136 - The Cell 



Kentucky Medical School, working in con- 

 junction with H. M. Dint/is, of the Massa- 

 chusetts Institute of Technology. During the 

 synthesis of the red blood-cell protein, hemo- 

 globin, according to this study, about two 

 amino acids per second are added to the 

 growing peptide chain, so that a finished 

 hemoglobin molecule, constituted of 150 

 amino acids, is produced within less than 

 one and a half minutes. 



Still another problem of protein synthesis 

 must be mentioned. Before affiliation with a 

 particular unit of transport RNA, each 

 amino acid must be activated (Fig. 7-5), as 

 first was shown by M. B. Hoaglancl, of Har- 

 vard University. A distinctively different 

 activating enzyme appears to be required lor 

 each of the different amino acids being util- 

 ized for protein synthesis. But the coding in- 

 structions are carried by the RNA. Thus, for 

 example, Severo Ochoa and co-workers at the 

 New York University Medical School devised 

 a method by which RNA of known composi- 

 tion can artificially be synthesized; and M. 

 W. Nurenberg, working with J. H. Mathaei, 

 of a National Institute of Health, in Be- 

 thesda, Mel., utilized such a sample of RNA 

 for protein synthesis. In (his case, the arti- 

 ficial (template) RNA consisted entirely of 

 — UUU — triplets and consequently only one 

 amino acid, phenylalanine, became incorpo- 

 rated during the formation ol the peptide 

 chain. However, introducing a few — UUA — 

 triplets into the template structure permitted 

 the — AAU — units of the transfer system to 

 bring a few molecules of another amino acid, 

 isoleucine, into the peptide series (Fig. 7-5). 



Undoubtedly the mechanisms ol protein 

 synthesis are generally similar in plant and 

 annual cells, but there is one important dif- 

 ference. Plant cells cati synthesize all of the 

 different amino acids, starting entirely at the 

 inorganic level — from water, carbon dioxide, 

 and inorganic salts (particularly nitrates, 

 sulfates, and phosphates). The animal cell, 

 on the other hand, can utilize inorganic 

 forms ol nitrogen (ammonia and ammonium 

 salts) only to a limited extent. Animal cells 



CH- 



I 

 H-C-OH + HNH 2 



I 

 COOH 



lactic acid ammonia 



deamination j 



CH, 



amination 



I 

 H— C— NH 2 + HOH 



COOH 



amino acid water 



(alanine) 



Fig. 7-6. The animal cell has the ability to utilize 

 inorganic nitrogen (ammonia or ammonium salts) for 

 the formation of a limited number of amino acids. 

 The amino acids in this group, which can be synthe- 

 sized and which, therefore, do not need to be 

 acquired as preformed substances, are designated as 

 the nonessential amino acids. The essential amino 

 acids must be present as preformed substances, in 

 the food or its derivatives (see Table 7-2). 



can manufacture some kinds of amino acids 

 in this manner (Fig. 7-6); and these kinds are 

 called nonessential. The other kinds, in con- 

 trast, cannot be synthesized by animal cells. 

 They must be obtained by the animal as 

 ready-made, or preformed substances, and 

 consequently they are referred to as the 

 essential amino acids (Table 7-2). Moreover, 

 since all of the different amino acids are 

 needed for the sxnthesis of any complete pro- 

 tein molecule, ever)' animal must include 

 some protein food in its basic diet. Only by 

 eating plants or other animals that have fed 

 directly or indirectly upon plant proteins can 

 the animal obtain, through the media of in- 

 gestion, digestion, and absorption, an ade- 

 quate supply of the essential amino acids; 

 these cannot be generated in the cells of its 

 own body. 



Other Synthetic Processes. Although the 

 cells ol animals, as compared to those of 

 typical plants, have a relatively lower capa- 



