The Kinetics of Haemoglobin in Solution 



k$ k$ 



Hb 4 X 2 + X^ Hb 4 X 3 , — = K 3 = equilibrium constant of reaction 



kz kz ....(2.3) 



/C 4 k 4 



Hb 4 X 3 + X^ Hb 4 X 4 , — = K 4 = equilibrium constant of reaction 



k * k * (2.4) 



ki, k 2 ', k 3 ', k,[ are the respective combination velocity constants of 

 the four successive reactions, and k lt k 2 , k 3 , k x the corresponding dis- 

 sociation velocity constants. These equations lead to the equilibrium 

 equation 



y {percentage saturation) _ [XHb] 



100 ~ [XHb] + [Hb] 



K t p + 2K!K 2 p 2 + 3K!K 2 K 3 p 3 + 4K 1 K 2 K 3 K 4 p* 

 ~ 4 (1 + K lP + K^p 2 + K 1 K 2 K 3 p 3 + K^K^p 4 ) 



....(3) 

 where p = partial pressure of X. 



In an adjoining paper, one of us (F. J. W. R.) makes an attempt to 

 describe both the kinetics and equilibria of the haemoglobin reactions 

 in terms of the intermediate compound hypothesis. 



The range of validity of equation (1) — Equation (1) has been most 

 thoroughly tested as regards the dissociation kinetics by Millikan 8 * 9 , 

 who has shown that over the range 75 per cent 2 Hb to per cent 

 2 Hb the experimental data yield constant values of k. The range 

 above 75 per cent 2 Hb has not, however, been properly considered 

 (see below). As regards the combination kinetics, the reactions of 

 haemoglobin with CO are more suited for study than the reactions with 

 2 , since the velocities of the former are much slower, and larger 

 variations in concentrations of the reagents are feasible. Roughton's 

 kinetic data 7 conform very satisfactorily with equation (1) for the 

 first half of the process of combination with CO, but the agreement 

 was not seriously tested over the second half of the reaction. 



Unfortunately the kinetic data in the missing ranges are few and far 

 between, but the analysis of them now to be given points to the need 

 of important modifications of equation (1) at higher values of [XHb], 

 both as regards the dissociation and the combination kinetics. 



The rate of dissociation of 2 Hb in presence of Na 2 S 2 4 is known 

 to show a lag period at the beginning of the reaction 2 , but this has 

 hitherto been attributed entirely to the fact that the dissociation is a 

 two-stage process, 

 viz. 2 Hb^0 2 + Hb ....(4.1) 



2 + Na 2 S 2 4 -» oxidation products o/Na 2 S 2 4 . . . .(4.2) 

 69 



