64 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



(a) General considerations. First, I should bring to your attention 

 certain general considerations which apply to all the work which has 

 been done. In researches with bacteria as source material, amounts 

 of protein available rarely exceed a few milligrams. Even this minute 

 amount is obtained only after rather arduous culture and isolation 

 procedures. For instance, to obtain 10 milligrams of the pure c-type 

 cytochrome ironx Rho do spirillum rubrum — a. microorganism very rich 

 in this heme protein— 10 grams dry weight of bacteria are required. 

 Most work on protein fine structure— such as sequence determination, 

 x-ray analyses— demands the sacrifice of literally kilograms of ma- 

 terial. Another uncertainty, less often encountered in conventional 

 aerobic systems where function is rather obvious, is the determination 

 of function in bacterial metabolism. Bacterial systems employ heme 

 proteins in a variety of ways which are quite different from those 

 commonly associated with the aerobic processes of mammals. Hence, 

 assays for activity cannot always be applied. 



Future researches will tend to the development of procedures for 

 bulk culture so that greater amounts of material are available. In ad- 

 diton, intensified enzymic analyses of particle, or insoluble, electron 

 transport systems coupled to energy storage and reduction of substrates 

 other than molecular oxygen will be likely, 



(b) c-Type cytochromes, Theseoccur most frequently in the greatest 

 concentrations of all the bacterial heme proteins. Pure specimens have 

 been obtained in varying amounts, ranging from a few micrograms up 

 to hundreds of milligrams, from all species of the photosynthetic 

 bacteria available. In general, they show no marked variance from 

 cytochrome c, as indicated by the usual spectrochemical criteria, 

 sequence, and other structural studies, but as a rule are wholly in- 

 active in the classic cytochrome c oxidase system of mitochondrial 

 tissues. Moreover, they usually exhibit acid isoelectric points, owing 

 to a preponderance of aspartic and glutamic acid residues. Another in- 

 teresting difference is that none of the bacterial proteins show an 

 acetylated N-terminal group, as in cytochrome c. There are great 

 variations in size, oxidation potentials, and associated properties. 

 Some data (9,10,11) which relate to the composition of c-type cyto- 

 chromes are exhibited in Table 1, 



Functionally, they differ completely from the conventional cyto- 

 chrome c, in that they appear to be associated wholly with the photo- 

 oxidase system of chromatophores. That is, they do not act as ter- 

 minal catalysts in the reduction of molecular oxygen. The most in- 

 formed guess, at present, based on the very extensive accumulation of 

 data from enzymic studies, dynamic spectrophotometric and flash 

 spectrophotometric observations (3,12-15), implicate the c-type cyto- 

 chromes (including the analogous heme proteins of plant chloroplasts) 

 as substrates for the primary photochemistry of the photosynthetic 

 process. The reaction is not certain, but involves one of two alterna- 



