Phenoloxidases: catechol oxidase - the temporary employer and laccase - the rising star of vascular plants.

Phenolics are vital for the adaptation of plants to terrestrial habitats and for species diversity. Phenoloxidases (catechol oxidases, COs, and laccases, LACs) are responsible for the oxidation and polymerization of phenolics. However, their origin, evolution, and differential roles during plant development and land colonization are unclear. We performed the phylogeny, domain, amino acids, compositional biases, and intron analyses to clarify the origin and evolution of COs and LACs, and analysed the structure, selective pressure, and chloroplast targeting to understand the species-dependent distribution of COs. We found that Streptophyta COs were not homologous to the Chlorophyta tyrosinases (TYRs), and might have been acquired by horizontal gene transfer from bacteria. COs expanded in bryophytes. Structural-functionality and selective pressure were partially responsible for the species-dependent retention of COs in embryophytes. LACs emerged in Zygnemaphyceae, having evolved from ascorbate oxidases (AAOs), and prevailed in the vascular plants and strongly expanded in seed plants. COs and LACs coevolved with the phenolic metabolism pathway genes. These results suggested that TYRs and AAOs were the first-stage phenoloxidases in Chlorophyta. COs might be the second key for the early land colonization. LACs were the third one (dominating in the vascular plants) and might be advantageous for diversified phenol substrates and the erect growth of plants. This work provided new insights into how phenoloxidases evolved and were devoted to plant evolution.

Quercetin as a tyrosinase inhibitor: Inhibitory activity, conformational change and mechanism.

Quercetin, a flavonoid compound, was found to inhibit both monophenolase and diphenolase activities of tyrosinase, and its inhibition against diphenolase activity was in a reversible and competitive manner with an IC50 value of (3.08±0.74)×10-5molL-1. Quercetin bound to tyrosinase driven by hydrophobic interaction, thereby resulted in a conformational change of tyrosinase and its intrinsic fluorescence quenching. Tyrosinase had one binding site for quercetin with the binding constant in the order of magnitude of 104Lmol-1. The molecular docking revealed that quercetin bound to the active site of tyrosinase and chelated a copper with the 3', 4'-dihydroxy groups. It can be deduced that the chelation may prevent the entrance of substrate and then inhibit the catalytic activity of tyrosinase. These findings may be helpful to understand the inhibition mechanism of quercetin on tyrosinase and functional research of quercetin in the treatment of pigmentation disorders.

An inhibition mechanism of dihydromyricetin on tyrosinase and the joint effects of vitamins B6, D3 or E.

Dihydromyricetin (DMY), a natural flavonoid, was found to effectively inhibit tyrosinase activity in a mixed-type manner with an IC50 value of (3.66 ± 0.14) × 10-5 mol L-1. DMY combined with the dietary vitamin D3 at lower concentrations exhibited a synergistic effect on the inhibition of tyrosinase. The formation of a DMY-tyrosinase complex led to fluorescence quenching and conformational changes of tyrosinase, which was driven mainly by hydrophobic interactions and hydrogen bonds. The molecular simulation further found that DMY inserted into the active pocket of tyrosinase interacted with amino acid residues Tyr78, His85, and Ala323, occupying the catalytic center of tyrosinase to hinder entrance of the substrate, leading to the inhibition of tyrosinase. This study may provide a scientific foundation for screening effective tyrosinase inhibitors.