Pfizer Handbook of Microbial Metabolites 310 



isoleucine. D-Alanine occurs in bacterial cell walls and spores 

 and frequently in antibiotics. Some bacteria even require an 

 exogenous source of D-alanine, particularly on a medium devoid 

 of pyridoxine, since pyridoxal phosphate is a coenzyme for the 

 racemase. ^-Alanine is a component of coenzyme A. A re- 

 lated substance, ^-nitropropionic acid has been isolated from an 

 aspergillus species. 



Glycine and serine are reversibly interconvertible in most or- 

 ganisms, tetrahydrofolic acid transferring the hydroxymethyl 

 group. Glycine also is formed by amination of glyoxylate in 

 some microorganisms. 



pyridoxal phosphate 

 THFA CH2OH 

 CH2— COOH ( : =± HOCH2— CH— COOH 



I I 



NH2 NH2 



Glycine Serine 



glutamate 

 or 



ammonia 

 OHC— COOH — ==^ NHo— CH2— COOH 

 Glyoxylic Acid Glycine 



In E. coll serine is probably to be regarded as the precursor of 

 glycine. The origin of serine is still obscure. There is a possi- 

 bility that it may arise from phosphoglyceric acid from the gly- 

 colysis scheme: 



COOH COOH COOH COOH 

 CH— OH ;=iC=0 ;=±CH— NH2 ^ CH— NH2 



CH2OPO3H. CH2OPO3H2 CH2OPO3H2 CH2OH 



3-Phospho- 3-Phospho- Phospho- Serine 



glyceric hydroxy- serine 



Acid pyruvic Acid 



Glycine is a precursor of the porphyrins, purines, glutathione 

 and sarcosine. 



Serine contributes the carbon skeleton of cysteine in most or- 

 ganisms. Most microorganisms can use sulfate but not methio- 

 nine as a sulfur source, while mammals require methionine for 

 this purpose but cannot use sulfate. The conversion route of 

 methionine to cysteine has been worked out for higher animals, 

 but is not entirely understood in microorganisms. 



Thiosulfate is used by some molds, and cysteine-S-sulfonate 

 has been found to be an intermediate. Hydrogen sulfide has 

 been reported as a precursor in yeast. Threonine has been iso- 

 lated as an intermediate to cysteine in a neurospora auxotroph. 



