114 INTERMEDIARY METABOLISM AND GROWTH I 



found with acetate-2-^''G as substrate. Glucose is also a precursor of the ribityl 

 portion of riboflavin as indicated in the discussion of ribose synthesis (Plaut and 

 Broberg, 1956). 



The coenzymes, Flavin Adenine Dinucleotide (FAD) and Flavin Mononucleo- 

 tide (FMN), are important in various oxidative reactions in the cell (Snell, 1953). 

 These substances can be synthesized from riboflavin by animal tissues, plants, and 

 microorganisms. The synthesis of FAD from riboflavin has been demonstrated 

 with enzymes of yeast and erythrocytes (Schrecker and Kornberg, 1950). 



i) Riboflavin + ATP > FMN + ADP 



2) FMN + ATP ' — r FAD + PP 



5. Pyridoxal phosphate 



Pyridoxal is synthesized by green plants and many microorganisms. The pre- 

 cursors of the vitamin have not, however, been defined. The conversion of pyri- 

 doxine, to the active coenzyme, pyridoxal phosphate, has been demonstrated in 

 yeast extracts (Hurwitz, 1952). Pyridoxine, pyridoxamine, and the antimetabolite 

 deoxypyridoxine, are also phosphorylated by the same enzyme system. 



Pyridoxal + ATP — » ADP + pyridoxal phosphate 



Pyridoxal phosphate serves as a coenzyme in several reactions involving amino 

 acids as substrates. These include i) transamination reactions (Meister, 1955), 

 2) the decarboxylation of amino acids to amines (Snell, 1953), 3) the serine and 

 threonine dehydrase and the cysteine desulfhydrase reaction, 4) the formation and 

 the cleavage of cystathionine, 5) the conversion of glycine to serine, 6) the for- 

 mation of tryptophane from serine and indole, and 7) the breakdown of tryp- 

 tophane to indole, pyruvate, and ammonia by bacterial tryptophanase. 



6. Biotiti 



Pimelic acid and desthiobiotin may be intermediates in biotin synthesis (Fig. 53). 

 Desthiobiotin satisfies the biotin requirements of some microorganisms, is inactive 

 with others, and is an antimetabolite with still others. Growing yeast or Aspergillus 

 niger cells can convert desthiobiotin to biotin. In the presence of pimelic acid, 

 desthiobiotin accumulates in the medium during the growth of a biotin requiring 



COOH 



COOH II II 



CH3 CH2— (CH2)4— COOH H2C CM — (CH 2)4— COOH 



s 



Pimelic Desthiobiotin Biotin 



Fig. 53. Structure of biotin, desthiobiotin, and pimelic acid. 



