferase functioning in the formation of acetoacetyl-CoA. Reaction chain from acetoacetyl-CoA to mevalonate continues to form 3-hydroxy-3 methylglutaryl-CoA (HMG-CoA) via the condensation in the presence of 3-hydroxy-3 methylglutaryl-CoA synthase (HMGS) [33]. In the presence of HMG-CoA reductase (HMGR) plus NADPH as the cofactor, HMG-CoA is later reduced to mevalonate [21], which is subsequently phosphorylated by mevalonate kinase (MK) in the presence of adenosine triphosphate (ATP) to form mevalonate 5-phosphate and is further phosphorylated by phosphomevalonate kinase (PMK) in the presence of ATP to form mevalonate 5-diphosphate and then decarboxylated by mevalonate 5-diphosphate decarboxylase (PMD) to form isopentenyl diphosphate (IPP), which is used as the chemical base to build most known polyprenyl compounds. IPP is then isomerized to DMAPP by isopentanyl diphosphate isomerase (IDI). The condensation of IPP with DMAPP by farnesyl diphosphate synthase (FPS) results in geranyl diphosphate (GPP), and the subsequent condensation with another IPP to form farnesyl diphosphate (FPP). Finally, squalene synthase has been identified as the enzyme that catalyzes the NADPH-mediated formation of squalene using FPP as the substrate [66] (Fig. 2A). Besides these,
Recently, Streptomyces peucetius ATCC27952 originated Spterp25 was successfully heterologously characterized in E. coli. The experimental results show that it is an enzyme functioning as a squalene-hopene cyclase [19] (Fig. 3).

Squalene synthases and its application
Squalene synthase is a very important enzyme catalyzing the first step of sterol/hopanoid biosynthesis in various organisms. Many studies have been reported on SQS such as in plant (Dioscorea zingiberensis (71), Withania somnifera (22), fungi (Candida tropicalis (38)). Detailed characteristics of squalene synthases are described in Table 1.
A Poria cocos – originated SQS was isolated and characterized using degenerate and inverse PCR. Transcriptional level of this SQS gene was increased about four folds by the treatment with P. cocos cell with 300 μM methyl jasmonate, enhancing the content of cellular squalene (128.62 μg/g) after 72 h induction [67].
Recently, directed evolution is a powerful technique to screen and recruit an