HEME PROTEINS 63 



(4) Degree of saturation of side chains. The presence of electron- 

 withdrawing side- chains (unsaturated groups) provides one further sub- 

 division, which may be termed "U," whereas their absence gives a 

 subdivision, termed "S" (saturated). 



It follows thatthirty-sixpossible combinations exist, twelve for each 

 of the three main classes; i.e., for the P class there are the following 

 subclasses: HFU, HFS, HCU, HCS, LFU, LFS, LCU, LCS, MFU, MFS, 

 MCU, MCS. 



The conventional classes of heme proteins, derived from studies 

 of aerobic tissues (beef, yeast, etc.) are bunched into a relatively few 

 of the above subclasses. Thus, the oxygen storage and transport pro- 

 teins (myoglobins, hemoglobins, chlorocruorins) fall into the PHFU 

 class. So do the catalases and some peroxidases. Myeloperoxidase 

 probably belongs in the class PHCU, as does lactoperoxidase. The 

 oxidation catalysts, cytochromes b and c, appear in the classes PLFU 

 and PLCS, respectively. The oxidases, or "a" type cytochromes, 

 appear to belong to the class, PMFU. Thus, just five combinations 

 appear to include all the conventional heme proteins. 



The bacterial heme proteins supply a few more. Thus, the so-called 

 "fl^" ^^^ "^4" cytochromes, from E. coll and Acetobacter s^. (4) (5), 

 belong somewhere in the "D" class. In addition, a diheme protein 

 which occurs in Pseiidomonas sp. (6) appears to fall into both classes 

 «D. . ." and "PLCS". 



The photosynthetic bacteria, together with the plant systems, pro- 

 vide representatives of the "b" and "c" type cytochromes, as well as 

 peroxidases and catalases. In addition, the purple photosynthetic bac- 

 teria have a type of oxidase— called "cytochrome o" (7) (8)— which 

 appears to be a particle-bound member of the same class as the per- 

 oxidases. It is uncertain whether it is similar in spin type, so it may 

 conceivably fall into a "PM" category, like the "a" type cytochromes. 

 Finally, there is the authentic new class of heme proteins— provision- 

 ally called "cytochromoids" and known previously as "RHP"-type 

 proteins (3e)— which falls into the class PMCS. Thus, we see that the 

 bacteria have added at least three new combinations, on the basis of 

 structure alone. More than this, they have alerted biochemists to the 

 possibility that new combinations may exist, not only in bacteria and 

 in plants, but also in animal tissues. 



PROPERTIES AND FUNCTIONS 



I wish now to summarize briefly present notions about the structures 

 and functions of heme proteins in photosynthetic bacteria. I shall deal 

 with these in terms of the various general classes known. 



