ON THE NATURE OF HEMOPROTEIN REACTIONS 



The data required are very simple, but scrupulous attention 

 must be paid to the control of temperature, pH, and ionic 

 strength. The possibility of specific salt effects complicating 

 the reactions should be investigated and buffer solutions chosen 

 to minimize them, although, fortunately, the hemoprotein 

 reactions dealt with below appear to be free from such complica- 

 tions (15). The reaction mechanism can be decided from the 

 pH variation of equilibrium constants {K^^^g) and forward and 

 back velocity constants (k^, kj) ; and the solution of various 

 algebraic equations gives equilibrium constants (iC) and velocity 

 constants {k) for the individual reaction steps. A typical example, 

 the ferrimyoglobin-cyanide reaction, will be discussed later. 

 Having obtained A" at a series of ionic strengtlis, extrapolation to 

 zero ionic strength gives Kq, from which the standard free-energy 

 change for the particular step, AF°, can be calculated using the 

 van't Hoff isotherm, 



AFo = -RTlnKo 



From values of K at several temperatures, AHq, the difference in 

 heat content between products and reactants follows from tlie 

 van't Hoff isochore, 



d/dt InK = AH'/RT\ 



and AS^, the entropy change accompanying the reaction, can be 

 obtained from the thermodynamic expression 



AFo = A//0 - TASo. 



In addition, heats and entropies of activation may be calculated 

 from the variation of velocity constants with temperature, using 

 the equation 



k = 6.2 X \0^^e^S*/Rg-AH*/RT 



where A//* = E — /?r, £ being the Arrhenius activation energy. 

 However there are fewer data of this kind available, so the pres- 

 ent discussion will be restricted to equilibrium and velocity 

 constants, and the values of AF°, A//°, and AS'^. 



It will appear that the AW and AS^ values are the most 



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