334 



Enzymes: Kinetics of Oxidations /IS: I 



The alternating single and double bonds give rise to a so-called 

 "resonance phenomenon," in which several different structures have the 

 same energy levels. For instance, in Figure lb, if one exchanges all the 

 single and the double bonds, and the dotted and solid chelating bonds, 

 one has an equally possible molecule. Modern quantum mechanics, 



H 



V I 



% 



c — c=c 



I 



C NH N 



I 

 -C 



,C— N 

 C— I 



C— H 

 II 

 HN — (1 



C — 



(a) 



(b) 



o= 



Figure I. (a) Basic porphyrin structure is common to the prosthetic groups of 

 hemoglobin, myoglobin, catalase, peroxidase, and cytochromes. It is also found 

 in chlorophyll. Various groups are attached to the eight "dangling" bonds. 

 (b) Ferroprotoporphyrin IX is the basic group of many of the heme proteins. 

 Hemoglobin, myoglobin, and catalase contain this group. The other heme proteins 

 contain either this porphyrin or ones derived from it by simple substitutions. 

 The iron atom is in the ferrous state in both reduced and oxyhemoglobin and 

 reduced and oxymyoglobin. In peroxidase, it is in the ferric state. Other heme 

 proteins have their iron alternately reduced and oxidized during reactions. 



discussed in Chapter 27, demands that one think of the molecule as 

 existing not in either one or a second state, but as partly in one and partly in 

 the other. The orbitals of the bonding electrons, so-called " it electrons " 

 (see Chapter 27) must be considered to include the entire structure. 

 These lead to absorption bands in the visible and the near ultraviolet 

 regions of the spectrum. All of the heme proteins have one or two absorp- 

 tion bands in the yellow-green region of the spectrum and a very strong 

 absorption band in the blue-violet region. The last region is referred 

 to as the Soret band or y-band. Typical spectra are shown in Figure 2. 

 When catalase catalyzes either the peroxidatic or catalatic reactions, 

 the absorption in the Soret region decreases during the reaction, indicat- 

 ing the formation of an enzyme complex with peroxide. The enzyme- 

 peroxide complex appears as green. On prolonged standing in the 



