2.6—Amino acids, proteins and protein structure

Amino acids share an amino group, carboxyl group and variable R group. Peptide bonds join amino acids by condensation to form a polypeptide; its sequence is the primary structure.

Hydrogen bonds, ionic interactions, disulfide bonds and hydrophobic interactions fold the chain into secondary, tertiary and sometimes quaternary structure. Shape determines binding and function.

Replacing one amino acid can alter charge or hydrophobicity, changing folding and a protein’s activity. A denatured enzyme may keep its peptide bonds but lose the shape of its active site.

Fibrous proteins form long structural arrangements; globular proteins fold into compact shapes with functional sites. Both depend on the same chemical principle that sequence constrains folding.

Primary structure is the amino-acid sequence, not the gene itself. A change in DNA can affect a protein, but environment and folding conditions also influence the final phenotype.