Post-translational modification of protein structures

Another example of a peptide sequence in a protein forcing it into a biologically useful conformation is found with collagen. This consists of a triple helix with chains of more than 1000 amino-acid residues, many of which are post-translation-ally modified. The latter steps, consisting inter alia of hydroxylation of Pro and Lys residues and 5-hydroxylysine residues, occur before the triple helix is formed, because the enzymes involved do not act on the helical structure. When the individual peptide chains of collagen are synthesised, there are N- and C-terminal sequences each containing about 100 amino-acid residues. These sequences favour the formation of a triple helix. When this has been achieved, the terminal sequences are removed. The sequences of these temporary terminal sequences are quite different from the main body of the collagen monomers, which consists of triads of the type Gly—X—Y, where X and Y are often proline or 3- or 4-hydroxyproline. 

Another example of post-translational modification that is important in confer- 

Proteins may be acetylated, usually on the a-amino group s TV-terminal residue but occasionally on the e-amino group of a lysine residue. Acetylation is effected with acetylcoenzyme A (8.3), although other biochemical acetylating agents are well known. a-Melanotropin or a-melanocyte-stimulating hormone (a-MSH) provides an example of post-translational acetylation  

A more complicated example is afforded by histone H4 from calf thymus. The TV-terminal serine residue is TV-acetylated and may be O-phosphorylated. e-TV- 

Acetylation can also affect Lys5, Lys8 and Lys12. The e-amino group of Lys20 is also modified, but by mono- or di-methylation. The biochemical methylating agent is S-adenosylmethionine (Section 8.5). The biosyntheses of coenzyme A and. S -adcno-sylmethionine provide additional examples of the biochemical utilisation of cysteine and methionine, respectively. 

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