Protein synthesis posttranslational modification

Pantothenic acid (vitamin B5) is both present in many nutrientcients and it is also produced by intestinal bacteria. Deficiency is therefore thought to be unlikely. Its active form, 4-phosphopantetheine, is an element of both coenzyme-A and acyl-carrier protein and thus participates in fatty acid synthesis and in the posttranslational modification of proteins. Acetylcoenzyme-A is important for the synthesis of the neurotransmitter acetylcholine. 

The pathway of protein synthesis translates the three-letter alphabet of nucleotide sequences on mRNA into the twenty-letter alphabet of amino acids that constitute proteins. The mRNA is translated from its 5 -end to its 3 -end, producing a protein synthesized from its amino-terminal end to its carboxyl-terminal end. Prokaryotic mRNAs often have several coding regions, that is, they are polycistronic (see p. 420). Each coding region has its own initiation codon and produces a separate species of polypeptide. In contrast, each eukaryotic mRNA codes for only one polypeptide chain, that is, it is monocistronic. The process of translation is divided into three separate steps initiation, elongation, and termination. The polypeptide chains produced may be modified by posttranslational modification. Eukaryotic protein synthesis resembles that of prokaryotes in most details. [Note Individual differences are mentioned in the text.]... 

Many polypeptide chains are covalently modified, either while they ae still attached to the ribosome or after their synthesis has been completed. Because the modifications occur after translation is initiated they are called posttranslational modifications. These modifications maj include removal of part of the translated sequence, or the covalent add-tion of one or more chemical groups required for protein activity. Som< types of posttranslational modifications are listed below. 

The enzymatic specificity of diphtheria toxin deserves special comment. The toxin ADP-ribosylates EF-2 in all eukaryotic cells in vitro whether or not they are sensitive to the toxin in vivo, but it does not modify any other protein, including the bacterial counterpart of EF-2. This narrow enzymatic specificity has called attention to an unusual posttranslational derivative of histidine, diphthamide, that occurs in EF-2 at the site of ADP-ribosylation (see fig. 1). Although the unique occurrence of diphthamide in EF-2 explains the specificity of the toxin, it raises questions about the functional significance of this modification in translocation. Interestingly, some mutants of eukaryotic cells selected for toxin resistance lack one of several enzymes necessary for the posttranslational synthesis of diphthamide in EF-2 that is necessary for toxin recognition, but these cells seem perfectly competent in protein synthesis. Thus, the raison d etre of diphthamide, as well as the biological origin of the toxin that modifies it, remains a mystery. 

Posttranslational Protein Modification a mechanism of biological control of protein activity by covalent modification after protein synthesis... 

Although the total synthesis of a protein allows complete control over the structure, including posttranslational modifications and introduction of labels at desired sites in the sequence, it is still a major undertaking for which most laboratories whose main interest is in the biology of their target proteins are not equipped. In certain cases, for example when the site of introduction of a specific chemical modification is near the C-terminus, a combination of molecular biological and chemical methods has proved to be very powerful. 

Figure 29.35. Blocking of Translocation by Diphtheria Toxin. Diphtheria toxin blocks protein synthesis in eukaryotes by catalyzing the transfer of an ADP-ribose unit from NAD+ to diphthamide, a modified amino acid residue in elongation factor 2 (translocase). Diphthamide is formed by a posttranslational modification (blue) of a histidine residue.

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