Posttranslational modification reactions, biological

The biological function of many of these posttranslational modification reactions is also tenuous at this stage, even if we can rationalize some of them as components of well understood processes. The most obvious example of such established processes is the oxidation of sulfhydryl groups resulting in the formation of disulfide bridges which are essential structural... 

Even when the purpose of a given posttranslational modification is understood, an examination of how and where it occurs is also likely to yield only limited information. The cell biological sites and processes involved in the reactions, the specificity by which certain amino acid residues or specific peptide bonds are selected for chemical modification and the mechanism by which the transformations are carried out remain obscure for a large number of these reactions. 

After posttranslational modification of the precursor peptide, the unmodified leader sequence is cleaved by a protease to reveal the biologically active compound. Depending on the lantibiotic, this cleavage occurs either just prior to or after transport outside the cell. For most class 1 lantibiotics, the cleavage reaction is... 

Once correct positioning occurs, and the match is made between the anticodon of the met-tRNA and the start codon, the GTP molecule bound to eIF-2 is hydrolyzed in a reaction promoted by eIF-5. The physical nature of this reaction remains controversial. There are thought to be two forms of eIF-5 with molecular masses of 125 kDa and 60 kDa without, however, any differences in their biological properties (Hershey, 1991 Merrick, 1992). The hydrolysis of GTP causes the release of the initiation factors from the surface of the 40S ribosomal subunit, and allows attachment of the 60S subunit by triggering the release of eIF-6 from it. The formation of the SOS initiation complex culminates in the formation of the first peptide bond at the ribosomal P site. The initiation factor eIF-4D is required for the formation of the first peptide bond. eIF-4D is a small protein (about 16 kDa), and has a unique posttranslational modification of its lysine-50 residue by the action of a polyamine, spermidine, to form a hypusine residue essential for its activity (Hershey, 1991 Merrick, 1992). Furthermore, in order to allow efficient and catalytic use of eIF-2 after GTP hydrolysis and its release from the complex, another factor, eIF-2B, facilitates the exchange of eIF-2 bound GDP for GTP. 

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