Evolution of Enzymes 127 



do so will disappear, since they can no longer reproduce themselves. Such a 

 process of gradual depletion of available substrates with the evolution of contin- 

 ually longer and longer chains of synthesis would then constitute the origin of the 

 synthetic sequences which we are now finding in present-day living orgaismns. 



With these two ideas we can see how an enzyme, or, I should say, a highly 

 efficient catalyst, could be built. It is worthwhile to trace such a development in 

 a specific instance with which we might be famihar, for example the iron 

 porphyrin. Fortunately, the essential steps in the present-day biosynthetic route 

 to porphyrin have been unravelled for us. One could begin with succinic acid 

 and glycine, two compounds which we already know can be made by the various 

 random methods which have been discussed earlier, such as gaseous discharge 

 or ionizing radiation or ultraviolet fight. From these (succinic acid and glycine) 

 an a-amino-|S-keto acid is formed, followed by decarboxylation to S-aminolae- 

 vufinic acid, two of which then condense to form a pyrrole nucleus, porphobi- 

 linogen. This molecule then passes through a series of steps, involving a number 

 of oxidations, leading finally to protoporphyrin 9. The skeleton of this sequence 

 is shown in Fig. 2. 



With the introduction of iron into the protoporphyrin, or perhaps a better 

 way to view it would be, the surrounding of iron by the protoporphyrin grouping, 

 the iron becomes a better oxidation catalyst, and, as you can see, there are several 

 oxidation steps along the biosynthetic route as we now know it. Thus, one has 

 only to suppose that one or more of the sequences of steps leading to protopor- 

 phyrin is dependent upon an iron-catalysed oxidation, and this is almost certainly 

 so, to arrive at the conclusion that once such an iron protoporphyrin is manu- 

 factured it will itself accelerate its own synthesis from such precursors as succinic 

 acid and glycine and thus tend to build up the supply of the material and improve 

 the iron catalyst that will be available for a variety of other fimctions as well. 



The second point that I would like to make concerns how the development of 

 the photosynthetic apparatus as we now know it might have occurred: In order to 

 do this we review briefly what our present state of knowledge might appear to 

 be with respect to the existing mechanism by which the photosynthetic apparatus 

 in the green plants and in the lower organisms can convert electromagnetic 

 energy into chemical potential as reduced carbon and molecular oxygen. One 

 need hardly do more than point out the essential features of the process to recog- 

 nize its present-day separation into several rather distinct parts. The reduction 

 of carbon dioxide, we now have every reason to suppose, occurs in a series of 

 reactions which can take place entirely in the dark. In fact, all of the enzyme 

 systems which we now know to participate in the conversion of CO2 to carbo- 

 hydrate have been found in a wide variety of organisms, many of which are not 

 photosynthetic. For example, the Thiobacillus contains very nearly all of the neces- 

 sary enzymes and Escherichia coli grown in xylose will contain not only 'carb- 

 oxydismutase' but a number of other enzymes involved in tlie carbon reduction 

 cycle. The final step was indeed taken when Racker was able to make a mixture 

 of all the requisite enzymes and the energy-storing compounds [reduced tri- 

 phosphopyridine nucleotide and adenosine triphosphate (ATP)] which produced 

 hexose phosphate from carbon dioxide, all in the dark. 



