68 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



is: H2N-AIa-Gly- Leu (lieu) •Ser-AspNH2. . .; for litis: H2N- Ala- AspNHg 

 •Val-Ala-Gly. . . Thus, both end groups are identical; there are simple 

 cross-overs between the asparagine residues andthe glycine residues, 

 while leucine-valine, and serine-alanine are comparable pairs. The 

 carboxyl end group for I is glutamic acid, and for II is alanine. It is 

 of interest that the two residues which separate the cysteinyl groups, 

 presumably holding one of the hemes in II, are serine and glutamine, 

 just as in chicken cytochrome c. In all other mammalian and animal 

 c-type cytochromes, these two residues are alanine and glutamine. 



These are only a few of the results at hand; space prohibits pre- 

 sentation of others, which in any case are still very preliminary, I 

 have included the results shown only to provide examples of the sort 

 of information coming out of our present studies. 



The most remarkable properties of cytochromoids are exhibited in 

 their behavior with ligands (2,21), with which they would normally be 

 expected to react; such reagents as azide, cyanide, hydrosulfide, 

 methylimidazole, nitrosobenzene, etc., fail to attach to the central iron 

 atom even at extremes of pH, The proteins in the oxidized state appear 

 to be accessible only to protons, CO and NO, and not even to protons 

 when in the reduced state (21,22), Reactions with NO occur with both 

 reduced and oxidized forms, but only marginally. Affinities are many 

 orders of magnitude less than for usual NO-heme interactions, 



CO reacts more strongly than NO and, of course, only with the re- 

 duced form, with which it forms an easily photodissociable complex. 

 The fact that cytochromoids attach NO less firmly than CO is anomal- 

 ous, and so is another finding— that the photodissociability is pH- 

 dependent. 



These ligand interactions are sufficiently weak so that taken with 

 the absolute lack of reaction with ligands in general they weight the 

 similarity to "c" cytochromes greater than to myoglobins or per- 

 oxidases, as based on the spectral and magnetic susceptibility data. 

 The sequence studies provide more firm evidence for the close rela- 

 tion to the "c" cytochromes. 



Finally, the testimony of data on functional involvement of cytochro- 

 moids as intermediates in the photoactivated electron-transport chain 

 coupled to photophosphorylation (23,24,25), rather than as oxidases or 

 peroxidases, completes the similarity to the "c" cytochromes. The 

 relevance of the term "cytochromoids" is obvious. 



Other counts against an oxidase function for cytochromoids are: 

 (1) One of them occurs, as in (H), in a strict anaerobe; (2) R. riibrum 

 oxidase from dark-grown aerobic cultures contains no extractable 

 cytochromoids and shows little (26) or no (27) detectable bound cyto- 

 chromoids; (3) The oxidase activity of R. ruhrum is inhibited by 

 approximately 10"5 M cyanide (2), whereas cytochromoid I does not 

 combine with cyanide even at a cyanide concentration of 10-2 m (2,21). 

 There have been data on action spectra for the relief of the CO inhibi- 



