CHAPTER VII 



COMPARATIVE BIOCHEMISTRY 

 OF HEMOGLOBINS 



1. INTRODUCTION 



In the previous chapter our attention has been chiefly directed 

 toward building up a physicochemical picture of the structure and 

 reactions of an ideahzed hemoglobin. The biological origin of the 

 hemoglobin was considered only if it assisted in this task. In the 

 present chapter, we propose to adopt the reverse procedure. Our 

 knowledge of the structure and reactions of the idealized hemoglobin 

 will be used as a basis for the critical consideration of individual 

 differences. 



In discussing hemoglobin, we distinguished between the respective 

 influence of environmental and constitutional factors in its affinity 

 for oxygen. It is convenient to retain a similar distinction when 

 examining different hemoglobins. The crystal form, spectrum, oxygen 

 affinity, and perhaps molecular weight, may be influenced by the 

 treatment the pigment has received during extraction, as well as by 

 the environment in which the observations are made. On the other 

 hand, the structure of the prosthetic group or the amino acid com- 

 position of the protein are not subject to adventitious alterations 

 and, together with the origins of the hemoglobins, provide a far more 

 certain basis for classification. 



Up to this point, our discussion of hematin compounds has pro- 

 ceeded from the point of view of structural and physical chemistry. 

 The present chapter marks a shift in emphasis toward a more bio- 

 logical point of view, when we discuss the functional role played by 

 the class of hemoglobins in living organisms. We do not attempt, at 

 this juncture, to consider in detail the mode of evolution of hemo- 

 globin, or the evolutionary importance of its functions. 



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