PROTEINS 



141 



combine with either retinene, but it is not certain that they are identical 

 proteins (p. 204) . 



Lipoproteins 



As a class, these proteins are not well-defined and are regarded by some 

 investigators as mixtures of lipides and proteins rather than as chemical 

 entities. They occur in cell nuclei, blood, milk, bacteria, etc. Lipovitellin 

 of egg yolk, thromboplastic protein (functioning in blood clotting) of 

 lung tissue, and the polymyxin antibiotics are examples of such proteins. 

 The prosthetic groups that have been obtained from lipoproteins include 

 lecithin, cephalin, and fatty acids. 



GENERAL PROPERTIES OF THE PROTEINS 



Form 



As usually obtained, proteins are amorphous, but, under carefully con- 

 trolled conditions, they can be made to ciystallize. Great progress has 

 been made in the preparation of crystalline proteins in recent years, 

 especially of those that are enzymes. About 150 proteins have been 

 crystallized and, of these, approximately 40 are enzymes, respiratory 

 pigments, toxins, viruses, or hormones. The most commonly prepared 

 crystalline proteins are the animal albumins and the plant globulins. 



Size of the protein molecule 



From several lines of evidence it is known that the protein molecule 

 is large. A minimum molecular weight can be determined in various 

 ways: from the percentage of some element, such as sulfur, phosphorus, 

 or iron, which occurs in small cjuantities; from the percentage of some 

 amino acid found in the protein. The true molecular weight is obtained 

 by physical methods such as ultracentrifuge measurements and osmotic 

 pressure determinations. 



The following example illustrates the method of calculating the mini- 

 mum molecular weight of hemoglobin from the iron content, 0.33 per 

 cent. Assuming one atom of iron, atomic weight 56, a proportion is 

 set up as follows: 



0.33 : 56 = 100 : a;; x= 17,000 minimum m.w. 



Since there are four atoms of iron in hemoglobin, the true molecular weight 

 becomes 68,000, which is in excellent agreement with ultracentrifuge 

 data. 



