Quercetin and its derivatives are important metabolites that belong to the flavonol class of flavonoids. Quercetin and some of the conjugates have been approved by the FDA for human use. They are widely distributed among plants and have various biological activities, such as being anticancer, antiviral, and antioxidant. Hence, the biosynthesis of novel derivatives is an important field of research. Glycosylation and methylation are two important modification strategies that have long been used and have resulted in many novel metabolites that are not present in natural sources. A strategy for modifying quercetin in E. coli by means of glycosylation, for example, involves overexpressing respective glycosyltransferases (GTs) in the host and metabolic engineering for increasing nucleoside diphosphate sugar (NDP-sugar). Still others have used microorganisms other than E. coli, such as Streptomyces sp., for the biotransformation process. The overall study of the structural activity relationship has revealed that modification of some residues in quercetin decreased one activity but increased others. This review summarizes all of the information mentioned above.

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Anthraquinone and its derivatives show remarkable biological properties such as anticancer, antibacterial, antifungal, and antiviral activities. Hence, anthraquinones derivatives have been of prime interest in drug development. This study developed a recombinant Escherichia coli strain to modify chrysazin to chrysazin-8-O-α-l-rhamnoside (CR) and chrysazin-8-O-α-l-2′-O-methylrhamnoside (CRM) using rhamnosyl transferase and sugar-O-methyltransferase. Biosynthesized CR and CRM were structurally characterized using HPLC, high-resolution mass spectrometry, and various nuclear magnetic resonance analyses. Antimicrobial effects of chrysazin, CR, and CRM against 18 superbugs, including 14 Gram-positive and 4 Gram-negative pathogens, were investigated. CR and CRM exhibited antimicrobial activities against nine pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA) in a disk diffusion assay at a concentration of 40 µg per disk. There were MIC and MBC values of 7.81–31.25 µg/mL for CR and CRM against methicillin-sensitive S. aureus CCARM 0205 (MSSA) for which the parent chrysazin is more than >1000 µg/mL. Furthermore, the anti-proliferative properties of chrysazin, CR, and CRM were assayed using AGS, Huh7, HL60, and HaCaT cell lines. CR and CRM showed higher antibacterial and anticancer properties than chrysazin.

Proteases were isolated from leaf, bark and root of Choerospondias axillaris, locally called Lapsi. Choerospondias axillaris is a dioecous, deciduous fruit bearing plant with multiple daily uses. The protease was extracted with 0.1 M phosphate buffer of pH 7 and then precipitated successively with TCA and ammonium sulfate. The protease from leaf showed maximum activity at pH 9, temperature at 20°C. While, bark protease showed optimum pH 5 and temperature 60°C. In case of root the optimum pH was 10 and the optimum temperature 35°C. The optimum time of incubation leaf, bark and root was 15 minute. The bell shaped curve was obtained for the effect of enzyme conc. with optimum enzyme conc. 50 µg for leaf, 30 µg for bark and 50 µg for root. The K m and V max value of leaf were 5.61 µM and 185.18 pmol/ min, K m and V max value of bark were 2.36 µM and 82.64 pmol/min. While for root, the K m and V max values were 1.53 µM and 52.91 pmol/min.

Spinosad, a combination of spinosyn A and D produced by Saccharopolyspora spinosa, is a highly efficient pesticide. There has been a considerable interest in the improvement of spinosad production because of a low yield achieved by wild‐type S. spinosa. In this study, we designed and constructed a pIBR‐SPN vector. pIBR‐SPN is an integrative vector that can be used to introduce foreign genes into the chromosome of S. spinosa. Different combinations of genes encoding forasamine and rhamnose were synthesized and used for the construction of different recombinant plasmids. The following recombinant strains were developed: S. spinosa pIBR‐SPN (only the vector), S. spinosa pIBR‐SPN F (forosamine genes), S. spinosa pIBR‐SPN R (rhamnose genes), S. spinosa pIBR‐SPN FR (forosamine and rhamnose genes), S. spinosa pIBR‐SPN FRS (forosamine, rhamnose, and SAM [S‐adenosyl‐L‐methionine synthetase] genes), and S. spinosa MUV pIBR‐SPN FR. Among these recombinant strains, S. spinosa pIBR‐SPN FR produced 1394 ± 163 mg/L spinosad, which was 13‐fold higher than the wild‐type. S. spinosa MUV pIBR‐SPN FR produced 1897 (±129) mg/L spinosad, which was seven‐fold higher than S. spinosa MUV and 17‐fold higher than the wild‐type strain.

Type III polyketide synthase (PKS) found in bacteria is known as 1,3,6,8-tetrahydroxynaphthalene synthase (THNS). Microbial type III PKSs synthesize various compounds that possess crucial biological functions and significant pharmaceutical activities. Based on our sequence analysis, we have identified a putative type III polyketide synthase from Nocardia sp. CS682 was named as ThnA. The role of ThnA, in Nocardia sp. CS682 during the biosynthesis of 1,3,6,8 tetrahydroxynaphthalene (THN), which is the key intermediate of 1-( α -L-(2- O -methyl)-6-deoxymannopyranosyloxy)-3,6,8-trimethoxynaphthalene (IBR-3) was characterized. ThnA utilized five molecules of malonyl-CoA as a starter substrate to generate the polyketide 1,3,6,8-tetrahydroxynaphthalene, which could spontaneously be oxidized to the red flaviolin compound 2,5,7-trihydroxy-1,4-naphthoquinone. The amino acid sequence alignment of ThnA revealed similarities with a previously identified type III PKS and identified Cys 138 , Phe 188 , His 270 , and Asn 303 as four highly conserved active site amino acid residues, as found in other known polyketide synthases. In this study, we report the heterologous expression of the type III polyketide synthase thnA in S. lividan TK24 and the identification of THN production in a mutant strain. We also compared the transcription level of thnA in S. lividan TK24 and S. lividan pIBR25- thnA and found that thnA was only transcribed in the mutant.