Theory for the observed isotope effects from enzymatic systems that form multiple products via branched reaction pathways: cytochrome P-450

• Published in: Biochemistry (Easton) Vol. 28; no. 23; pp. 9012 - 9018
• Main Authors: KORZEKWA, K. R, TRAGER, W. F, GILLETTE, J. R
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 14-11-1989
• Subjects: Analytical, structural and metabolic biochemistry; BASIC BIOLOGICAL SCIENCES; Binding, Competitive; BIOCHEMICAL REACTION KINETICS; Biological and medical sciences; BIOLOGICAL PATHWAYS; Cytochrome P-450 Enzyme System; CYTOCHROMES; ENZYMES; Enzymes and enzyme inhibitors; Fundamental and applied biological sciences. Psychology; General aspects, investigation methods; ISOTOPE APPLICATIONS; ISOTOPE EFFECTS; Isotopes; KINETICS; MIXED-FUNCTION OXIDASES; Models, Chemical; ORGANIC COMPOUNDS; OXIDOREDUCTASES; OXYGENASES; PIGMENTS; PROTEINS; REACTION KINETICS 550201 > Biochemistry > Tracer Techniques; reviews; Substrate Specificity; TRACER TECHNIQUES; Isotope effect; Theoretical model; Enzymatic activity; Cytochrome P450; Reaction mechanism; Enzymatic system; Enzyme model; Kinetics; Kinetic parameter
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00449a009

By use of cytochrome P-450 as the prototype, kinetic descriptions are derived for the observed isotope effects for several models of enzymatic systems which are capable of generating multiple products from single substrates. The models include rapid and slow equilibria between enzyme-substrate orientations as well as multiple simultaneous and multiple sequential isotope effects. When an equilibrium is established between enzyme-substrate complexes that are responsible for the oxidation of different positions of the substrate, the kinetics can be represented by competing pathways from the same intermediate. When direct interchange between the complexes does not occur, the alternate pathway mimics the presence of a competitive inhibitor in the substrate solution. In general, the presence of alternate pathways in competition with the isotopically sensitive step will tend to unmask the intrinsic isotope effect.