624 PHYSIOLOGICAL CHEMISTRY. 



acids l and their derivatives, e. g. y a-amino-acetic acid or glycocoll ; 

 a-amino-propionic acid or alanine ; phenyl-a-amino-propionic acid or 

 phenylalanine ; guanidine-amino- valeric acid or arginine, etc., and are, 

 therefore, essentially polypeptides. The various proteins resemble one 

 another closely in their properties. Their composition is so complex 

 that, as yet, no chemical formula has been assigned to them with any cer- 

 tainty; they all contain carbon, hydrogen, oxygen, and nitrogen; most 

 contain sulphur, several phosphorus, and some iron. Various other 

 metallic and non-metallic elements have been found in certain proteins. 



Nearly all proteins are amorphous, non-diffusible, colorless, odor- 

 less, nearly tasteless, and non-volatile. When heated under such 

 conditions that the volatile products formed are not burnt at all, or 

 only partially, a disagreeable odor is noticed, due chiefly to ammonia 

 derivatives. Proteins vary in solubility ; all are optically active, 

 most of them being laevorotatory, while haemoglobin and the nucleo- 

 proteins are dextrorotatory. Proteins are distinguished by the ease 

 with which they undergo chemical change under the influence of 

 reagents, ferments, or variations in temperature ; they all undergo 

 the process of putrefaction, which has been considered in Chapter 41. 



By boiling with dilute acids or alkalies, and also by the action of 

 certain enzymes, the proteins undergo hydrolytic cleavage, forming 

 many simpler compounds. (The terms hydrolytic cleavage and 

 hydrolysis -will be explained more fully later on ; they refer to a 

 splitting up of complex molecules, while water or its constituents are 

 taken up at the same time.) 



Classification. The classification here used is the one which has 

 been adopted by the American Society of Biological Chemists and 

 the American Physiological Society, and is as follows : 



I. SIMPLE PROTEINS. a, Albumins; b, Globulins; c, Glutelins ; 

 d, Prolamines, or Alcohol-soluble proteins; e, Albuminoids; f, His- 

 tones ; g, Protamines. 



II. CONJUGATED PROTEINS. a, Nucleoproteins ; b, Gtycoproteins ; 

 c, Phosphoproteins ; d, Haemoglobins ; e, Lecithoproteins. 



III. DERIVED PROTEINS : 



1. Primary protein derivatives: a, Proteans ; b, Metaproteins ; c, 

 Coagulated proteins. 2. Secondary protein derivatives : a, Proteases; 

 b, Peptones; c, Peptides. 



1 Greek letters are, in some cases, used to indicate the position of substituting groups in a 

 molecule. In organic acids, the carbon atom next to the carboxyl group (COOH) is designated 

 by a, the next one by /3, the next one by y, etc. Thus, lactic acid is a-hydroxy-propionic 

 acid, CH 3 .CHOH.COOH; alanine is a-amino-propionic acid, CH 3 .CHNH 2 .COOH; glycocoll is 

 a-amino-acetic acid, CH 2 NH 2 .COOH ; hydracrylic acid is -hydroxy-propionic acid, CH 2 OH.- 

 CH 2 COOH. 



