84 - The Cell 



teins play an absolutely essential role in 

 determining both structure and activity in 

 ever)' cell. 



Chemically, proteins are the most compli- 

 cated of all substances. The molecules — 

 even in such relatively simple proteins as 

 gelatin and egg albumen — are huge, con- 

 sisting of thousands of atoms. The constitu- 

 ent atoms of natural proteins always include 

 carbon, hydrogen, oxygen, nitrogen, and sul- 

 fur. Moreover, proteins can chemically unite 

 with any or all of the various acidic and 

 basic components of the protoplasm (p. 74), 

 and consequently all the various ions listed 

 in Table -1-4 can be found in affiliation with 

 any protein molecule. 



Amino Acids. Protein chemistry was greatly 

 simplified when it was discovered that each 

 large protein molecule represents a chain of 

 simpler units — the amino acids. The amino 

 acids are liberated as separate molecules 

 when a protein is completely broken down 

 by hydrolysis. This can be achieved by boil- 

 ing a protein preparation for several hours 

 in strongly acid medium. When the protein 

 is fully hydroly/ed, it yields 25 different 

 amino acids in varying amounts, according 

 to the specific composition ol the initial 

 protein. 



The amino acids are relatively small mole- 

 cules. Almost all may be designated by 

 a single general jormala, R — CH(NH L ,)- 

 COOH. In expanded form, this may be 

 written as shown at top of next column. 



The body of an amino acid molecule, des- 

 ignated by R, is different for each different 



H— C— NH 2 

 I 

 C— OH 



II 

 O 



amino acid (Fig. 4-13); but the head is al- 

 most always the same. This head displays 

 two important radicals: (1) the carboxyl 

 ( — COOH), which determines the acidic 

 properties ol the amino acid, and (2) the 

 amino radical ( — NIT), which accounts for 

 die basic behavior of the amino acid (see 

 below). 



The three simplest of the twenty-five dif- 

 ferent amnio acids are shown in Figure 4-13. 

 In glycine, R represents simply — H; in 

 alanine, R = - -CH,; and in cysteine, 7^ = 

 — CHo-SH. 



The acidic behavior of the carboxyl radi- 

 cal is the same in an amino acid as in other 

 organic acids (Fig. 4-14). But the basic be- 

 havior of the amino ( — N'H 2 ) group requires 

 further explanation. Amino compounds act 

 as bases, not by liberating hydroxyl ions in a 

 solution, but by removing hydrogen (H + ) 

 ions from the solution, as may be seen in Fig- 

 ure 4-11. 



The degree to which an amino acid acts 

 as a base or acid depends upon the acidity or 

 alkalinity ol the solution in which it is dis- 

 solved. In acid solutions, the amino radicals 

 lend to neutralize the acidity by accepting 

 hydrogen ions. But in basic solutions, the 



H 



CH, 



SH 



I 

 CH, 



H— C— NH 2 



I 

 COOH 



glycine 



H— C— NH 2 



COOH 

 alanine 



H— C— NH 2 

 I 

 COOH 



cysteine 



Fig. 4-13. Three amino acids. 



