CHAPTER 4 



Protein Specificity 



I HE PROTEINS OCCUR in greater variety than other organic compounds 



■ and are responsible for many specific properties ol colls and tissues. 



■ Proteins exist as huge molecules which vary in amino Acid com 

 position and in their physical form (stereochemistry). Most of the structural 

 proteins are called fibrous— collagen from c(mncciive tissue, keratin from hair, 

 myosin from muscle, and fibrin from blood clot. Most of the metabolic pro- 

 teins are called globular— hemoglobins, hemcx-yanins, albumins, and the glo- 

 buhns. Fibrous proteins occur in long slender chains which can undergo 

 folding and which show reproducible geometric patterns as seen by x-ray 

 diffraction; they may be similar among different species. Ihc structure ot 

 globular proteins is only slightly known; they vary in molecular weights from 

 tens of thousands to a few million, and characteristically they are specific for 

 different kinds of animal. Some proteins, such as actin, are reversibly fibrous 

 or globular. 



Protein Concentration in Blood. Protein molecules are, for the most part, 

 so large that they cannot diffuse through cell membranes, f fence in body 

 fluids they are responsible for a "colloid osmotic pressure" which limits the 

 amount of fluid which can diffuse across gill and kidney membranes. Blood 

 proteins also constitute a nitrogen reserve for an animal. In addition, certain 

 proteins have specific chemical functions in the body fluids. 



The absolute concentration of protein in blood is highest in animals with 

 well-developed circulatory systems ('Table 16). This^ may be related to the 

 higher blood pressures in such animals. Very low protein concentrations (about 

 0.1 gm. per cent) are found in the blood of bivalve molluscs. In annelids 

 coelomic fluid contains much less protein than does the blood. Protein in the 

 blood of arthropods varies from 1 to 6 gm. per cent, and in mammals protein 

 concentration in the blood is usually 5 to 8 gm. i^er cent. Insects have an 

 unusually high amino acid content in their blood. 



Electrophoretic Separation of Proteins. The proteins which have been 

 most studied from the viewpoint of comparative j^hysiology are blood proteins. 

 These can be separated by various means. One method of separating the 

 proteins of blood serum is to observe their migration in an electrophoretic field. 

 In the Tiselius electrophoresis apparatus, protein components can he separated 

 visually. In mammalian sera the proteins, in order of decreasing mMbility, are 

 albumin, the or- and ^-globuHns, and finally fibrinogen and y-globulm. 

 The relative proportion of these comixments ditters among diflcrent species 

 and in various pathological conditions. ' Not only can the various protein 

 fractions be separated visuallv, but some electrophoresis cells arc so arranged 

 that the particular protein fractions can be removed after separation. In human 



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