title: The association reaction of yeast alcohol dehydrogenase with coenzyme is partly diffusion-controlled in solvents of increased viscosity.
abstract: The steady-state kinetics of the yeast and liver alcohol dehydrogenase catalyzed reduction of aldehydes were examined in solvent mixtures of increased viscosity. This was done to investigate the effects of diffusion control on the fast association of NADH with the enzymes. Both glycerol and sucrose were unsatisfactory as viscosogens, as they inhibited the enzyme, but poly(ethylene glycol)/water mixtures were satisfactory. The 5-fold faster reaction of yeast alcohol dehydrogenase with NADH is partly diffusion controlled, whereas the slower liver alcohol dehydrogenase reaction showed no diffusion effects. These results are consistent with a yeast alcohol dehydrogenase active site that has relatively little steric hindrance to NADH binding. It is estimated that contributions to this association reaction from diffusion control and chemical activation control are equal at a solvent viscosity of 10 cP. Thus, under physiological conditions of increased viscosity the NADH association may be significantly affected by diffusion effects. In order to estimate accurately the maximum diffusion-controlled rate constant from diffusion theory, the diffusion coefficients of NADH were measured in poly(ethylene glycol)/water mixtures and were found to vary inversely as the solvent viscosity raised to the power of 0.5. The non-Stokesian behaviour of molecules as large as NADH in polymer/water mixtures may be a serious limitation to the routine use of poly(ethylene glycol) as a viscosogen for diffusion studies.
mesh_terms: D000426:Alcohol Dehydrogenase / Q000378:metabolism*; D001665:Binding Sites; D004058:Diffusion; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D009243:NAD / Q000378:metabolism*; D011092:Polyethylene Glycols; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D012997:Solvents; D014783:Viscosity
chemical_list: D012997:Solvents; D009243:NAD; D011092:Polyethylene Glycols; D000426:Alcohol Dehydrogenase