no GENERAL BIOCHEMISTRY 



peptide bond between cysteine and valine plus two additional link- 

 ages not usually encountered in peptides. 



Another type of peptide which has been isolated from the fungus 

 Fiisarium lycopersici produces the symptoms of a tomato blight and 

 kills this plant. A number of metabolic disturbances occur, although it 

 is difficult to decide which are primary and which are secondary. 

 Though not yet conclusively proved, the structure is thought to in- 

 volve asparagine and an addition product of glycine and pyruvic acid. 



H2NOCCH2CHNH 



1 

 COOH 



OCCH.NH 



CH3 



I 

 -C— OH 



I lycoinarasiiiin 



COOH 



By contrast, other peptides serve as growth factors and either are 

 required for growth or increase it. One of these, the citrovorum factor 

 and its relatives, is discussed as folic acid on page 344. Another 

 group stimulates the growth of a variety of microorganisms. These 

 peptides are called strepogenins and have been isolated after the par- 

 tial hydrolysis of any one of several proteins. One strepogenin is the 

 pentapeptide serylhistidylleucylvalylglutamic acid, and another is a 

 heptapeptide having the same sequence attached to alanylleucine. The 

 mechanism by which growth is stimulated is still unknown, but re- 

 placement by a simple mixture of the corresponding amino acids does 

 not suffice. In a similar way certain peptides may be desirable, if not 

 essential, in the diets of higher animals, but conclusive experiments 

 have not been performed. 



The higher animals do synthesize certain peptides essential to their 

 welfare which serve as hormones. The possible medical importance of 

 these compounds has stimulated interest in them and led to a great 

 deal of study, including successful structural and synthetic work. 

 These peptides are discussed along with other hormones on pages 311 

 to 325. 



A final example is the peptide glutathione, believed to occur in all 

 cells. It is a tripeptide with the structure 



-OOCCHCHoCH.CONHCHCONHCHoCOO- 



. 1 " ! 



NH3+ CHoSH 



7-L-glutamyl-L-cysteinylglycine 



Note the unusual peptide bond involving the y-carboxyl group of 

 glutamic acid instead of the much more common a-peptide bond. Glu- 

 tathione is believed to be important in controlling the oxidation- 



