208 VI. HEMOGLOBIN 



Within the class of ferroporphyrin globins, however, a certain varia- 

 tion is possible without loss of oxygen combining power. A globin 

 from one species may combine with a hematin found in another, such 

 as Spirographis hematin, or with a synthetic hematin such as meso- 

 hematin, and the resulting compound may be transformed into a 

 true oxyhemoglobin. Conversely, the hematin most frequently found, 

 protohematin, is combined with a slightly different globin in every 

 species. 



The ability to combine reversibly with oxygen may be lost by 

 alteration of the porphyrin nucleus without denaturation of the 

 protein, as in azahemoglobin or verdoglobin, as well as by denaturation 

 of the globin in which the porphyrin nucleus remains unaltered.* 



The present chapter endeavors to give a coherent picture of the 

 hemoglobin molecule in the light of modern chemistry. Differences 

 between hemoglobins from various sources are considered in this 

 chapter only in so far as they throw light on the reactions of an ideal 

 hemoglobin molecule which is an abstraction from the numerous 

 hemoglobins found in nature. The difference between the hemo- 

 globins found in nature may thus be left until Chapter VII, which 

 deals with the comparative biochemistry and physiology' of this 

 pigment class. 



1.2. Nomenclature 



With the increasing number of hemoglobin derivatives reported 

 during the last twenty years, the nomenclature has become rather 

 confused. A number of amendments recently put forward have been 

 in the direction of choosing names which reflect the composition of 

 the compound more closely than does the older nomenclature (Keilin, 

 1475). The most detailed nomenclature, developed by Clark (4-52, 

 453) and Drabkin (617,620) for use with hemochromes, has not so 

 far been extended to the hemoglobins and would make the nomencla- 

 ture of the hemoglobin derivatives unnecessarily unwieldy. Here it 

 may become necessary, for example, to distinguish myohemoglobin 

 (muscle hemoglobin, myoglobin) from the hemoglobin contained in 



* Hemocyiiniii, the re.si)iratory blood pigment of some invertebrates, is a copper 

 protein wliifh does not contain the porj)hyrin nucleus. 



While synthetic iron or copper compounds able to unite reversibly with oxygen 

 are so far unknown, the cobaltous complexes of salicylaldehyde-ethylene diimine and 

 related sul>stances, and also of histidine, have recently been shown to have this prop- 

 erty (rf. -iSla, J9oa, '/Ma, 19S6a). In these compounds one molecule of oxygen is 

 bound between two cobalt atoms. 



