Cytochrome b in the Respiratory Chain 58? 

 The second step is described in reactions (8)-(12) 



Z>++ - L, + ci+++ ^ 6+++ - I2 + q++ (8) 



Z>+++ - I2 + c^+ + X ^ 6H+ -\ + r^x + c^+ (9) 



/,+++ _ 1^+ ^ X + Pi ^ ^+++ - I2+ + X -^ P (10) 



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



C++ + C+++ ^ Ci+++ + C++ (1 1) 



b++ - I2 + ^^-++ - I2+ ^ 2Z?+++ - I2 (12) 



Sum 2b^+ - I2 + 2c+++ + Pi + ADP ^ lb+++ - I2 + 2c++ + ATP 



Reaction (9) is analogous to reaction (5), but I2, unlike I^, remains bound 

 to (or is a part of) cytochrome b throughout the reaction. In this scheme, the 

 iron atom of cytochrome b is oxidized by either cytochrome q (reaction (8)) 

 or by I2+ (reaction (12)). lo is oxidized by cytochrome c, and is reduced by 

 cytochrome b. 



It is important to note that a fully oxidized preparation in the absence of 

 substrate would be expected to have all its cytochrome b in its fully oxidized 

 form Z>+++ — I2+ which is not shown to react in reactions (4)-(7). It is, there- 

 fore, necessary to postulate an initiating reaction, e.g. the reduction of the 

 I2+ by fpH2 



fpH2 + ^+++ - I2+ ^b^-\^ + fpH + H+ (13) 



Once the reaction was initiated, it would proceed with increasing speed since 

 each molecule of Z)++ — I2 formed by reactions (4)-(7) from the Z?+++ — Ig 

 formed in reaction (13) would produce two molecules of ^+++ — I2 by 

 reaction (12). Indeed, this could be the explanation of the marked delay in 

 reacliing a steady rate of oxidation when DPNH is mixed with heart-muscle 

 preparation (Slater, 1950). 



It seems likely that the loss of phosphorylating activity in the Keilin and 

 Hartree heart-muscle preparation is a consequence of the susceptibility of 

 'energy-rich' intermediates to hydrolysis. Thus, it would be expected that 

 the DPN '-^ I^H formed in reaction (4) would be hydrolysed 



DPN -- I^H -t- H+ -^ DPN+ + I1H2 (14) 



and no DPN ^-' I^H would be available to reduce cytochrome b. We suggest 

 that, under these circumstances, the reaction follows the following course 



DPNH 4- fp + I1H2 ^ DPN -^ IiH 4- fpHg (4) 



♦ DPN -- IiH + H+ -^ DPN+ -h I1H2 (14) 



fpH2 + &+++ - I2+ ^ b+++ - I2 + fpH + H+ (13) 



