Flavin-Nicotinamide Biscoenzymes: Models for the Interaction between NADH (NADPH) and Flavin in Flavoenzymes.

1. Flavin-nicotinamide biscoenzymes covalently linked by two, three or four methylene groups through positions N(1) of the flavin (Fl) and N(1) of the nicotinamide (Nic) form long-wave- length-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 Nicox+-(CH2)3-FlredH- (&lambdamax= 600nm; ϵ= 1200 M-1 × cm-1). 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-wave- length-absorbing complexes. The energy of the long-wavelength transition is minimal and its intensity maximal for the complex NicredH-(CH2)3-Flox (λmax = 575 nm; ϵ = 650 M-1 × cm-1). 4. The rate of intramolecular flavin-dependent dihydronicotinamide dehydrogenation is highest for NicredH-(CH2)3-Flox (345 s-1),about 3 times slower for NicredH-(CH2)4-Flox and 100 times slower for NicredH-(CH2)2-Flox 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. [ABSTRACT FROM AUTHOR]