These bridges increase toughness and resistance to unfolding of the protein structure. Glycine is the smallest amino acid, with hydrogen as its R group, and it fits into tight places within a protein's structure. Proline has a cyclic ring involving the central carbon, and it causes kinks to occur in a protein chain. Both proline and glycine are common at the corner of turns in the protein foldings.
Primary Structure
The unique sequence of amino acids in a protein is termed the primary structure. When amino acids form a protein chain, a unique bond, termed the peptide bond, exists between two amino acids. The sequence of a protein begins with the amino of the first amino acid and continues to the carboxyl end of the last amino acid.
The unique sequence of amino acids results from the translation of codons present in messenger RNA (mRNA). The mRNA, in turn, is a complementary copy of the gene that codes for that protein. Protein structure and function can change when "misspellings" occur in the order of amino acids during their transcription and translation. Sickle-cell hemoglobin, for example, is "misspelled" in only one amino acid; the sixth amino acid in the beta chain, where a valine is substituted for a glutamic acid.
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