94 Discussion 



REFERENCES 



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DISCUSSION 



The Haem-binding Groups in Haemoproteins 



The Nature of Haem-binding, and the Bohr Effect 



By J. H. Wang and Y. N. Chiu (Yale) 



Wang: I would like to suggest a possible interpretation of the extremely interesting 

 results found by Kaziro and Tsushima that while native haemoglobin shows a Bohr 

 effect in its reactions with both oxygen and alkyl isocyanide, myoglobin and urea- 

 denatured haemoglobin show the Bohr effect only in their reactions with oxygen and 

 not with alkyl isocyanide. 



For convenience of the present discussion we may arbitrarily divide the low-spin 

 complexes of haem into two classes: (1) Those in which "back-7r-bonding" contributes 

 substantially to the thermodynamic stability of the complex, and (2) those in which 

 "back-TT-bonding" is much less important for the ground state of the complex. Com- 

 plexes formed by combining haem with CO, NO and presumably O2 belong to the 

 first class. If we use K^ and K^ to denote the formation constants for the complexes 

 Z-haem-OHj and Z,-haem-Z,, i.e., 



[L-haem-OHj] [L-haem-L] 



^^ ^ [HaO-haem-OHalEL] ^"^ ^ [L-haenvOHj[Z]' 



where L represents the ligand, we have K^ ^ 4K2 for complexes of the first class. 

 Complexes formed by combining haem with ammonia, pyridine, imidazole, etc., 

 belong to the second class, for which K^ < ^K^. The complex ~NC-haem-CN~ 

 may be an intermediate case. 



St. George and Pauling (Science, 114, 629, 1951) found that each haem can combine 

 with two alkyl isocyanides and that complexes of the type RNC-haem-CNR are 

 probably even more stable than the corresponding monoisocyanide complex RNC- 

 haem-OHg. This observation indicates that "back-7T-bonding" does not contribute 

 substantially to the thermodynamic stability of these complexes, i.e., the binding of 

 alkyl isocyanide by haem is essentially that represented by structure A. 



