Amino acids imparting structural stability

Hydrogen bonds are also invaluable as mechanisms by which secondary and tertiary structure can be imparted to proteins. The most well-known examples are the formation of a-helices and p-shccls. In the former, linear sequences of amino acids form a spiral that is stabilized through multiple interactions between aligned amine hydrogen atoms and carbonyl oxygen atoms. In the latter the same interactions are found between amino acid sequences that are aligned in the same plane. 

Hydrophobicity is a property shared to varying degrees by most proteins and is imparted primarily through the side chains of neutral aromatic amino acids phenylalanine, tyrosine and tryptophan. By their lower attraction for water molecules, these amino acids tend to link to one another, thus expelling water from the molecule. While hydrophobicity is one of the natural forces that confer stability on the tertiary structure of peptides, it also imparts stability to formed immune complexes and depending on environmental factors, can exist also between different protein molecules. 

If export competence is associated with a loosely folded precursor, then parameters that accelerate folding or stabilize folded states should impede export. This relationship has also been explored in E. coli using a DHFR fusion protein. At low levels of synthesis, a hybrid protein consisting of the signal peptide and the first 153 amino acid residues of OmpA joined to DHFR is efficiently secreted. However, addition of trimethoprim imparts a kinetic defect in the secretion rate of the hybrid protein. The effect of trimethoprim is dependent on the presence of a full length, presumably active, DHFR moiety, indicating that secretion in vivo is inhibited by stabilization of the native DHFR structure (FreudI et ai, 1988). 

Cytokines have very stable structures with the presence of carbohydrates and disulfide bonds imparting solubility, stability, and resistance to cleavage. Despite the lack of amino acid sequence similarity, the limited variation in their overall three-dimensional structure may reflect the evolutionary origins of cytokines. This structural variation has made possible the definition of at least the following four different structural families. ... 

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