ENZYME COMPLEXES 



release to a nucleotide acceptor may lead to the discovery of an 

 enzyme system capable of carrying out oxidative phosphoryla- 

 tion,* Another attractive mechanism for oxidative phosphoryla- 

 tion during metallofiavoprotein oxidation by cytochromes, 

 involving a phosphate bridge between the two components, has 

 been postulated by Boyer (5). 



A POSSIBLE ROLE OF METAL IONS IN ELECTRON TRANSPORT 



Studies with metalloflavoproteins have demonstrated that 

 the metals involved in this type of enzyme are capable of under- 

 going reversible valence changes, of contributing to the light 

 absorption of the molecule, and of influencing not only the rates 

 and the type of catalytic action but also the energy content of 

 the molecule. These different lines of evidence taken in con- 

 junction have been interpreted to mean that the metal orbitals 

 are in direct electronic interaction with the other parts of the 

 catalytically active molecular structure and that they form part 

 of a resonating, mobile 7r-electron system consisting of flavin, 

 acceptor, and those parts of the protein adjacent to and inter- 

 acting with the catalytically active site (21,28). Oxidation of 

 reduced flavin by cytochrome c therefore entails simply the 

 m/ra-molecular movement within the enzyme-acceptor complex 

 of one of the 7r-electrons from the flavin system to the acceptor 

 system, with the metal as an essential feature of the circuit. The 

 metal functions as the site of attachment for the acceptor and 

 provides a linking place or nexus for the two x-electron systems — 

 that of the flavoprotein and that of the acceptor. This concept 

 is capable of extension first of all to the hemoflavoproteins, the 

 group of flavoproteins containing a "built-in" cytochrome 



* A possible variant of this scheme (suggested by Dr. B. M. Strehler) 

 may involve uptake of a nucleoside pyrophosphate instead of phosphate by the 

 metal enzyme. After reoxidation the original aquoenzyme would then be 

 reformed by phosphorolysis leading to the simultaneous production of a 

 nucleoside triphosphate. The corresponding cycle would then be: 

 reduction > nucleoside pyrophosphate uptake > 



oxidation > nucleoside triphosphate release. 



267 



