BIOCHEMISTRY OF MICRO-ORGANISMS 



111 



ing with micro-organisms whose metabolism 

 again seems to conform to the general reac- 

 tion scheme 



AH + B -> A + BH, 



in which the substrate AH which is under- 

 going dehydrogenation, may be a fatty 

 acid. The acceptor B can be either oxygen 

 or carbon dioxide. The metabolism in the 

 presence of oxygen has not yet been 

 studied extensively; but an important fea- 

 ture with respect to the fatty acid function 

 is that there is good evidence to show that 

 not all fatty acids are used through the 

 same enzyme system. For example it can 

 be demonstrated that two fatty acids as 

 closely related as acetic and propionic acid, 

 or propionic and butyric acid, behave as 

 if they were used independently of each 

 other. The same holds true for acetic and 

 formic acids. To mention an example: if 

 it takes a given number of organisms n 

 minutes to completely use a certain amount 

 of acetate, and p minutes to use a definite 

 quantity of propionate, the time required 

 for the utilization of both when added to- 

 gether is not (n + p) minutes, but consid- 

 erably shorter, frequently equal to or 

 approaching p, though never less. This 

 would imply that while the organisms are 

 using the acetate, they can simultaneously 

 use the propionate, or formate as well, and 

 frequently the rate at which the combined 

 substrates are oxidized is accurately the 

 sum of the rates at which the two acids are 

 used when given separately. Inasmuch as 

 the concentration of the individual acids, 

 in the range employed, has no effect upon 

 the rate of their utilization, this would 

 mean, in our present language, that, 

 although the enzyme system which decom- 

 poses the acetate is constantly saturated in 

 the presence of acetate, there is still 

 another enzyme system present which is re- 

 sponsible for the decomposition of pro- 

 pionate, and which works independently of 

 the acetate-decomposing system. 



Through studies of this type in the 

 course of time it will be possible to gain 

 information, concerning the enzyme sys- 

 tems operative in the breakdown of related 



substances. By a similar mode of approach 

 Sperber and Runnstrom (1939) have re- 

 cently made plausible the idea that pyruvic 

 acid and alcohol employ at least in part 

 the same enzyme system in the oxidation 

 mechanism of yeast. 



The decomposition of fatty acids by 

 purple bacteria with carbon dioxide as ac- 

 ceptor is an example of a curious process, 

 because it is entirely dependent upon the 

 supply of radiant energy. It has been 

 shown that the purple bacteria perform a 

 sort of photosynthesis which differs from 

 that of green plants in two respects: (a) 

 The reduction of carbon dioxide is here not 

 accompanied by the evolution of oxygen; 

 (b) The process is completely dependent 

 upon the presence of certain oxidizable 

 compounds. 



These facts were incorporated, ten years 

 ago, into a generalized formulation of 

 photosynthesis according to the equation 

 (van Niel 1930, 1931, 1936) : 



2AH2 + CO2 -> (CH2O) + H2O + 2A. 



This would leave room for the existence of 

 a number of different photosynthetic reac- 

 tions depending upon the specific hydrogen 

 donors AH2 which the different organisms 

 can use. The photosynthetic metabolism 

 of the purple bacteria has recently made 

 it possible to show that formic acid, long 

 considered by many investigators as the 

 most probable first intermediate product 

 in photosynthetic carbon dioxide reduction, 

 does not play this role, at least in bacterial 

 photosynthesis. It is true that many of 

 the purple bacteria can use formate; but 

 this might as well be explained as being 

 due to an AH2 function of the formic 

 acid. It has now become evident that 

 all strains which can use formic acid can 

 also utilize molecular hydrogen, a property 

 first established by Roelofsen (1934, 1935). 

 Conversely, the species which are incapable 

 of photosynthesizing with the last-men- 

 tioned gas do not attack formic acid either. 

 This correlation is reminiscent of similar 

 observations with other micro-organisms 

 (cf., e.g., Stephenson 1939, p. 96.) and 

 definitely indicates that the acid is used 



