AMINO ACIDS, PEPTIDES AND PROTEINS 217 



proteins. These are further subdivided into endosperm and embryo proteins for the seed, 

 and chloroplastic proteins for those found in leaf tissue. A better scheme rests on a sub- 

 division into simple proteins which on hydrolysis yield only alpha-amino acids, and con- 

 jugated proteins, which on hydrolysis yield amino acids plus other substances. The ma- 

 jor subdivisions of the simple proteins are listed below. 



Albumins - proteins that are soluble in water and in dilute salt solutions and are co- 

 agulable by heat. An example of an albumin is the leucosin from wheat. 



Globulins - proteins that are insoluble in water but soluble in dilute salt solutions. 

 Globulins are very prevalent in vegetable seeds. 



Glutelins - proteins that are insoluble in all neutral solvents but readily soluble in 

 very dilute acids and bases. Examples are the glutenin from wheat and oryzenin of rice. 



Prolamines - proteins that are insoluble in water but soluble in 70-80% ethanol. 

 Typical examples are zein from corn, gliadin from wheat, and hordein from barley. The 

 peculiar solubility of these proteins is probably due to their high proline content. Proline 

 itself has unusually high solubility in ethanol. 



The name given to the conjugated proteins is determined by the nature of the non- 

 amino acid moieties of the protein. Thus, nucleoproteins contain nucleic acids, glyco- 

 proteins are composed partially of carbohydrate, and chromoproteins are colored due to 

 the presence of porphyrin ring systems, flavins or other pigmented fragments. 



One of the characteristics of proteins is their great lability under very mild conditions. 

 This lack of stability manifests itself by changes in the physical properties, the detection 

 of functional groups or the side chains of the constituent amino acids, and the loss of the 

 catalytic properties of the enzymes. These changes in the properties of proteins are 

 usually called denaturation. Another characteristic of proteins is the great precision 

 with which they are synthesized in nature. Thus, a pure protein from a particular species 

 always contains the same number of amino acids in the same sequence. Mention should 

 also be made of the fact that contrary to many other polymers, proteins in aqueous solu- 

 tion are not in equilibrium with the solvent. This means that in solution the proteins re- 

 tain definite shapes and orientations which are held together by the disulfide bridges, 

 hydrogen bonds and other attractive forces. 



The proteins are such complex molecules that the determination of their structures 

 is among the most challenging problems encountered by chemists. Yet, because of the 

 vital role that these compounds play as biological catalysts it is imperative that we fully 

 understand the nature of these compounds. A tremendous effort is therefore being made 

 by chemists to increase our knowledge of the structure, the properties and biosynthesis 

 of the proteins. So voluminous is the literature and so rapid are the advances in this 

 field that only a very incomplete look at the exciting developments taking place can be 

 presented here. A great deal is known about the covalent structure of proteins. All the 

 evidence indicates that the amide bonds in the protein chains are formed only between 

 alpha amino and alpha carboxyl groups. The only other type of covalent linkage that is 

 well established is the disulfide bridge formed by oxidation of the sulfhydryl groups of 

 two cysteine molecules. This can produce crosslinks between two or more peptide chains, 

 or introduce folds into a long single chain. Insulin is an example of the first type (33) 

 and ribonuclease of the second (34). Several methods for the determination of the se- 

 quence of amino acids are available. The most important of these are end group deter- 

 minations, stepwise degradations, random partial hydrolysis and specific cleavage. 

 Methods for the location of the disulfide bridges have also been developed. Application 

 of these techniques has led to the elucidation of the entire amino acid sequence of ribo- 

 nuclease, a single chain protein composed of 124 amino acid units (34) and of the protein 

 from tobacco mosaic virus, a high molecular weight substance with repeating units com- 

 posed of 158 amino acids (35). 



