title: Flavin-nicotinamide biscoenzymes: models for the interaction between NADH (NADPH) and flavin in flavoenzymes. Reaction rates and physicochemical properties of intermediate species.
abstract: 1. Flavin-nicotinamide biscoenzymes covalently linked by two, three or four methylene groups through positions N(10) of the flavin (Fl) and (N1) of the nicontinamide (Nic) form long-wavelength-absorbing, intramolecular complexes when the flavin part of the molecule is reduced specifically. The energy of the long-wavelength transition is minimal and its intensity maximal for (see journal for formula). 2. The increasing proximity of the positively charged nicotinamide lowers the pK-value of dihydroflavin deprotonation up to 1.7 units and the flavin oxidation-reduction potential becomes more positive up to 116 mV. 3. Specific reduction of the nicotinamide part of the biscoenzymes yields transient, long-wavelength-absorbing complexes. The energy of the long-wavelength transition is minimal and its intensity maximal for the complex (see journal for formula). 4. The rate of intramolecular flavin-dependent dihydronicotinamide dehydrogenation is highest for (see journal for formula), about 3 times slower for (see journal for formula) and 100 times slower for (see journal for formula). 5. The results obtained in this study are consistent with a reaction mechanism that involves formation of a charge transfer complex between reduced nicotinamide and oxidized flavin and rate-limiting heterolytic breakdown into products.
mesh_terms: D001665:Binding Sites; D004798:Enzymes / Q000378:metabolism; D005415:Flavins; D005420:Flavoproteins; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008968:Molecular Conformation; D009243:NAD; D009536:Niacinamide; D010084:Oxidation-Reduction; D013053:Spectrophotometry; D013056:Spectrophotometry, Ultraviolet; D013329:Structure-Activity Relationship
chemical_list: D004798:Enzymes; D005415:Flavins; D005420:Flavoproteins; D009243:NAD; D009536:Niacinamide