ENZYMES AS SYNTHETIC AGENTS 115 



to the attempts that had been made to synthesise sugars from 

 carbon dioxide and water, pointed out that in addition to the 

 small yields obtained by these chemical methods they also failed 

 to realise the condition of producing only optically active 

 sugars. Since then in more recent experiments (Stoklasa, 

 Sebor and Zdobnicky l ) the yields have been improved by the 

 use of the ultra-violet rays of the quartz mercury vapour 

 lamp, but the difficulty of producing the right optically active 

 sugar still remains. All the naturally occurring sugars in the 

 plants are optically active, having different powers of rotating 

 the plane of polarised light, and all are what are termed d forms, 

 that is of the same general type of constitution as the sugar that 

 Fischer has termed (^-glucose. The difference in the power of 

 rotating polarised light is traced to the different arrangement 

 of the asymmetric carbon atoms within the isomeric sugars. 

 The problem then is to produce in vitro not only a sugar but 

 the sugar with the natural arrangement of the asymmetric carbon 

 atoms, not merely an isomer of this sugar but the correct stereo- 

 isomer, as it is called. 



Enzymes, themselves probably asymmetric organic bodies, 

 are in most cases extremely restricted in reference to the 

 reactions they can accelerate and can usually only react with a 

 certain class of stereo-isomer. This fact, which is of great 

 biological significance, is probably to be traced to the method in 

 which they produce their accelerating effect; they are usually 

 regarded as combining with the reacting substances, and if these 

 are asymmetric, then in all probability this temporary combina- 

 tion is facilitated by their own asymmetric constitution. The 

 same fact should hold good in relation to their activities in 

 synthesis, and they should therefore produce optically active 

 bodies instead of inactive mixtures containing equal quantities 

 of both stereo-isomers. They therefore provide a possible agent 

 by which this necessary asymmetry should be introduced in the 

 course of the process of synthesis known as photosynthesis. 

 The starting-point for this synthesis is, of course, carbon dioxide, 

 but when the substance has diffused into the chloroplast the next 

 substance in the transition to carbohydrate is still a matter for 

 speculation. 



Considerable, but not conclusive, evidence has accumulated 

 that formaldehyde is produced within the plant, and the passage 

 1 Biochem. Zeitschr. 1912, vol. 41, p. 333. 



