## 2 Specialized Metabolic Pathways for Supply of Alternative Precursors for Secondary Metabolism
### Exoenzymes and Proteases  
_Streptomyces_ typically act as saprophytes in soil. The nutrients found in this habitat are very diverse and many different polymers that can be used in primary and secondary metabolisms are present there. These include starch, cellulose, chitin, keratin, lignin, xylan and many others. Such compounds are mostly of plant origin, and therefore, the secretion of extracellular enzymes is required for the breakdown of these polymers, especially insoluble ones such as starch and cellulose. Soil environments are typically carbohydrate-rich but relatively deficient in nitrogen and phosphate [6; 72].  
In addition to taking up available building blocks or compounds that can be converted into building blocks, _Streptomyces_ can rely on macromolecules that are broken down by exoenzymes, and whose fragments can then be picked up and processed as precursors. Their use of non-catalytic substrate-binding proteins and hydrolytic enzymes to obtain soluble nutrients from carbohydrates enables _Streptomyces_ to interact with other organisms and promote cell growth. _Streptomyces_ have a large repertoire of exoenzymes that have been extensively investigated and includes xylanases, proteases, cellulases, ligninases, proteases, chitinases, lipases, nucleases, keratinases and others [73; 74; 75]. Such extracellular enzymes are a collection of enzymes with a vital role in cell viability, stress responses and pathogenicity.  
The insoluble nitrogen-containing polysaccharide chitin is a major nutrient source for streptomyces. Chitases have been isolated from various _Streptomyces_ strains [73]; most species contain chitinase genes, enabling them to hydrolyze chitin. For instance, the _S. coelicolor_ genome encodes 11 deduced family-18 chitinases and two family-19 chitinases. Furthermore, streptomyces have a variety of extracellular enzymes for xylan hydrolysis [76] and can degrade carboxymethyl cellulose using secreted enzymes; 25 of the 180 _Streptomyces_ strains can hydrolyze Avicel (microcrystalline cellulose). Most streptomyces are saprophytic, but some phytopathogenic species like _S. scabies_ are dependent on the inhibition of plant cellulose synthesis that is achieved though the production of thaxtomins and 4-nitroindol-3-yl-containing 2,5-dioxopiperazines [77]. _Streptomyces_ spp. are large producers of proteases [78]. For example, in _S. coelicolor_, 21 genes code for aminopeptides, 27 for serine proteases and 8 for metalloproteinases [79]. Extracellular proteases are involved in assimilating extracellular proteinaceous nitrogen sources, such as keratin by using keratinases [80]. Some of the numerous secreted proteases participate in developmentally significant extracellular cascades, which lead to the cannibalization of the substrate mycelium biomass to support aerial growth and sporulation [72; 78].  
Numerous reviews are dedicated to the exoenzymes in _Streptomyces_ (e.g., see [72]). In this paper, the focus is on the interconnection between the exoenzymes and secondary metabolism in _Streptomyces_, especially the supply of the main building blocks for secondary metabolism (N, C, P and S).