586 E. C. Slater and J. P. Colpa-Boonstra 



X^I + Pi^X'-P + I (3a) 



X -- P + ADP ^ X + ATP (3b) 



Reaction (1) describes a hydrogen transfer from AHo to B, in which the 

 energy available from the oxido-reduction is retained in the energy-rich 

 compound A '--' 1. Reactions (2)-(3b) describe energy-transferring reactions 

 leading eventually to the synthesis of ATP from adenosine diphosphate 

 (ADP) and inorganic phosphate (Pi). I and X stand for unknown compounds 

 of the system, which are necessary for the conversion of oxidation energy to 

 phosphorylation, but which do not themselves undergo oxidation and 

 reduction. The different Fs required for the first two phosphorylation steps 

 of the respiratory chain are indicated by I^ and Ig. 



Some recent experiments which have been described elsewhere (Slater, 

 1959) have brought the possibility to the fore that an oxidation is also neces- 

 sary for the transfer reaction (2), and that 1 takes part in reaction (1) in a 

 reduced form, and is oxidized in reaction (2). Slightly modifying the details 

 of the reaction mechanisms proposed recently (Slater, 1959), we should hke 

 to suggest the following mechanisms for the phosphorylating steps between 

 DPNH and cytochrome b, and between cytochrome b and cytochrome c, 

 respectively. 



The first phosphorylation step is described by reactions (4)-(7) 



DPNH 4- fp + IiHo v^ DPN ~ IjH -f fpHg (4) 



DPN ~ IiH -1- 2Z) ' ++ - I2 + X ^ DPN+ + 2^++ - I2 -h H+ -h X ~ I^ (5) 



X ^ Ii + Pi ^ X -- P -Mi (6) 



X -- P + ADP ^ X H- ATP (3b) 



fpH^ + I, - fp -f I^H^ (7) 



Sum DPNH + 2b+++ -h + ?i + ADP 



^ DPN+ -f 2b++ - lo 4- H+ + ATP 



Reaction (4) is the same as reaction (1) for the specific case where AH2 and 

 B are DPNH and fp, respectively, while I is written in the form IHg. Reaction 

 (5) describes the oxidation of IiH, (bound to the DPN) by cytochrome b, 

 which is shown here already bound to Ig. It is possible that Ig is a part of the 

 cytochrome b molecule (e.g. a vinyl side chain of the porphyrin ring). Simul- 

 taneous with the reduction of cytochrome b by I^Ho in reaction (5), there is a 

 transfer of I^ to X. Reaction (6) is the same as (3a). Reaction (7) postulates 

 the reduction of I^ by the reduced flavoprotein formed in reaction (4). It 

 should be noted that cytochrome b is reduced by I^Ha, and I^ by flavoprotein. 



