490 APPENDIX 



practical, the sacrifice of a small amount of pure material for deter- 

 mining the extinction per heme gives a useable approximation to the 

 true extinction coefficient of cytochromes which contain c-type heme 

 groups. 



Properties of c-type cytochromes . 



Cytochrome C2 is a heme protein of approximately 12,000 molecu- 

 lar weight (2) which contains one heme bound to the peptide chain by 

 two thioether bonds as in cytochrome c (10), Cytochrome c2 isolated 

 from various sources has a relatively high oxidation- reduction po- 

 tential, E^ 7 = +0.29-0.33 volt (2,11). Inasmuch as cytochrome c2 is 

 the only c-{ype cytochrome found in R. rubrum and Rps . palustris, it 

 is probably the c-type cytochrome which is rapidly oxidized when the 

 bacteria are illuminated (12), Thus cytochrome C2 may be considered 

 to be a bacterial analog of green plant cytochrome / and may function 

 as the primary reductant of light- activated bacteriochlorophyll. 



Cytochromes with spectroscopic and chemical properties different 

 from those of C2 have been isolated from certain of the photosynthetic 

 bacteria, Chlorobium thiosulphatophilmn contains two, cytochrome 

 554 and cytochrome 553, In addition to cytochrome C2, Rps. spheroides 

 also contains a cytochrome 553 which may function in the aerobic 

 metabolism of the organism (5), Chromatium may contain a c2-type 

 cytochrome, as is suggested elsewhere in this volume, p, 315, but 

 such a cytochrome has not yet been isolated in soluble form. There is 

 present in Chromatium a complex cytochrome 553 which contains 

 three cytochrome c-type heme groups plus one FMN in a molecule 

 of about 97,000 molecular weight (3,13). This heme protein forms a 

 complex with carbon monoxide, perhaps involving but one of the three 

 hemes, with resultant formation of an anomalous absorption spectrum 

 (see Fig, 3). There is evidence that a cytochrome with these spectro- 

 scopic properties undergoes light-induced reactions in the C/zroma^eww 

 cell (14). 



Properties of cytochromoids. 



The cytochromoid C-type cytochromes are diheme proteins of about 

 26-28,000 molecular weight which form a complex with carbon mon- 

 oxide (15,3,2) and are found in all the purple bacteria examined to 

 date. The molecular weight of 37,000 (3) reported for Chromatium 

 cytochromoid C may be too high on the basis of quantitative end- group 

 analysis (personal communication from Dr. K, Dus). The heme groups 

 possess many of the properties of c-type hemes as is indicated by 

 heme splitting experiments (16) and by the properties of the diheme 

 peptide prepared from Chromatium cytochromoid C (17). These heme 

 proteins are autoxidizable with Em, 7 - 0,0 volt (2,3,15). Molecules 

 with cytochromoid C spectroscopic properties do not engage directly 



