Chemical structures amino acids, substituted

The apoproteins are distinct physically, chemically, and immunochemically and have important roles in lipid transport and metabolism (Table 20-1). In keeping with their individual metabolic functions, they have specific structural domains. Amino acid substitutions or deletions in critical domains result in functional abnormalities. The apoproteins share a common structure in the form of an amphipathic helix, in which the amino acid residues have hydrophobic side chains on one face of the helix and hydrophilic polar residues on the other. The hydrophilic face is believed to interact with the polar head groups of the phospholipids, while the hydrophobic residues interact with their fatty acid portions. 

The relationship between nucleotide frequencies and protein secondary structures is associated not only with the physico-chemical properties of these structures, but also with the organisation of the genetic code. In fact, this organisation seems to have evolved so as to preserve the secondary structures of proteins by preventing deleterious amino acid substitutions that could modify the physico-chemical properties required for an optimal structure Chiusano et al., 2000). 

---