296 



2. ANALOGS OF ENZYME REACTION COMPONENTS 



Photolysis of water 



SH, 



(nitrogenase) 



N2 (forming NH3) 



available hydrogen atoms, while others are based on direct competition 

 at the enzyme active sites. 



PHENOL OXIDASES 



These enzymes usually hydroxylate monophenols in the oriho position 

 and further oxidize these to o-quinones. They have often been named ac- 

 cording to the particular substrate chosen: e.g., catechol oxidase (cate- 

 cholase), cresolase, tyrosinase, phenolase, o-diphenol oxidase, or polyphenol 

 oxidase. Different specificities are observed with enzymes from various 

 sources. The role such enzymes play in tyrosine metabolism will be dis- 

 cussed in the following section. 



The competitive inhibition of potato catechol oxidase by resorcinol was 

 first observed by Richter (1934). He noticed that the enzymes from various 

 sources exhibited quite different susceptibilities to resorcinol, those from 

 elder (Sambucus) and lilac {Syringa) being more sensitive than the potato 

 enzyme, and those from mushroom (Polyporus) and mealworm {Tenebrio) 

 less sensitive. The respiration of apple skin is inhibited about 35% by 

 50 mM resorcinol and thus Hackney (1948) studied an extracted catechol 

 OH OH OH 



"OH H3C ^-^ OH 



Resorcinol Orcinol 



COOH 



C^ 



CH-CH— COOH 



Nicotinic acid 



Cinnamic acid 



