CYANIDE INHIBITION OF PHOTOSYNTHESIS 307 



sible, it faces certain difficulties. If the cyanide-affected enzyme were 

 the one which Hmits the rate of photosynthesis under ordinary conditions, 

 {i. e. in strong Hght and in the presence of abundant carbon dioxide but 

 in the absence of inhibitors), differences in the available quantity of this 

 enzyme could not explain why a hundred or a thousand times more 

 cyanide is required to produce a certain proportional inhibition in 

 Hormidium than is necessary to bring about the same results in Scene- 

 desmns. Since the cyanide is always added in a large excess compared 

 with the enzyme, a certain added concentration, [HCN]o, should always 

 deactivate the same fraction of the enzyme, regardless of the absolute 

 concentration of the latter: 



(12.2) ^'°J''^'*"'^ = K [HCN] ^ K [HCN]o 



where K is the dissociation constant of the enzyme-inhibitor complex. 



If we want to explain the different response of different species to 

 cyanide by differences in enzyme concentration, we must assume that the 

 cyanide-sensitive enzyme is not responsible for rate limitation under ordinary 

 conditions, but becomes limiting when its active concentration is reduced 

 by poisoning. If we assume, for example, that both Hormidium and 

 Scenedesmus contain the cyanide-sensitive enzyme in excess of the 

 maximum requirement of photosynthesis, but that this excess is larger in 

 the second organism, then we can understand why a larger fraction of 

 this enzyme can be inactivated by cyanide in the second species before 

 the rate of photosynthesis becomes affected by its deficiency. 



This point of view has been stressed by Franck (c/., for example, 

 Franck, French, and Puck 1941). Its importance for the understanding 

 of the effects of inhibitors in all complex biochemical processes, is obvious. 

 In the case of photosynthesis, this explanation is supported by inde- 

 pendent considerations of Weller and Franck (1941), based on experi- 

 ments in flashing light. These observations confirmed that the catalytic 

 reaction which limits the rate of photosynthesis in strong light in absence 

 of inhibitors is not sensitive to cyanide, and that the source of cyanide 

 sensitivity of the photosynthetic process as a whole is the inhibition of 

 an ordinarily nonlimiting catalytic process, more specifically, of the 

 formation of the acceptor-carbon dioxide complex, {CO2}. 



The complete argument will be presented in volume II, chapter 34, 

 dealing with phenomena in flashing light. The basic fact (discovered by 

 Emerson and Arnold in 1932) is that cyanide does not affect the oxygen 

 yield per light flash, provided the dark intervals between the flashes are 

 sufficiently long. Two interpretations of this fact will be discussed in 

 chapter 34; and it will be shown that the shape of the curves represent- 

 ing the yield per flash as a function of the length of the dark interval 



