228 AMINO ACIDS, PEPTIDES AND PROTEINS 



31 xn 



PROTEIN BIOSYNTHESIS 



A dramatic development, the demonstration of protein synthesis in stable cell-free 

 systems, by Nirenberg and Matthai (82) promises to lead to the complete elucidation of 

 the reactions involved in the biosynthesis of these compounds and at the same time con- 

 firms a great deal of the earlier hypotheses concerning their mode of formation. 



The preponderance of the available evidence indicates that proteins are synthesized 

 from free amino acids and not by the condensation of preformed peptides or keto acids. 

 The free amino acids are believed to be activated by a reaction with ATP to yield enzyme- 

 bound amino-acyladenylates (83). In this complex the carboxyl group of the amino acid 

 is linked to the 5' -phosphate of AMP as a mixed anhydride. Each individual amino acid 

 has its own activation enzyme. This type of reaction has been shown to occur in many of 

 the higher plants including spinach, rye, asparagus and tobacco (84). The next major 

 step forward was the finding that amino acids from the AMP-complexes are transferred 

 to soluble ribonucleic acids (sRNA) yielding amino acyl-sRNA complexes (85). No sep- 

 arate enzyme is required for the transfer of the amino acid from the AMP complexes to 

 the sRNA. The reaction sequence is shown below. There seems to be at least one sRNA 

 for each 



ATP + amino acid + enzyme - enzyme - (AMP - amino acid) + PP 

 enzyme - (AMP - amino acid) + sRNA ^ sRNA - amino acid + enzyme 



amino acid and these substances all have molecular weights of approximately 30, 000. 

 Partial separations have been achieved by several procedures but counter current dis- 

 tribution appears to be particularly suitable (86). In addition to the sRNA at least two 

 other nucleic acid fractions are involved in protein biosynthesis, messenger RNA (mRNA) 

 and ribosomes. mRNA is believed to carry the information necessary to synthesize a 

 given protein. Because the mRNA is only a small portion of the total RNA present in the 

 cell it is difficult to get precise information concerning its composition and properties. 

 However, evidence does exist that it acts as the template for protein synthesis, that it is 

 rapidly synthesized and metabolically unstable and that it may mimic the N-base compo- 

 sition of DNA. The hypothesis therefore is that mRNA acts as an information carrier 

 from DNA and plays the role of a template in protein synthesis. 



The ribosomes are associated with the particulate fractions of the cell and have been 

 obtained from microsomes, nuclei and mitochondria (87). They are isolated by ultracentrif- 

 ugation and their size depends on the magnesium ion concentration of the medium. The 

 major constituents (40 to 60 percent) of the ribosomes are proteins that resemble histones 

 and RNA. Experimental evidence indicates that although ribosomes are required for pro- 

 tein synthesis they do not determine the nature of the protein produced. Presumably the 

 ribosomes represent a non-specific part of the total machinery necessary for protein 

 production and only after they have interacted with a specific mRNA will a given protein 

 be synthesized. 



