c Cytochrome P450 monooxygenases (P450s), which constitute a superfamily of heme-containing proteins, catalyze the direct oxidation of a variety of compounds in a regio-and stereospecific manner; therefore, they are promising catalysts for use in the oxyfunctionalization of chemicals. In the course of our comprehensive substrate screening for all 27 putative P450s encoded by the Streptomyces griseus genome, we found that Escherichia coli cells producing an S. griseus P450 (CYP154C3), which was fused C terminally with the P450 reductase domain (RED) of a self-sufficient P450 from Rhodococcus sp., could transform various steroids (testosterone, progesterone, ⌬ 4 -androstene-3,17-dione, adrenosterone, 1,4-androstadiene-3,17-dione, dehydroepiandrosterone, 4-pregnane-3,11,20-trione, and deoxycorticosterone) into their 16␣-hydroxy derivatives as determined by nuclear magnetic resonance and high-resolution mass spectrometry analyses. The purified CYP154C3, which was not fused with RED, also catalyzed the regio-and stereospecific hydroxylation of these steroids at the same position with the aid of ferredoxin and ferredoxin reductase from spinach. The apparent equilibrium dissociation constant (K d ) values of the binding between CYP154C3 and these steroids were less than 8 M as determined by the heme spectral change, indicating that CYP154C3 strongly binds to these steroids. Furthermore, kinetic parameters of the CYP154C3-catalyzed hydroxylation of ⌬ 4 -androstene-3,17-dione were determined (K m , 31.9 ؎ 9.1 M; k cat , 181 ؎ 4.5 s ؊1 ). We concluded that CYP154C3 is a steroid D-ring 16␣-specific hydroxylase which has considerable potential for industrial applications. This is the first detailed enzymatic characterization of a P450 enzyme that has a steroid D-ring 16␣-specific hydroxylation activity. C ytochrome P450 monooxygenases (P450s or CYPs) are found in all three domains of life, i.e., archaea, bacteria, and eukarya, and are classified into more than 1,000 families by their amino acid sequence homology (1). They have the ability to hydroxylate inactive carbons of hydrocarbons or aromatic compounds regioand stereospecifically (2, 3). In animals, P450s are mainly involved in xenobiotic metabolism and steroid biosynthesis (4, 5). In plants, many P450s are involved in the biosynthesis of plant hormones and secondary metabolites (6). In contrast, the functions of bacterial P450s are largely unknown, although some bacterial P450s are required for secondary metabolite biosynthesis and assimilation of terpenoids, such as cineol or terpineol (7-10). In spite of their unknown physiological functions, however, bacterial P450s have attracted much attention as a rich resource for new biocatalysts for use in the oxyfunctionalization of chemicals (11, 12). For example, microbial conversion of steroid precursors is widely used for large-scale synthesis of steroid drugs (13-17).CYP154 family members are widely found in actinomycetes, and many putative CYP154 genes have been found in the genomes of Streptomyces species. The structu...

The pathway from beta-carotene to astaxanthin is a crucial step in the synthesis of astaxanthin, a red antioxidative ketocarotenoid that confers beneficial effects on human health. Two enzymes, a beta-carotene ketolase (carotenoid 4,4'-oxygenase) and a beta-carotene hydroxylase (carotenoid 3,3'-hydroxylase), are involved in this pathway. Cyanobacteria are known to utilize the carotenoid ketolase CrtW and/or CrtO, and the carotenoid hydroxylase CrtR. Here, we compared the catalytic functions of CrtW ketolases, which originated from Gloeobacter violaceus PCC 7421, Anabaena (also known as Nostoc) sp. PCC 7120 and Nostoc punctiforme PCC 73102, and CrtR from Synechocystis sp. PCC 6803, Anabaena sp. PCC 7120 and Anabaena variabilis ATCC 29413 by complementation analysis using recombinant Escherichia coli cells that synthesized various carotenoid substrates. The results demonstrated that the CrtW proteins derived from Anabaena sp. PCC 7120 as well as N. punctiforme PCC 73102 (CrtW148) can convert not only beta-carotene but also zeaxanthin into their 4,4'-ketolated products, canthaxanthin and astaxanthin, respectively. In contrast, the Anabaena CrtR enzymes were very poor in accepting either beta-carotene or canthaxanthin as substrates. By comparison, the Synechocystis sp. PCC 6803 CrtR converted beta-carotene into zeaxanthin efficiently. We could assign the catalytic functions of the gene products involved in ketocarotenoid biosynthetic pathways in Synechocystis sp. PCC 6803, Anabaena sp. PCC 7120 and N. punctiforme PCC 73102, based on the present and previous findings. This explains why these cyanobacteria cannot produce astaxanthin and why only Synechocystis sp. PCC 6803 can produce zeaxanthin.

BackgroundCyanobacteria possess several cytochrome P450s, but very little is known about their catalytic functions. CYP110 genes unique to cyanaobacteria are widely distributed in heterocyst-forming cyanobacteria including nitrogen-fixing genera Nostoc and Anabaena. We screened the biocatalytic functions of all P450s from three cyanobacterial strains of genus Nostoc or Anabaena using a series of small molecules that contain flavonoids, sesquiterpenes, low-molecular-weight drugs, and other aromatic compounds.ResultsEscherichia coli cells carrying each P450 gene that was inserted into the pRED vector, containing the RhFRed reductase domain sequence from Rhodococcus sp. NCIMB 9784 P450RhF (CYP116B2), were co-cultured with substrates and products were identified when bioconversion reactions proceeded. Consequently, CYP110E1 of Nostoc sp. strain PCC 7120, located in close proximity to the first branch point in the phylogenetic tree of the CYP110 family, was found to be promiscuous for the substrate range mediating the biotransformation of various small molecules. Naringenin and (hydroxyl) flavanones were respectively converted to apigenin and (hydroxyl) flavones, by functioning as a flavone synthase. Such an activity is reported for the first time in prokaryotic P450s. Additionally, CYP110E1 biotransformed the notable sesquiterpene zerumbone, anti-inflammatory drugs ibuprofen and flurbiprofen (methylester forms), and some aryl compounds such as 1-methoxy and 1-ethoxy naphthalene to produce hydroxylated compounds that are difficult to synthesize chemically, including novel compounds.ConclusionWe elucidated that the CYP110E1 gene, C-terminally fused to the P450RhF RhFRed reductase domain sequence, is functionally expressed in E. coli to synthesize a robust monooxygenase, which shows promiscuous substrate specificity (affinity) for various small molecules, allowing the biosynthesis of not only flavones (from flavanones) but also a variety of hydroxyl-small molecules that may span pharmaceutical and nutraceutical industries.

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