PROTEINS 115 



are colored. Color may be due to metals like copper and vanadium 

 or metals with organic groups as in the iron and magnesium porphy- 

 rins. Include many respiratory pigments and a large number of 

 enzymes. 



D. Lipoproteins. Conjugates of lipides and proteins, the lipide not 

 extractable with ether. They are widely distributed, especially in 

 animal tissues like eggs, brain, and blood. The lipides may be both 

 large and complex aggregates. 



E. Nucleoproteins. Conjugates of nucleic acids and proteins. Con- 

 sidered in a separate section on page 120. 



Another group of related materials consists of derived proteins 

 formed by modifying the native or natural proteins. Coagulated 

 proteins are the insoluble products obtained by heat or alcohol treat- 

 ment. Metaproteins are the slightly modified products obtained on 

 treating native proteins with dilute acid or alkali at temperatures 

 below about 60°C. Proteoses are formed when degradation by acid, 

 alkali, or enzymes proceeds beyond the metaprotein stage. Protesoses 

 are not coagulated by heat, tend to be soluble in most aqueous systems 

 but not in saturated ammonium sulfate, and are often toxic when 

 injected into animals. Peptones are formed by still more extensive 

 hydrolysis, cannot be salted out, and are of relatively small size. They 

 correspond to peptides of moderate size. 



Other systems of classification employ terms with fairly obvious 

 meanings to indicate source, function, shape, and the like. As exam- 

 ples, one may list, respectively, blood proteins, respiratory proteins, 

 and globular proteins. No particular discussion of such terminology 

 appears necessary. 



Chemical and Physical Properties 



The known chemical properties of proteins are fundamentally those 

 of the functional groups present. Thus the properties depend upon 

 those of the amino acid components with some modification in reac- 

 tivity and steric factors resulting from the presence of a peptide chain. 

 As expected, the free amino groups of proteins may be acylated, re- 

 placed with hydroxyl groups, combined with formaldehyde, converted 

 to substituted ammonium ions by treatment with acids, or undergo 

 many of the other standard reactions of amines. Carboxyl groups can 

 be used as proton donors, esterified, incorporated into mixed anhy- 

 drides, reduced, or converted into amides, peptides, or acyl halides. 



Hydroxyl groups of serine and threonine are typical of the alcohols 

 and react in the same ways. The sulfhydryl group of cysteine and the 

 sulfide group of cystine are typical of the corresponding alkyl sulfur 



