SPECTRA AND STRUCTURE IN ENZYME COMPLEXES 

 OF PYRIDINE AND FLAVIN NUCLEOTIDES 



Sidney F. Velick. 



Department of Biological Chemistry, Washington 

 University, School of Medicine. SI. Louis. Missouri 



Of the many coenzymes that might profitably be discussed in a 

 symposium on Hght and Hfe I have selected two, the pyridine and 

 the flavin nucleotides. The spectra and excited states ol these com- 

 pounds are neither simpler nor better understood than those of other 

 coenzymes, but we are in a particularly good position to view them 

 in relation to the substrates and enzymes with which they act. A 

 coenzyme may be considered alternatively as a substrate or as a pros- 

 thetic group of an enzyme, depending upon the dissociability of the 

 complex; and the distinction is often blurred. Once the position of 

 a coenzyme in a metabolic pathway is established, it is the detailed 

 relation of the coenzyme to the enzyme and substrate that is of in- 

 terest to the biochemist. These are the relations that determine speci- 

 ficity and reaction mechanism and that are the focal points for 

 metabolic integration and control. Absorption and emission spectra 

 provide an approach to these problems and I sliall discuss a Icav 

 of the applications. 



Free PvRmiNE NucLEOTmES 

 We may begin by considering the three-dimensional structiue of 

 diphosphopyridine nucleotide (DPNH) in solution. Fig. 1 illustrates 

 two isomers, botli of which are found in nature. The isomerism 

 is at the glycosidic linkage between the nicotinamide ring and the 

 ribose. Only the fi isomer is active with known enzymes. The a 

 isomer, first isolated and characterized by Kaplan and coworkers (13) , 

 is a bit of a puzzle, and we will make use of it only l)y the contrasts 

 which it presents. The functional part of the pyridine micleotides 

 is the substituted pyridine ring. Warburg and Christian (37) es- 

 tablished the function as a reversible reduction and oxidation; and 

 Vennesland, Wcstheimer, and coworkers (8) provided the evidence 



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