18: 1/ Enzymes: Kinetics of Oxidations 333 



namely catalase, is not known. In its purified form, it catalyzes two 

 types of reactions. The first of these, the so-called "catalatic reaction," 

 is the destruction of hydrogen peroxide 



2H 2 2 -> 2H 2 + 2 



This is the oldest known biologically catalyzed reaction; its discovery 

 was responsible for the name "catalase." Another type of reaction 

 catalyzed by catalase, the peroxidatic reaction, is an oxidation of any 

 of a variety of reduced substrates. The over-all reaction can be repre- 

 sented by 



H 2 2 + AH 2 -> H 2 + A + H z O 



where AH 2 is the reduced substrate (hydrogen donor) and A is its oxidized 

 form. This type of oxidation is catalyzed by both catalases and per- 

 oxidases. Both are often grouped under the more general name 

 "hydroperoxidase." The kinetics of the peroxidases are, however, 

 different from those of the catalases. 



Catalase occurs in many mammalian cells, including red blood cells 

 and liver cells; it is also found in large amounts in certain bacteria. One 

 role of catalase is to protect other proteins from destruction by hydrogen 

 peroxide. For example, a mutant human was discovered whose red 

 blood cells lacked catalase; the hemoglobin in these cells was rapidly 

 destroyed by hydrogen peroxide at concentrations often used for anti- 

 septic purposes. The hemoglobin of a normal person is unaltered under 

 these conditions. Thus, erythrocyte catalase protects hemoglobin from 

 hydrogen peroxide. 



Under normal physiological conditions, a limited amount of hydrogen 

 peroxide is produced within living cells by reactions catalyzed by certain 

 respiratory enzymes, such as xanthine oxidase. Just how much peroxide 

 is formed is not known, nor is its fate certain. Accordingly, it is hard 

 to guess at the physiological role of catalase, or to understand the reason 

 for the very high catalase content of some species of bacteria. In the 

 extreme case, 1 per cent of the dry weight of the bacterial species, 

 Micrococcus lysodeikticus, is catalase. 



Catalases obtained from different types of cells, or from the same 

 types of cells in different species, are different. These differences lie in 

 the protein (apoenzyme) portion of the molecule. They alter the 

 molecular weight, the molecular shape, and the reaction rates of the 

 enzyme. All catalases contain the same type of prosthetic group, called 

 a heme. The 2 -transport protein, hemoglobin, as well as the enzymes, 

 myoglobin, peroxidase, and cytochromes, all contain heme groups. A 

 heme is a chelated Fe compound containing the tetrapyrrole (porphyrin) 

 ring structure shown in Figure la. The porphyrin ring also occurs in 

 chlorophyll. 



