ABSTRACT
The mechanism of the H2O2 disproportionation catalyzed by the manganese catalase (MnCat) KatB was studied using the hybrid density functional theory B3LYP and the quantum chemical cluster approach. Compared to the previous mechanistic study at the molecular level for the Thermus thermophilus MnCat (TTC), more modern methodology was used and larger models of increasing sizes were employed with the help of the high-resolution X-ray structure. In the reaction pathway suggested for KatB using the Large chemical model, the O-O homolysis of the first substrate H2O2 occurs through a µ-η1:η1 coordination mode and requires a barrier of 10.9 kcal/mol. In the intermediate state of the bond cleavage, two hydroxides form as terminal ligands of the dimanganese cluster at the Mn2(III,III) oxidation state. One of the two Mn(III)-OH- moieties and a second-sphere tyrosine stabilize the second substrate H2O2 in the second-sphere of the active site via hydrogen bonding interactions. The H2O2, unbound to the metals, is first oxidized into HO2· through a proton-coupled electron transfer (PCET) step with a barrier of 9.5 kcal/mol. After the system switches to the triplet surface, the uncoordinated HO2· replaces the product water terminally bound to the Mn(II) and is then oxidized into O2 spontaneously. Transition states with structural similarities to those obtained for TTC, where µ-η2-OH-/O2- groups play important roles, were found to be higher in energy.
Subject(s)
Anabaena/metabolism , Bacterial Proteins/metabolism , Catalase/metabolism , Hydrogen Peroxide/metabolism , Anabaena/chemistry , Bacterial Proteins/chemistry , Catalase/chemistry , Crystallography, X-Ray , Density Functional Theory , Manganese/chemistry , Manganese/metabolism , Models, Molecular , Oxidation-Reduction , ThermodynamicsABSTRACT
The total phenolic content (TFC), total flavonoid content (TFC), antioxidant activity, and polyphenol composition of extracts of rabbiteye blueberry fruit polyphenols (BBFPs), rabbiteye blueberry pomace polyphenols (BBPPs), and rabbiteye blueberry leaves polyphenols (BBLPs) were investigated. The highest TPC and TFC were found in the extract of BBLPs, followed by the extracts of BBPPs and BBFPs. The extract of BBLPs exhibited the highest antioxidant activity according to five antioxidant methods. All three samples exhibited significantly higher antioxidant activity than the positive control rutin. HPLC-DAD-MS(n) analysis showed that the extract of BBFPs contained nine different anthocyanins, while the extract of BBPPs contained only four of them. In addition, four caffeoylquinic acids, quercetin, and its three derivatives were detected in the extract of BBLPs. The results indicate that the fruits, pomace, and leaves of rabbiteye blueberry were good sources of polyphenols and natural antioxidants. These could be useful as a functional food ingredient beneficial to human health.
