58 



PHYSIOLOGY OF THE FUNGI 



glutathione, to reactivate phosphoglyceral dehydrogenase which had been 

 inactivated by X rays. 



Whether radiation is absorbed or not depends upon the chemical con- 

 stitution of the absorbing molecule and the wave length of the radiation. 

 The energy thus obtained may disrupt the molecule or may merely 

 increase its ability to react. These generalizations are not very helpful in 

 either predicting the effect of light upon living fungi or interpreting the 

 observed effects of light on growth and reproduction. It is probable that 

 light acts on various enzyme systems. Light is known to affect one 

 specific enzyme system (cytochrome-cytochrome oxidase). Warburg 

 (1926) showed that the respiration of baker's yeast was inhibited to the 

 extent of 70 per cent in the dark when exposed to carbon monoxide con- 

 taining 5 per cent oxygen, while respiration was inhibited only 14 per cent 

 in light. The same effect of light on carbon monoxide inhil)ition of 

 respiration has been demonstrated with larvae of Tenehris molitor and the 



heart of embryo trout. 



Ultraviolet radiation and X rays have a 

 lethal effect on fungi. A small percentagr^ 

 of the spores which survive exposure 

 to ultraviolet radiation may produce 

 mutants. It has been noted recently 

 (Kelner, 1949) that the lethal effect of 

 ultraviolet radiation upon spores of Strep- 

 tomyces griseus is overcome to a considera- 

 ble extent by exposing irradiated spores 

 to visible light. Whether this is due to 



Fig. 11. Diagrammatic illustra- 

 tion of the mechanism of enzy- 

 matic hydrolysis. The substrate 

 molecules are represented by 

 small circles, the products of 

 hydrolysis by semicircles. (Cour- 

 tesy of Van Slyke, Advances in 

 Enzymol. 2 : 38, 1942. Published 

 by permission of Interscience 

 Publishers, Inc.) 



reactivation of certain enzyme systems is 

 not known. 



MECHANISM OF ENZYME ACTION 



The most generally accepted theory of 

 enzyme action postulates that the enzyme 

 and substrate unite to form a molecular 

 compound or complex (enzyme-substrate 

 complex). In favorable instances the ex- 

 istence of such enzyme-substrate complexes has been demonstrated (Stern, 

 1936) . During this temporary union the substrate molecule is " strained" 

 or activated so that it undergoes reaction. The products of the reaction 

 have less affinity for the enzyme surface than the substrate molecules and 

 hence diffuse away, and other substrate molecules unite temporarly with 

 the enzyme and the process continues. If the product molecules are 

 present in excess, they may compete more successfully for the enzyme 

 surface than the substrate does. During synthesis, when the reactants 



