Theoretical Study Of The Mechanisms Of Some Prototype Chemical Reactions Mediated By Cytochrome P450

The ubiquitous cytochrome P450 (CYP450) enzymes, which are widely distributed in plants, bacteria, insects, and mammals, are stated to be among"the most versatile biological catalysts known", owing to their vital roles in the metabolism of xenobiotics, such as drugs, and in the biosynthesis of endogenous compounds, such as steroids. A wide variety of chemical transformations including hydroxylation of inert C–H bond, epoxidation of C=C bond, dealkylations, etc., are all catalyzed by these monooxygenases. Despite extensive experimentally mechanistic explorations of these processes there remain many intriguing questions, on which theory can offer the missing insights and reveal new features. This is precisely the case for the mechanism of CYP450-catalyzed N-dealkylation of tertiary amines, oxidations of alcohols and aldehydes.Cytochrome P450 2E1 (CYP2E1) enzyme is considered as one of the major human hepatic cytochrome P450 enzymes and plays a key role in ethanol metabolism at high blood alcohol concentrations. The most significant role of CYP2E1 is its adaptive response to high blood ethanol concentrations with a corresponding acceleration of ethanol metabolism. We propose a new reversed dual hydrogen abstraction (R-DAH) mechanism that can account for the adaptive response of CYP2E1 to high ethanol concentration. Our work demonstrates that solvent effects make a difference in the choice of reaction channels: in conditions of low polarity the new mechanism and traditional ones are competitive; however, under polar environment, the barriers for the R-DHA mechanism become thesmallest, thus making the R-DHA mechanism the most likely one of ethanol oxidation. As such, we proposed that this R-DHA reaction channel might gradually be predominant when the ethanol concentration in blood became sufficiently high. Therefore, CYP2E1 can adapt its ethanol metabolism as a response to growing blood ethanol level. This mechanistic switch is an alternative hypothesis to the conventional explanation of global conformational change.Moreover, to the best of our knowledge, this study provides the first theoretical insights into the mechanistic details of alcohol oxidation mediated by CYP450. Acetaldehyde hydroxylation by CYP2E1 is a very hot and interesting topic because acetaldehyde is considered to be responsible for many of the toxic effects caused by alcoholism, such as acidosis and fatty liver. Ethanol metabolism by CYP2E1 is now considered as a dangerous channel for the more reducing acetaldehyde radical generated in the reaction process. But out work establishes that acetaldehyde hydroxylation by CYP2E1 is in accord with the effectively concerted mechanisms and the acetaldehyde radical intermediate is very short-lived, and thus its effect to biosystem is negligible. It's a novel result, and will have an influence on the alcoholism therapeutics The mechanism of N-demethylation of N,N-dimethylanilines (DMAs) by cytochrome P450 is a highly debated topic in mechanistic bioinorganic chemistry.Two alternatives, i.e., the hydrogen-atom transfer (HAT) mechanism and the single-electron transfer (SET) mechanism, have been surviving for more than 30 years of discourse, despite of extensive experimental explorations. Our study presented systematic DFT calculations on the mechanisms and kinetic isotope effects of these series reactions. Our work not only resolves mechanistic controversies, offers a consistent mechanistic view, but also reveals the following new features for the first time: a) There is a mechanistic switch from spin-selective reactivity for the DMA and p-Cl-DMA oxidations to two-statereactivity for the p-CN-DMA and p-NO2-DMA oxidations b) The HAT step possesses a"polar"character and the HAT energy barriers correlate with the energy level of the HOMO of the substrates, which cause the observations of large negative Hammettρparameters and good correlation of reaction rates with substrate redox potentials, which are taken as evidence for the SET mechanism. c) The non-enzymatic carbinolaniline decomposition process involves water assisted proton shift process. These mechanistic insights and new findings are very useful for the corresponding drug design and the understanding of disease states, such as Parkinson's disease.In contrast to the amine N-dealkylation, N-dealkylation of amides by cytochrome P450 has received very litter mechanistic attention. The conventional concern is that amide can be regarded as an amine with strong electron-withdrawing acyl group. Because the oxidative potential is raised a lot by the acyl group, amide dealkylation is believed to proceed in the HAT mechanism like alkane hydroxylation. However, our results revealed that the mechanism is more complex than both of the alkane hydroxylation and amine dealkylation. The reaction is of the state-of-the-art spin-selective regiochemistry, so we must take a refreshed eye on these subtle reactions and present more explorations on it in future.