shikonin and quinone

Shikonin derivatives are red naphthoquinone pigments produced by several boraginaceous plants, such as Lithospermum erythrorhizon. These compounds are biosynthesized from p-hydroxybenzoic acid and geranyl diphosphate. The coupling reaction that yields m-geranyl-p-hydroxybenzoic acid has been actively characterized, but little is known about later biosynthetic reactions. Although 3″-hydroxy-geranylhydroquinone produced from geranylhydroquinone by CYP76B74 has been regarded as an intermediate of shikonin derivatives, the next intermediate has not yet been identified. This study describes a novel alcohol dehydrogenase activity in L. erythrorhizon cell cultures. This enzyme was shown to oxidize the 3″-alcoholic group of (Z)-3″-hydroxy-geranylhydroquinone to an aldehyde moiety concomitant with the isomerization at the C2'-C3' double bond from the Z-form to the E-form. An enzyme oxidizing this substrate was not detected in other plant cell cultures, suggesting that this enzyme is specific to L. erythrorhizon. The reaction product, (E)-3″-oxo-geranylhydroquinone, was further converted to deoxyshikonofuran, another meroterpenoid metabolite produced in L. erythrorhizon cells. Although nonenzymatic cyclization occurred slowly, it was more efficient in the presence of crude enzymes of L. erythrorhizon cells. This activity was detected in both shikonin-producing and nonproducing cells, suggesting that the aldehyde intermediate at the biosynthetic branch point between naphthalene and benzo/hydroquinone ring formation likely constitutes a key common intermediate in the synthesis of shikonin and benzoquinone products, respectively.

Topics: Alcohol Dehydrogenase; Aldehydes; Benzoquinones; Lithospermum; Metabolic Networks and Pathways; Naphthoquinones; Terpenes

Sortase A (SrtA) anchors surface proteins to the cell wall and aids biofilm formation during infection, which functions as a key virulence factor of important Gram-positive pathogens, such as Staphylococcus aureus. At present researchers need a way in which to validate whether or not SrtA is a druggable target alternative to the conventional antibiotic targets in the mechanism. In this study, we performed a high-throughput screening and identified a new class of potential inhibitors of S. aureus SrtA, which are derived from natural products and contain the quinone skeleton. Compound 283 functions as an irreversible inhibitor that covalently alkylates the active site Cys184 of SrtA. NMR analysis confirms the direct interaction of the small-molecule inhibitor towards SrtA protein. The anchoring of protein A (SpA) to the cell wall and the biofilm formation are significantly attenuated when the S. aureus Newman strain is cultured in the presence of inhibitor. Our study indicates that compound 283 could be a potential hit for the development of new anti-virulence agents against S. aureus infections by covalently targeting SrtA.

Topics: Aminoacyltransferases; Bacterial Proteins; Benzoquinones; Cell Wall; Cysteine Endopeptidases; Cysteine Proteinase Inhibitors; Fluorescence Resonance Energy Transfer; Inhibitory Concentration 50; Magnetic Resonance Spectroscopy; Peptides; Protein Binding; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; Staphylococcus aureus