Energy Transfer and Conservation in the Respiratory Chain 623 



217, 439, 1955), also referred to in a footnote by Chance and Williams (1956, loc. cit.), 

 that the portion of the oxidation-reduction cycle at which energy is conserved need 

 not be the reduction phase alone, but could also be the oxidation phase. This I 

 regard as a most useful suggestion, which we have made use of in our most recent 

 mechanism. 



3. I should prefer not to refer to C (or I) as a ligand, which I understand has a 

 special connotation in co-ordinate chemistry. C (or I) is a group bound to a carrier. 

 Chance: The following comments concern Equations 2, 3, and 4 of the preceding remarks 

 by Slater. 



We agree most heartily that Equation 2 is irrelevant and that it is unnecessary to con- 

 sider this case; we have never proposed it. 



Equation 4 was introduced by us to provide an explanation for inhibition of the 

 ATPase activity by increasing reduction of the carriers. Although this hypothesis 

 suggests that a reduced carrier can bind I in a low-energy form, it does not exclude 

 binding of a reduced carrier in a high-energy form and does not lead to conclusions 

 about high-energy forms of the oxidized carrier. 



The statements that Equation 3 will cause no change in the amount of bound I may 

 be incorrect because Equation I clearly indicates that more I can be bound by removal 

 of X '~ I and formation of C ■~ I. The situation is more concisely expressed by the 

 chemical equations. (A more complete analysis is given by Boyer (Arch. Biocheni. 

 Biophys. 82, 387, 1959).) 



From Equation 1, the rate of utilization of ATP is 



- ^^^ = ArJATP] [X] [I] - A'_,[ADP] [P] [X -^ I] (a) 



dt 



If X '-' I is regulated by the reduced carrier level, c" 



X ~ 1 + c" v-^ c" ~ I + X (b) 



^ _ [C-- ^ 1] [X] [c" -^ I] [X] 



^-[CnX^lV ""-'= K[c"] ^'^ 



Substituting the value of X ~ 1 into (a), letting ki = -^, and factoring X out of 

 both terms, ^ 



- ^^^ = X [a-JATP] [\]-k'., [ADP] [P] ^^^j^] (d) 



The ATP''^ exchange reaction will be regulated by whichever term of (d) is smaller. 

 Since there appears to be an effect of the carrier concentration on the exchange reaction 

 and since ADP and P do regulate the rate of oxidative phosphorylation and ATP does 

 not (Chance and Williams, Advanc. Enzymol. 17, 65, 1956), the second term is import- 

 ant for an exchange reaction that is relevant to oxidative phosphorylation. 



Exchange velocity a [ADP] [P] [X] ^^-f^ (e) 



Since the concentration of reduced carrier is observed to increase in anaerobiosis, a 

 diminution of the exchange velocity is consistent with the above equations. 

 If reaction (b) had been written in terms of the oxidized carrier, 



X ~ I -f c'" ^ c'" ~ I + X (f ) 



Exchange velocity a [ADP] [P] [X] ^^-f^ (g) 



Since the concentration of oxidized carriers is observed to decrease in anaerobiosis, a 

 diminution of the exchange velocity is inconsistent with the above equation (g). Thus, 

 the "reduced carrier" mechanism is consistent with the exchange data for the conditions 

 specified. The result does not prove that the exchange reaction follows the pathways of 



