Amino acids glycine replacement

Much work has been done therefore, to try and stabilize this bond towards hydrolysis. It is possible to replace the amino acid glycine with an unnatural D-amino acid such as D-alanine. Since D-amino acids are not naturally occurring, peptidases do not recognize the structure and the peptide bond is not attacked. The alternative tactic of replacing L-tyrosine with D-tyrosine is not possible, since this completely alters the relative orientation of the tyrosine aromatic ring with respect to the rest of the molecule. As a result, the analogue is unable to bind to the analgesic receptor and is inactive. 

Proteins that cause disease may do so by having an incorrect structure, as we will see later with an in-born error in hemoglobin where a small amino acid, glycine, is replaced by a large amino acid, phenylalanine, that cannot fit into the small space available within the normal hemoglobin structure. In this case, the structure of hemoglobin is destabilized and anemia results. 

However, it is clear from this that activation of an oncogene may only require a point mutation in the proto-oncogene sequence (e.g., H-ras, G to T in reading frame). This would mean that glycine was replaced with valine. All Ras oncoproteins have amino acid substitution in residues 12, 61, or 13. 

To get acquainted with amino acid structures, leam first those of glycine, alanine, serine, aspartic acid, and glutamic acid. The structures of many other amino acids can be related to that of alanine (R=CH3) by replacement of a P hydrogen by another... 

A number of residues may be replaced with other amino acids without apparent change in enzymic activity. These replacements include nor-leucine for methionine at positions 26 and 32 87), phenylalanine for tyrosine 27 92), and glycine for histidine 46 85). As mentioned above, deletion of histidine 8 is without deleterious effect. The two remaining residues of histidine at positions 121 and 124 may also be ruled out as components of the active site since the former is, stereoehemically, far on the other side of the molecule and the latter is replaced by leucine in nuclease from the Foggi strain of Staphylococcus aureus. 

Formally, however, the amino acids contained either in peptides or proteins are named in a fashion related to that used for alkanes. The -ine suffix is replaced by -yl to give, for GAG, glycyl-alanyl-glycine. Peptide and protein sequences are always written and named from the N-terminus to the C-terminus. The C-terminal amino acid retains its full name. 

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