S/I)XEY F. VELICK 117 



tioii. In model (a) the libosidic bond is out ol the pyridine plane. 

 In model (b), which is perhaps more probable, the ribosidic bond 

 is in plane but does not form a 120° angle with the bonds in the 

 ring. The chief point of these arguments is that in addition to the 

 change in the charge on the ring nitrogen there is also a conforma- 

 tion change that accompanies oxidation and reduction. Both the 

 oxidized and reduced forms of the coenzyme occupy the same binding 

 sites in enzyme complexes. If the enzyme is constructed to fit one 

 form closely, the fit will be less perfect and the binding interaction 

 weaker for the other form. In most of the cases that have been ex- 

 amined by equilibrium methods, the enzyme interacts much more 

 strongly with DPNH than with DPN. Specific instances are tabulated 

 later, in Table I, where they are correlated with other properties 

 of the complexes. 



Reduced PvRmiNE Nucleotide Enzyme Complexes 

 Glyceraldehyde-3-phosphate dehydrogenase — DPNH. The persis- 

 tence and the disappearance of a 260 ni/A excitation band of DPNH 

 fluorescence are criteria for closed and open conformations of the 

 dinucleotide. In the complex of DPNH wdth glyceraldehyde-3- 

 phosphate dehydrogenase (GPD) the coenzyme fluorescence, contrary 

 to its behavior in other known enzyme complexes, is partially 

 quenched (35) and the emission band shift, if significant at all, is 

 slightly toward the red. The excitation spectrum is shown in Fig. 

 7. At 340 m/x the excitation band is normal, but the short wave band 

 is strong and maximal at 280 instead of 260 m^. The 280 mfi band 

 occurs Avhere protein absorption is maximal and nucleotide absorp- 

 tion weak, and corresponds to direct excitation of the protein and 

 not of the coenzyme. However, emission was measured at 490 m^ 

 and comes chiefly from the coenzyme. This effect, with uncorrected 

 spectra, was described in an earlier report (35) and arises by the 

 transfer of excitation energy from protein to bound coenzyme. If 

 we superimpose on this excitation spectrum the excitation spectrum 

 of the protein fluorescence, measured at 350 m/^, the two curves, 

 normalized at 280 m^^i, are in agreement only near the maximum. 

 On the long wavelength side there is divergence due to the rise of the 

 340 m^ absorption band of the coenzyme. On the short wavelength 

 side there is divergence due to the persistence of the 260 m^ excita- 

 tion band of the coenzyme. The 260 uifi band is shown by the dif- 

 ference spectrum. The results thus indicate that the DPNH is bound 

 in folded conformation to GPD. Unlike the free coenzyme, the 



