Multiple-ligand binding in CYP2A6: probing mechanisms of cytochrome P450 cooperativity by assessing substrate dynamics

Biochemistry. 2008 Mar 4;47(9):2978-88. doi: 10.1021/bi702020y. Epub 2008 Feb 5.

Abstract

The contribution of ligand dynamics to CYP allosterism has not been considered in detail. On the basis of a previous study, we hypothesized that CYP2A6 and CYP2E1 accommodate multiple xylene ligands. As a result, the intramolecular ( k H/ k D) obs values observed for some xylene isomers are expected to be dependent on ligand concentration with contributions from [CYP.xylene] and [CYP.xylene.xylene], etc. To explore this possibility and the utility of kinetic isotope effects in characterizing allosteric CYP behavior, steady state kinetics, product ratios, and ( k H/ k D) obs values for CYP2E1 and CYP2A6 oxidation of m-xylene-alpha- (2)H 3 and p-xylene-alpha- (2)H 3 were determined. Evidence is presented that CYP2A6 accommodates multiple ligands and that intramolecular isotope effect experiments can provide insight into the mechanisms of multiple-ligand binding. CYP2A6 exhibited cooperative kinetics for m-xylene-alpha- (2)H 3 oxidation and a concentration-dependent decrease in the m-methylbenzylalcohol:2,4-dimethylphenol product ratio (9.8 +/- 0.1 and 4.8 +/- 0.3 at 2.5 microM and 1 mM, respectively). Heterotropic effects were observed as well, as incubations containing both 15 microM m-xylene-alpha- (2)H 3 and 200 microM p-xylene resulted in further reduction of the product ratio (2.4 +/- 0.2). When p-xylene (60 microM) was replaced with deuterium-labeled d 6- p-xylene (60 microM), an intermolecular competitive inverse isotope effect on 2,4-dimethylphenol formation [( k H/ k D) obs = 0.49] was observed, indicating that p-xylene exerts heterotropic effects by residing in the active site simultaneously with m-xylene. The data indicate that there is a concentration-dependent decrease in the reorientation rate of m-xylene, as no increase in ( k H/ k D) obs was observed in the presence of an increased level of metabolic switching. That is, the accommodation of a second xylene molecule in the active site leads to a decrease in substrate dynamics.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Algorithms
  • Aryl Hydrocarbon Hydroxylases / metabolism*
  • Binding Sites
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 Enzyme System / metabolism*
  • Humans
  • Kinesics
  • Kinetics
  • Ligands
  • Mixed Function Oxygenases / metabolism*
  • Models, Biological
  • Models, Chemical
  • Molecular Structure
  • Oxidation-Reduction
  • Protein Binding
  • Substrate Specificity
  • Xylenes / chemistry
  • Xylenes / metabolism

Substances

  • Ligands
  • Xylenes
  • Cytochrome P-450 Enzyme System
  • Mixed Function Oxygenases
  • Aryl Hydrocarbon Hydroxylases
  • CYP2A6 protein, human
  • Cytochrome P-450 CYP2A6