## 4 Bioactive Metabolites
### Proteins and Peptides  
Proteins are high-molecular-weight polymers made of amino acids linked together by peptide bonds and are essential for human health. Increased population and current dietary trends have resulted in increased demand for protein and meat alternatives, and it has been proposed that mushroom proteins can efficiently substitute meat. Fungal proteins contain all nine essential amino acids required by humans, in addition to carbohydrates, vitamins, and carotenes. Mycoprotein is considered a high-quality protein comparable to animal protein since it provides a high protein-to-energy ratio. At the same time, it contributes to minimizing animal slaughter, reducing the carbon footprint of food production, and can be produced with low total costs independently of climate and landscape limitations [117]. _Pleurotus_ spp. are well known for efficiently recycling agro-industrial wastes and producing biomass with a protein content of up to 49% of their dry weight [118]. Submerged cultivation of _P. sajor-caju_ in 5-L stirred tank bioreactor using 10 g/L glycose resulted in fast production of mycelium biomass (8.18 g/L) with a high content of protein (32%) [119]. Similarly, _Cordyces millitaris_ mycelium produced in submerged cultivation had higher protein content than that of the fruiting body (21.10% vs. 18.47%), and higher total essential amino acid concentration than that found in eggs [120]. _A. bisporus_ cultivated in shake flasks with synthetic media produced 12.67 g/L biomass containing 36% \(w/w\) protein and noteworthy amounts of IPS (2.57 g/L) in only 26 days [121]. The screening of Basidiomycetes: _A. aegerita_, _Pleurotus sapiens_, _L. edodes_, _Wolfporia cocos_, _Stropharia rugosaunulata_, _P. sajor-caju_, and _P. salmonestramineus_ grown in submerged cultures using industrial side streams as the sole carbon successfully converted the residues into biomass containing protein enhanced with lipids, carbohydrates, and vitamin D\({}_{2}\). _P. sapidus_ excelled in the screening and produced biomass containing 21% true protein, 4% lipids, and 115 \(\upmu\)g/g vitamin D\({}_{2}\) in 4 days [24]. Making use of statistical optimization, _Morchela fluvialis_ mycelium with 38% \(w/w\) protein content was produced in liquid culture, deeming it a promising source of essential amino acids for the human diet [122]. _Pleurotus_ species cultivated on agro-industrial by-products produced a total of 22 free amino acids in quantities up to 130.12 mg/g dw plus the neurotransmitter GABA and ornithine [123]. The connection between dietary protein, the gut microbiota, and overall human health is also being investigated [124], and a single band protein (HEP3) isolated from _H. erinaceus_ was found to improve the immune functions of mice by regulating the gut microbiota [125]. Apart from dietary proteins, macrofungi produce various bioactive proteins and peptides classified as lectins, fungal immunomodulatory proteins (FIP), ribosome-inactivating proteins (RIP), antimicrobial proteins, and ribonucleases. Lectins are nonimmune proteins or glycoproteins that bind specifically to carbohydrates and result in cell agglutination. This property of lectins makes them useful in studying the structure of cell surfaces and the isolation and purification of polymers with carbohydrate units [126]. In the last years, many lectins with antiproliferative, antitumor, cytotoxic, and immunomodulatory activities have been isolated from edible mushrooms such as _A. bisporus_, _F. velutipes_, _G. lucidum_, _G. fondose_, and _V. volvacea_[127]. FIPs are a novel family of bioactive proteins isolated from macrofungi that exhibit significant activity in suppressing the invasion and metastasis of tumor cells and show promise as immunologic adjuvants for tumor treatment [128]. RIPs are enzymatically active proteins that can inactivate ribosomes by eliminating adenosine residues from rRNA. They have been studied for their ability to inhibit HIV replication and viral disease prevention and are promising substances for developing drugs [129].