Applications of kinetics to auxin-induced growth 



Kj which tells us what concentration of this inhibitor is needed to half- 

 saturate available receptor sites with the substance. We will now make the 

 assumption that the dissociation constants for the two precomplexes ES^ and 

 ES2 are equal to the dissociation constants of the two related inhibitor 

 complexes EI^ and EI^. This assumption, which has been shown by Foster 



[^•^J 



Figure 5. The equilibrium constant. Kg, 

 for the formation of growth-active ES^ ^ " 

 composed of the product of the two constants 

 Kg^ {relating to the formation of complex ES^) 

 and Ks relating to the conversion of ES^ 



to ES 



1,2- 



A. 



et al. (1955) to be consistent with other data, provides us with a way of 

 measuring the individual equilibria of Figure 4. This way is indicated in 

 Figure 5. 



The over-all constant A'^. which describes the dissociation of growth- 

 active doubly attached auxin-receptor, ES^^^^ is made up of the product of 



Figure 6. The equilibrium constants 

 relating the varied auxin-receptor, ES, 

 complexes in the Avena coleoptile 

 section as calculated for the case of 

 2:4-D. 



£-^S 



£SA 



£S. 



1,2 



Growth =T<[ES^^^] 



constants K^ and K^ which characterize formation of singly attached 

 ES-^ and conversion of ES-^ to ES-^^^ respectively. Since we can measure A'^ 

 for the over-all process as well as A'^^ (on the assumption made above), we 

 can therefore calculate A^ . Similar considerations apply to A^ and to 

 Kg . The equilibrium constants relating the varied species of ES complexes 

 between 2 :4-D and coleoptile as calculated on this basis are summarized in 

 Figure 6. 



We are now in the position to estimate the relative contributions of each 

 form of complex for any given concentration of 2 :4-D. We note that 



(^total) = (Afree) + (A.Si) + (£^2) + (^'^l,2) + (^V2)> 

 (^free) = {ES^WsJS, {ES,) = {ES,^^)K g^^^, 



302 



(14) 



etc. 



