2. Biosynthesis
The biosynthetic pathway is greener and more efficient for producing structurally diverse natural products. This pathway acts as an alternative to chemical synthesis and microbial transformation, which suffer from complex routes, difficult isomer separation, and high cost. Biosynthesis, also known as anabolism, is a simple compound that is joined together and forms a macromolecule using enzymes. Extreme reaction conditions and many toxic chemicals are required for the chemical synthesis of flavonoids [23]. One of the advantages of biosynthesis is the production of rare and expensive natural products because of the development of molecular biological tools and availability of genome information from a variety of organisms. Biosynthesis can be used in simple and complex transformations without disturbing the blocking and deblocking steps, which are common in organic synthesis [24]. The mechanism of the flavonoid biosynthesis pathway in mulberry leaves is not yet clear, and currently, this is a much-needed topic for scientists to explore. Flavonoids are synthesized through the shikimate pathway followed by the phenylpropanoid metabolic pathway and possess approximately 15 carbon atoms arranged in three aromatic rings linked as C6-C3-C6 [25]. Several enzymes are involved in the shikimate pathway, which is a six-step reaction in the biosynthesis of shikimic acid. The reaction as shown in figure 5 begins with a simple aldol condensation reaction of phosphoenolpyruvic acid and D-erythrose 4-phosphate [Dias MC, Pinto DCGA, Silva AMS. (2021) Plant flavnoids: Chemical Characteristics and Biological Activity. Molecules, 26:5377.]. The end product of chorismic acid is converted into the amino acid phenylalanine by the action of prephenate-aminotransferase (PhAT) and aromate-dehydrate (ADT) enzymes [Tariq H, Asif S, Andleeb A, Hano C, Abbasi BH. (2023) Flavonoid Production: Current trends in plant metabolic engineering and de novo microbial production. Metabolites, 13:124.].