HAEM PROTEIN CONTENT AND FUNCTION IN RELATION TO STRUCTURE 29 1 



chemical potential. It cannot bind oxygen reversibly, but appears to 

 function as an oxidase or as an electron carrier in the photorespiratory 

 chain [27]. It contains 2 haems per molecule (MW = 28 000-35 °°°' 

 depending on the source). One or both of these haems may be bound by 

 only a single thio-ether linkage such as is characteristic of cytochrome c, 

 which has two such links to a single haem. 



It is evident that this protein provides a good test object for the present 

 theories about haem protein structure. For instance, Williams has pre- 

 dicted RHP would be a "high-spin" complex [62]. His prediction seems 

 to be correct on the basis of work by A. Ehrenberg (unpublished) using a 

 crystalline sample of pure RHP provided by Dr. Horio and myself. 



Further work on the amino acid sequence of haem peptides obtained 

 from RHP as well as from other bacterial cytochromes, should provide 

 important data for rationalizing the structural aspects of haem protein 

 chemistry, and is now proceeding in our laboratory. We expect that work 

 on the bacterial and plant cytochromes will greatly modify and extend 

 present concepts of the chemistry inherent in the combination of iron 

 tetrapyrrolic chelates and proteins. 



However, it is unlikelv that any future developments will support a 

 notion, such as put forward by Arnon [63], that chloroplast, or any, 

 cytochrome in its ¥e^ ^ state, will possess sufficient positive electro- 

 chemical potential to extract electrons from the hydroxyl ion or water. 

 Hill and Bendall [^2] point the fallacy of this notion properly in reference 

 to the cytochromes known at present to exist in chloroplasts — namely, the 

 r-tvpe haem protein, cytochrome /, and the 6-type haem protein — cyto- 

 chrome b^. George and Irvine have found [64] that mammalian cyto- 

 chrome r, as a representative of the haemochrome type haem proteins, 

 does not react with strong oxidizing agents to give the higher valence 

 (Fe'* +) form of haem, as appears to be the case with peroxidase or metmyo- 

 globin. It would seem, then, that we must search for the sort of haem 

 protein postulated in the scheme of Fig. i among the haem compounds of 

 plant chloroplasts which are contained in the haem fraction which is not 

 accounted for as either cytochrome / or cytochrome b^. 



So far, the only plausible haem compound found which resembles 

 myoglobin and other myohaematin proteins is the RHP of the bacterial 

 chromatophores. Its presence in chloroplasts remains to be demonstrated. 

 However, even if a myoglobin-type compound is absent, there are still 

 both peroxidases and catalases present in appreciable quantities in chloro- 

 plast tissues ; any of these may reveal the requisite properties upon isolation 

 and purification.* Even the attainment of a Fe^- state in cytochrome/ is 



* It is possible that the peroxidase and catalase activities found in chloroplasts 

 are functional, at least in part, in the manganese cycle suggested by Kenten and 

 Mann (see previous footnote). 



