308 ELECTRON TRANSPORT 



ceived little direct experimental support, the theoretical considera- 

 tions upon which it was based are of considerable utility. Of more 

 immediate relevance are studies by Massey and collaborators (7-9) on 

 the enzyme, lipoyl dehydrogenase. This enzyme is a protein in which 

 disulfides involved in maintenanceof the intact tertiary structure of the 

 enzyme also participate in the catalytic cycle (7). The enzyme is a 

 flavoprotein, and interactions between the enzyme sulfur and flavin 

 contribute to the formation of a variety of spectral peaks during the 

 catalytic cycle (8). Of greatest interest for bacterial photosynthesis, 

 however, is the presence of certain long wavelength bands in the 

 enzyme, recently attributed to charge transfer complexes between 



Light 



B— ChL — ► fp C.T. Complex-I^Chromatophore-Sj-S 

 [Chromatophore— S^S ► Chromatophore— S* 



N 



Chromatophore— S' + Pj — ►Chromatophore-S~P03H2 



Chromatophore-S'-P03H2 + ADP ~^> > ATP + 



S-^Chromatophore— S— S— Chromatophore^S 



Fig. 1. Speculative scheme of energy transfer and 

 phosphorylation in a chromatophore. Transfer of ex- 

 citation energy from bacteriochlorophyll to flavopro- 

 tein charge transfer complex, formation of sulfur 

 radical in interchromatophore disulfide bridge, and 

 oxidative phosphoryl transfer from thiol phosphate 

 formed by radical. 



reduced enzyme flavin and oxidized pyridine nucleotide (9). These 

 long wavelength bands appear in the 900 m/i region, and are conse- 

 quently desirably situated for energy capture via inductive resonance 

 transfer from bacteriochlorophyll. It is relevant, furthermore, that 

 photosynthetic cells contain relatively large amounts of lipoyl dehydro- 

 genase (diaphorase). One might envisage, as outlined in Fig, 1, a 

 process in which a disulfide-bonded chromatophore flavoprotein sub- 

 unit, forming a charge transfer complex analogous to that in lipoyl 

 dehydrogenase, accepts excitation energy from bacteriochlorophyll. 

 This energy is transferred to the interchromatophore disulfide to 

 form a sulfur radical "sink" that dehydrates orthophosphate to form a 

 chromatophore-thiol phosphate; i,e., a "high energy" phosphate linked 

 to the enzyme complex. Subsequent electron flow, in which the phos- 

 phate is transferred in an oxidative step by nucleophilic attack of ADP, 



