## 3 Biosynthetic Pathways and Key Enzymes of Mushroom Polysaccharides
### Effects of Key Enzymes on PS Production and Monosaccharide Composition  
Enzymes in the PS synthesis pathways of mushrooms play an important role in PS production and regulation of monosaccharide composition. These enzymes include Phosphoglucose isomerase, \(\alpha\)-phosphoglucomutase, UDP-Glc pyrophosphylase and so on. Phosphoglucose isomerase (PGI; EC 5.3.1.9) catalyzes the interconversion of Glc-6-phosphate and Fru-6-phosphate, and plays key roles in glycolysis and gluconeogenesis pathways. PGI catalyzes the reaction that generates Fru-6-phosphate, the precursor of GDP-Man in PS synthesis [16]. Thus, PGI activity level determines the direction of Glc-6-phosphate conversion. PGI may also control PS synthesis to some extent. [PERSON] et al. observed a strong correlation between PGI activity level and the amount of IPS produced [11]. [PERSON] et al. showed that the addition of coix lacryma-jobi oil (CLO) to culture medium enhanced EPS and IPS production by directly affecting synthesis levels of PGI and \(\alpha\)-phosphoglucomutase (PGM) at different stages [70]. In a study by [PERSON] et al., higher sucrose level increased PGM activity, and also enhanced PS production through inhibition of PGI activity [12]. Similar results were reported by [PERSON] and [PERSON] [71]. \(\alpha\)-phosphoglucomutes (PGM; EC 5.4.2.2), a key enzyme in carbohydrate metabolism, catalyzes the reversible transfer of a phosphate group between C-1 and C-6. PGM converts Glc-6-P and Glc-1-P; Glc-6-phosphate enters a glycolysis pathway to yield energy, and Glc-1-phosphate is the precursor of sugar nucleotides (e.g., UDP-Glc, dTDP-Rha, UDP-Gal) in PSs [10; 16]. In the study by [PERSON] and [PERSON], PGM activity was correlated with the amount of EPS produced by _G. lucidum_[71]. [PERSON] et al. investigated the effects of PGM gene overexpression on PS production and transcription levels of three genes, and demonstrated that the PGM gene is an important regulatory gene in PS biosynthesis in _G. lucidum_[14]. PGM gene overexpression in submerged culture resulted in increased PS production and upregulation of downstream genes encoding the enzymes involved in PS biosynthesis, including PGM, UDP-Glc pyrophosphorylase (UGP), and \(\beta\)-1,3-glucan synthase (GLS) [14]. PGM activity was also affected by the addition of coixenolide during PS production in _G. lucidum_[10]. In a submerged culture of _C. militaris_, PGM was the key enzyme for regulation of IPS biosynthesis [11]. UDP-Glc pyrophosphorylase (UGP; also known as UGPase or UTP: Glc-1-phosphate uridylyltransferase; EC 2.7.7.9), a member of the Pfam glycosyl transferase family (PF01702), catalyzes the reversible interconversion of Glc-1-phosphate and UDP-Glc [64]. UGP is a key enzyme involved in carbohydrate metabolism and cell wall biosynthesis [72]. Studies by [PERSON] et al. and [PERSON] et al. showed that the UGP gene (_ugp_) is associated with the synthesis of LPS and EPS surface structures, because its substrate/product UDP-Glc is the precursor of these PSs [73; 74]. [PERSON] et al. cloned the _ugp_ gene from _G. lucidum_, and examined the effect of RNAi silencing of _ugp_ on PS synthesis. In liquid culture, EPS concentration in _ugp_-silenced strains was much lower (30-56%) than that in wild type (WT) strains. In contrast, IPS concentration did not differ significantly between _ugp_-silenced and WT strains [64]. [PERSON] et al. showed that overexpression of homologous _ugp_ in submerged culture _G. lucidum_ enhanced PS production; maximal IPS and EPS levels in the _ugp_-overexpressing strains were respectively 42% and 36% higher than in WT strains [13]. [PERSON] et al. found that higher UGP activity was correlated with higher IPS biosynthesis rate in _Cordyceps militaris_[11], suggesting an important role of UGP in PS biosynthesis. However, a controversial conclusion was found by [PERSON] et al., [PERSON] et al., and [PERSON] et al.