ON THE NATURE OF HEMOPROTEIN REACTIONS 



The greater specificity of the hemoproteins Hes in the fact 

 that with the same prosthetic group, iron protoporphyrin, one 

 of four different reactions is especially favored depending on the 

 particular protein to which the prosthetic group is joined (32). 

 The cytochromes play a catalytic role in the main metabolic 

 process by undergoing rapid oxidation and reduction between 

 the ferrous and ferric states. The hemoglobins, of blood, and 

 of muscle tissue, combine reversibly with oxygen in the ferrous 

 state without undergoing rapid oxidation to the ferric state. 

 The peroxidases and catalases are very efficient catalysts for 

 the oxidation of certain reducing agents by peroxides and some 

 other strong oxidizing agents. Peroxidase action proceeds by 

 single equivalent oxidation steps, catalase action by what 

 appears to be a simultaneous two-equivalent oxidation step, each 

 of the enzymes showing a marked preference for one class of 

 reducing agent (10,11). The intermediate compounds formed 

 in both reactions were at one time regarded as "enzyme-sub- 

 strate complexes," according to the Michaelis-Menten theory of 

 enzyme action (5), but it seems very likely now that they are 

 higher oxidation states of the hemoproteins, in which a more 

 complicated oxidation process than electron transfer has occurred 

 (21). 



The interaction effects have been more fully investigated 

 with the hemoglobins. Interaction between prosthetic groups 

 themselves is, in fact, only well established in the case of hemo- 

 globin, which has four heme groups per molecule. The affinity 

 of one particular heme for, say, oxygen depends upon the 

 extent to which the other three are already combined (32,38,47). 

 Interaction between the prosthetic group and an ionizing group 

 on the protein was also first observed in the hemoglobin-oxygen 

 reaction, although this kind of interaction certainly occurs in 

 myoglobin reactions, and very probably in those of catalase and 

 peroxidase too. When hemoglobin combines with oxygen, the 

 acid strength of an ionizing group in the neighborhood of the 

 heme, believed to be on the amino acid residue to which the 

 heme is attached, increases from a pK. value of about 7.9 in 



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