116 GENERAL BIOCHEMISTRY 



compounds. The hyclroxyl group o[ tyrosine shows the color and 

 chemical reactions of simple phenols, while the functional groujis 

 of histidine, arginine, methionine, and tryptophan resemble those of 

 compounds not so familiar but forming well-understood chemical 

 families. Reactions of the purely aliphatic hydrocarbon structure exist 

 but are generally of little value since the conditions necessary for reac- 

 tion are usually so drastic as to break up the protein itself. 



As expected from the properties of peptides, proteins may be hy- 

 drolyzed to smaller fragments, depending on the conditions and dura- 

 tion of treatment. Peptide bonds may be split by acids, alkalis, or cer- 

 tain enzymes known as proteolytic enzymes. Hydrolyses may open 

 only one or all of the peptide bonds and serve in laboratory investiga- 

 tions, digestion of food proteins, or various transformations inside 

 cells. 



Proteins undergo one type of reaction, probably partly chemical and 

 partly physical, that is relatively peculiar to this group. When heated, 

 most proteins undergo irreversible changes in solubility and molecular 

 shape. Apparently a number of bonds are broken, at least some of 

 them being hydrogen bonds. When soluble, globular proteins are 

 heated, they become insoluble, and the molecules elongate or unfold to 

 resemble fibrous proteins. These changes are not well understood but 

 are widely encountered and are called denaturation or sometimes 

 coagulation when a curdling or extensive clumping results. Denatura- 

 tion often accompanies chemical alteration, including hydrolysis of 

 proteins. In fact, enzymatic hydrolysis of proteins may require dena- 

 turation as the first step before any peptide bonds are split. At any 

 rate heat denaturation (cooking) alters the digestibility of our food 

 proteins. 



The physical properties of proteins differ markedly in some respects 

 but not in all. Many proteins are very soluble in water, many are 

 not, and some are intermediate. None dissolve in non-polar solvents, 

 and only the prolamines are soluble in even moderate concentrations 

 of ethanol. Proteins are always solids. They cannot be heated re- 

 motely near to melting without complete decomposition. 



Protein molecules vary in size by a factor of about a thousand, and 

 they vary in shape from spheres to relatively linear peptide chains. 

 The latter type is usually not soluble without decomposition, and 

 bundles or sheets of such long molecules make up hair, horn, mem- 

 branes, and finger nails. Soluble proteins may often be crystallized, 

 the molecules fitting into regular arrays like those formed by com- 

 mon smaller molecules. These soluble proteins are often termed globu- 

 lar, though they may be ellipsoidal, that is, with elliptical cross sec- 



