Tropolone

About: Tropolone is a research topic. Over the lifetime, 774 publications have been published within this topic receiving 11117 citations.

Crystal structure of Agaricus bisporus mushroom tyrosinase: identity of the tetramer subunits and interaction with tropolone.

Abstract: Tyrosinase catalyzes the conversion of phenolic compounds into their quinone derivatives, which are precursors for the formation of melanin, a ubiquitous pigment in living organisms. Because of its importance for browning reactions in the food industry, the tyrosinase from the mushroom Agaricus bisporus has been investigated in depth. In previous studies the tyrosinase enzyme complex was shown to be a H(2)L(2) tetramer, but no clues were obtained of the identities of the subunits, their mode of association, and the 3D structure of the complex. Here we unravel this tetramer at the molecular level. Its 2.3 A resolution crystal structure is the first structure of the full fungal tyrosinase complex. The complex comprises two H subunits of ∼392 residues and two L subunits of ∼150 residues. The H subunit originates from the ppo3 gene and has a fold similar to other tyrosinases, but it is ∼100 residues larger. The L subunit appeared to be the product of orf239342 and has a lectin-like fold. The H subunit contains a binuclear copper-binding site in the deoxy-state, in which three histidine residues coordinate each copper ion. The side chains of these histidines have their orientation fixed by hydrogen bonds or, in the case of His85, by a thioether bridge with the side chain of Cys83. The specific tyrosinase inhibitor tropolone forms a pre-Michaelis complex with the enzyme. It binds near the binuclear copper site without directly coordinating the copper ions. The function of the ORF239342 subunits is not known. Carbohydrate binding sites identified in other lectins are not conserved in ORF239342, and the subunits are over 25 A away from the active site, making a role in activity unlikely. The structures explain how calcium ions stabilize the tetrameric state of the enzyme.

The effect of tropolone on the formation of 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylacetic acid in the brain of the mouse

Abstract: 1. The development of a very sensitive and specific fluorimetric assay for 3,4-dihydroxyphenylacetic acid has made it possible to measure how inhibitors of the enzyme catechol-O-methyl transferase affect the relative concentrations of this acid and its O-methylated derivative 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid) in the brains of mice treated with L-3,4-dihydroxyphenylalanine or probenecid.2. It was found that tropolone and tropolone-4-acetamide reduce the concentration of homovanillic acid in the brains of the treated mice to an extent dependent on the dose.3. The concentration of 3,4-dihydroxyphenylacetic acid in the brain was increased by the administration of tropolone or tropolone-4-acetamide but the dose and response were not simply related to one another.4. The results suggest that, in vivo, the formation of 3,4-dihydroxyphenylacetic acid is not always a simple alternative to the formation of homovanillic acid when the enzyme catechol-O-methyl transferase is inhibited.

Selective inhibition of HIV-1 reverse transcriptase-associated ribonuclease H activity by hydroxylated tropolones

Abstract: High-throughput screening of a National Cancer Institute library of pure natural products identified the hydroxylated tropolone derivatives β-thujaplicinol (2,7-dihydroxy-4-1(methylethyl)-2,4,6-cycloheptatrien-1-one) and manicol (1,2,3,4-tetrahydro-5-7-dihydroxy-9-methyl-2-(1-methylethenyl)-6H-benzocyclohepten-6-one) as potent and selective inhibitors of the ribonuclease H (RNase H) activity of human immunodeficiency virus-type 1 reverse transcriptase (HIV-1 RT). β-Thujaplicinol inhibited HIV-1 RNase H in vitro with an IC50 of 0.2 μM, while the IC50 for Escherichia coli and human RNases H was 50 μM and 5.7 μM, respectively. In contrast, the related tropolone analog β-thujaplicin (2-hydroxy-4-(methylethyl)-2,4,6-cycloheptatrien-1-one), which lacks the 7-OH group of the heptatriene ring, was inactive, while manicol, which possesses a 7-OH group, inhibited HIV-1 and E.coli RNases H with IC50 = 1.5 μM and 40 μM, respectively. Such a result highlights the importance of the 2,7-dihydroxy function of these tropolone analogs, possibly through a role in metal chelation at the RNase H active site. Inhibition of HIV-2 RT-associated RNase H indirectly indicates that these compounds do not occupy the nonnucleoside inhibitor-binding pocket in the vicinity of the DNA polymerase domain. Both β-thujaplicinol and manicol failed to inhibit DNA-dependent DNA polymerase activity of HIV-1 RT at a concentration of 50 μM, suggesting that they are specific for the C-terminal RNase H domain, while surface plasmon resonance studies indicated that the inhibition was not due to intercalation of the analog into the nucleic acid substrate. Finally, we have demonstrated synergy between β-thujaplicinol and calanolide A, a nonnucleoside inhibitor of HIV-1 RT, raising the possibility that both enzymatic activities of HIV-1 RT can be simultaneously targeted.