Secondary metabolites can be derived from primary metabolites through modifications, like methylation, hydroxylation and glycosylation. Secondary metabolites are naturally more complex than primary metabolites and are classified on the basis of chemical structure (e.g., aromatic rings, sugar), composition (containing nitrogen or not), their solubility in various solvents or the pathway by which they are synthesized (Table 31.1). They have been classified into terpenes (composed entirely of carbon and hydrogen), phenolics (composed of simple sugars, benzene rings, hydrogen and oxygen) and nitrogen and/or sulphur containing compounds (Figure 31.2). It has been observed that each plant family, genus and species produces a characteristic mix of these bioactive compounds.
All plants produce secondary metabolites, which are specific to an individual species, genus and are produced during specific environmental conditions which makes their extraction and purification difficult. As a result, commercially available secondary metabolites, for example, pharmaceuticals, flavours, fragrances and pesticides etc. are generally considered high value products as compared to primary metabolites and they are considered to be fine chemicals.
Table 31.1: Classification of secondary metabolites
Figure 31.2: The production of secondary metabolites is tightly associated with the pathways of primary/central metabolism, such as glycolysis, shikimate and production of aliphatic amino acids.