9] MYOGLOBIN II 145 



The values of K for the reaction MbOo+CO^MbCO+Og was found 



to be 106 at 20" and pH 7-4 and that of y (y=*-^^) 0-028. This figure 



is remarkably different from that obtained by Theorell for horse Mb^ and 

 by us for human Mb (y = 0-048). 



The difference in the properties of Aplysia and mammalian Mb could 

 arise from a substantial difference in the chemical structure of the two 

 proteins. The prosthetic group of Aplysia Mb is identical with that of horse 



mm Hg 



log pO^ 



Fig. 2. Oxygen dissociation curve of Aplysia Mb (~3 x 10~%) at 20° C and pH 7-2 Y= 

 MbOa 



Mb+MbOa 



Mb: this was demonstrated by spectrophotometry of pyridine haemochro- 

 mogen and by resynthesis of each pigment from its own globin and the 

 haem split from the other. We then analysed the amino acid composition 

 of crystallized Mb from Aplysia Depilans and Aplysia Limacina, by ion 

 exchange chromatography according to Moore and Stein. ^ The results ob- 

 tained are reported in Table 1, which includes for comparison the amino 

 acid composition of a mammalian (human) Mb.^'' Table 1 shows a notice- 

 able difference in the amino acid content between Aplysia and human Mb. 

 The former is richer in aspartic acid, serine, valine, phenyl alanine, arginine, 

 and poorer in threonine, glutamic acid, proHne, tyrosine, hystidine and 

 lysine. Aplysia Mb is characterized by a higher content of dicarboxylic 

 amino acids and by a small content of basic amino acids. Histidine is pre- 

 sent in such a quantity as to account for only one mole per mole of protein. 

 This result seems very interesting in connection with the role played by 

 imidazole as haem-linked group. 

 Kps 



