*2.6. Cyanobacterial Species P450s Functions and Features Resemblance to Eukaryotic P450s*

Functional analysis of a few cyanobacterial species P450s revealed that these P450s have some unusual catalytic diversity and resemblance to eukaryotic P450s. Based on our study, it can safely be predicted that the 27 cyanobacterial species P450s listed in Table 3 are involved in the production of different secondary metabolites. Some of the cyanobacterial species P450 functions against different compounds were elucidated. However, the biological relevance of these reactions is not clear. CYP120A1 from *Synechocystis* sp. PCC 6803 was found to be the first non-animal retinoic acid hydroxylase [66]. This P450-catalyzed reaction represents a novel modification of retinoids compared to vertebrate CYP26 family P450s. CYP120A1 hydroxylated all-*trans*-retinoic acid at C16 or C17 positons and converted *cis*-retinoic acids (9-*cis*-retinoic acid and 13-*cis*-retinoic acid), retinal, 3(*R*)-OH-retinal, retinol, β-apo-13-carotenone (C18) and β-apo-14- -carotenal (C22) resulted in the formation of the corresponding hydroxyl derivatives [66]. CYP120A1 had the highest preference for all-*trans* substrates compared to *cis*- substrates. Among the compounds analysed, CYP120A1 had the highest activity of all-*trans*-retinoic acids, followed by β-apo-13-carotenone (C18) [66]. CYP110C1 from *Nostoc* sp. PCC7120 was found to be germacrene A hydroxylase involved in sesquiterpene biosynthesis [57,67]. CYP110C1 converted germacrene A into two different products and the predominant product of this reaction was identified as 1,2,3,5,6,7,8,8aoctahydro-6-isopropenyl-4,8a-dimethylnaphth-1-ol [67]. CYP110E1 from the *Nostoc* sp. strain PCC 7120 was found to be flavone synthase, the first prokaryotic P450 with such activity [68]. CYP110E1 hydroxylated naringenin and (hydroxyl) flavanones into apigenin and (hydroxyl) flavones [68]. CYP110E1 also hydroxylated different compounds such as sesquiterpenes (zerumbone), drugs (ibuprofen and flurbiprofen), and aryl compounds (1-methoxy and 1-ethoxy naphthalene) into novel compounds that are usually difficult to synthesize chemically [68]. CYP110A1 from *Nostoc* sp. PCC 7120 was predicted to be a fatty acid ω-hydroxylase as the purified P450 binds to long-chain saturated and unsaturated fatty acids [69]. Unlike other prokaryotic P450s, CYP110A1 was found to be associated with membrane fraction, indicating its close resemblance to eukarotic P450s [69].

**Table 3.** Comparative analysis of P450s that are associated with secondary metabolites biosynthetic gene clusters (BGCs). Types of clusters, most similar known cluster and similarity were obtained by submitting individual P450 clusters to anti-SMASH (antibiotics & Secondary Metabolite Analysis Shell) [65]. Standard abbreviations representing type of clusters as indicated in anti-SMASH [65] were used in the table.


P450s' role in the synthesis of carotenoids, light-harvesting pigments, in cyanobacterial species will help in addressing an evolutionary link between these species and plants since cyanobacterial species considered as precursors of chloroplasts in plants [6,7]. It is well-known that members of the CYP97 P450 family are conserved across plant taxa [70] and involved in the synthesis of carotenoids in plants [71,72]. Furthermore, research in this direction will also help in identifying the biological relevance of P450s in cyanobacterial species.

### **3. Materials and Methods**
