380 R. W. Henderson and W. A. Rawlinson 



acid oxidase activity nor combination with CO. Furthermore, guanidation of 

 these residues (Takahashi et ah, 1958) left the pigment practically unaltered 

 in relation to these properties. These findings along with those of Nozaki 

 et al. (1957, 1958), who showed dependence of resistance to proteolytic 

 attack on the state of oxidation of the pigment, seem to be, as postulated by 

 these workers, an indication of the involvement of the secondary protein 

 structure in the oxidation-reduction reactions of cytochrome c. 



Eo CHANGE WITHIN THE CYTOCHROME C GROUP 



An explanation for the variation in E'q values quoted in Tables 1 and 2 

 may be found in terms of the following general equation derived by Clark 

 et al. (1940). This relates the Eq of a metal co-ordinate complex (of the type 

 with which we are dealing here) to change in degree of association of the 

 ligand with the central metal atom. 



An outline only of the derivation is given below, and reference should be 

 made to the original for full details of assumptions. 



From the basic equation— 



0, + ne=pR,, (1) 



where 0„ and R^ are the oxidant and reductant respectively in the fixed 

 states of aggregation designated by n and m, the usual electrode equation 

 may be derived : 



Co-ordination is then assumed to occur with a nitrogenous base B such that 

 there is no change in the respective states of aggregation ; then : 



0, + qB= 0,8, (3) 



R^ + iB = R^B, (4) 



from which the respective equihbrium constants Kq and A'^ may be obtained. 

 Under the conditions/? = m = n = 1 and at 30°C, the following equation 

 may be obtained : 



£, = £, -f 0-06 log f + 006 log ^ -f 0-06 log p^^j||^ (5) 



where S, = niOJ + n{OM 



and S^ = m(RJ + m(R,M 



It can be seen from equation (5) that, other things being equal, E^ is dependent 

 upon the values Kq and Kj^, i.e. the Eq value is dependent upon these two 

 constants. 



