charges and is called a zwitterion. Glycine may act either as an acid or 

 base by liberating or combining with a proton (H+), as shown in For- 

 mula (2-4). The fact that proteins differ in the relative number of acid 

 or base groups which they contain makes it possible by specific staining 

 methods to distinguish between certain types, particularly those which 

 are highly acidic or basic in character (see Chapter 1 1 for details). 



/NH..+ +H+ /NHo+ -H+ /NHo 



CHo< ^ ^ CHo< - - > CHo/ - (2-4) 



"\COOH -\C00- "\COO- 



POSITIVE GLYCINE NEGATIVE 



CHARGE (Zwitterion) charge 



The character of a protein is determined, in part, by the sequence of 

 amino acids in the molecule. Since there are some 20 amino acids which 

 may occur in any order with various repetitions and almost any number 

 in high polymers, the potential for different proteins on this basis alone 

 is astronomical. As indicated above, proteins may be classified as acidic 

 or basic depending on the balance of positive and negative charges pre- 

 vailing in a given medium. They may also be classified in terms of the 

 number and frequency of the amino acids yielded on hydrolysis. Finally, 

 they may be characterized by their molecular structure such as straight 

 chain, branched chain, or cyclic. 



Proteins are usually combined with other substances; in this form they 

 are known as conjugated proteins and provide the basis for the struc- 

 tural organization of the cell. The most common conjugated proteins are 

 the nucleoproteins (nucleic acid + protein) and the lipoproteins (lipid 

 + protein). Most known enzymes are also proteins. Some of these 

 are "soluble" and appear not to be directly attached to any particular 

 structural component of the cell, others are more "insoluble" and 

 more difficult to separate from cell parts, such as the mitochondria 

 and microsomes, to which they may be bound intimately. Enzymes may 

 be considered as organic catalysts which determine the rate of specific 

 biochemical reactions. In general, the evidence favors the view that 

 enzymatic activity is closely associated with the molecular structure of 

 the protein involved. This specificity of molecular structure determines 

 the particular substrate with which the enzyme will combine or inter- 

 act. The formation of substrate-enzyme complexes involves combination 

 of specific groupings of the substrate, say A and B, with corresponding 

 groups A' and B' which are present on the enzyme surface in a fixed 

 position (Figure 2-3). 



GENERAL MORPHOLOGY AND CHEMISTRY OF THE CELL / 15 



