22 INTERMEDIARY METABOLISM AND GROWTH I 



the amine oxidases, glucose oxidase, and glycolic oxidase are examples of this 

 class of enzymes (Table 2). The oxidation of leucine by amino acid oxidase is 

 shown in equations i and 2. 



FAD enzyme 

 i) Leucine + O, > a-ketoisocaproic acid + NH3 + H2O2 



catalase 

 2) H2O, > V2O2 ^ H2O 



It will be noted that hydrogen peroxide is one of the products of the reaction. The 

 enzymes catalase or peroxidase, catalyze the subsequent breakdown of the hydrogen 

 peroxide to water and oxygen. 



5. Glycolate as a hydrogen carrier 



Plant tissues contain two types of dehydrogenases which utilize glycolic acid as substrate. 

 One of these, glycolic acid reductase requires DPN"^ or TPN"^ as a prosthetic group while 

 the other, glycolic acid oxidase, functions in the presence of FAD. The glycolic oxidase 

 can be oxidized by O2. Both enzymes have been highly purified from spinach leaves 

 (Zelitch and Ochoa, 1953; Zelitch, 1953). It has been suggested that glycolate may function 

 in conjunction with the above two enzymes in a novel hydrogen carrier system from 

 reduced pyridine nucleotides to molecular oxygen. The system may be depicted as follows: 



dehydrogenase 

 i) Substrate-H, + DPN^ > DPNH + H^ + substrate 



glycolic reductase 



2) DPNH + H^ + glyoxylate > glycolate + DPN"" 



glycolic oxidase-FAD 



3) Glycolate + O2 > glyoxylate + H2O2 



catalase 



4) H2O2 > H2O + '/2O2 



Sum: substrate-Hj + '/2O2 * substrate + H2O 



6. Nitrate as a hydrogen acceptor 



In the above sections, we have considered hydrogen and electron transport systems in 

 which O2 is the terminal acceptor. Many bacteria utilize O2 as the terminal hydrogen 

 acceptor under aerobic conditions and alternatively substitute HNO3 as the acceptor under 

 anaerobic conditions, or under conditions when the oxygen available to cells is in short 

 supply. Anaerobic conditions and an adequate nitrate supply favor the adaptive develop- 

 ment of nitrate reductase enzymes. In some instances, as in Pseudomonas and Aiicrococcus 



2e" ■> 



NOS ^NOi ^NO '■ — =-N20 ^-Ng 



+5 +3 



4-2 +1 Nitrogen 



valence 



HNO \ 



HgNaOs/ ^-NHgOH =-NH3 



4-1 — 1 —3 Nitrogen 



valence 



Fig. 8. Nitrate reduction. 



