Metabolic mechanisms of caffeine catalyzed by cytochrome P450 isoenzyme 1A2: a theoretical study

Caffeine (CA), a universally used psychoactive substance in foods and drugs, can cause osteoporosis when taken in moderate-to-high doses. The metabolic mechanisms of CA catalyzed by cytochrome P450 isoenzyme 1A2 (CYP1A2) were systematically explored in this study based on DFT calculation. Four possible metabolic pathways were investigated, namely 1-N, 3-N, 7-N demethylations, and C-8 hydroxylation. The results determined the mechanistic details and revealed some notable features. The rate-limiting C-alpha-H hydroxylation for the N-demethylation mechanism proceeded predominantly through a hydrogen atom transfer mechanism with two-state reactivity. The generated carbinolamine decomposed in a nonenzymatic environment, especially through the adjacent heteroatom-assisted proton transfer. The rate-limiting step for C-8 hydroxylation involved the nucleophilic attack of the active Cpd I's oxygen atom. Intriguingly, CA metabolic performance depended on the multiplicity of Cpd I. The 3-N demethylation metabolic mechanism predominated over the C-8 hydroxylation on the high-spin quartet state. Paraxanthine was the most energetically feasible metabolic product of CA. On the low-spin doublet state, however, C-8 hydroxylation had the lowest activation energy; hence, 1,3,7-trimethyluric acid was the optimum metabolic product of CA. All the results were in agreement with the experimental observation and can supply rational clues for the different metabolic performances of CA catalyzed by CYP1A2 in humans and rats. The calculated results in this study can provide more implications for the controversial amine N-dealkylation mechanisms by CYP and offer essential insights into bio-decaffeination techniques.