Energy Transfer and Conservation in the Respiratory Chain 599 



afford electron transfer at the rate required by the oxidase activity of the 

 particle. 



MECHANISM OF ELECTRON TRANSFER IN THE PARTICLES 

 Thermal Collisions 



The reasonable agreement between the measured reaction velocity in the 

 'soluble' system and that calculated for a closely-packed molecular array in 

 the particles supports the possibility that a colhsion mechanism can operate 

 by rotational or vibrational motion of the carriers about their points of 

 attachment to the structure in wliich they are embedded (thermal motions 

 would give about 10^ collisions sec~i (Chance, 1959a). Further support is 

 provided by spectroscopic studies of the oxidation-reduction states of the 

 respiratory carriers at liquid nitrogen temperatures; the values correspond 

 closely to those observed at room temperature, as if the transfer processes 

 were 'immobilized' in the solid state (Chance and Spencer, 1959). (The 

 inhibition factor exceeds 10^-fold.) Another result of some relevance is the 

 considerable inhibition of electron transfer by glycerol with very little change 

 in the steady state of the carriers (Chance and Spencer, 1959). Although 

 alternate interpretations of these data are possible, these effects of temperature 

 and viscosity are consistent with the collision mechanism, which also appears 

 to be an acceptable hypothesis for energy transfer in the visual receptor as 

 reported by Hagins and Jennings (1959), who propose brownian rotation or 

 nutation in one or two degrees of freedom as an explanation of dichroism 

 and bleaching kinetics in the retinal rods. 



Conduction Bands 



Many other theories of electron transfer are under active consideration in 

 this laboratory and elsewhere, particularly that of the participation of electron 

 conduction bands in the protein portion of the cytochromes (Cardew and 

 Eley, 1959; Taylor, 1959). Two problems arise in considering this process. 

 First, it appears that all electrons transferred from substrate to oxygen pass 

 through the haem portion of the respiratory carriers. Although this observa- 

 tion still merits further experimental tests, it appears to require that all elec- 

 trons transferred in conduction bands be trapped in the haem groups before 

 transfer to the next member of the respiratory sequence. Second, the pro- 

 found effect of the haem upon the transfer activity of the protein does not 

 seem to be adequately explained by the conduction-band hypothesis. How- 

 ever, such an explanation is attractive because it should operate effectively 

 with completely immobihzed carriers. In this connexion, the low temperature 

 'steady states' of the cytochromes are of particular interest because of the 

 probability that a number of assemblies of respiratory carriers contain adja- 

 cent oxidized and reduced forms of the cytochromes in a condition that is 



