Leu Val Lys Phe Arg 35 40 45 50 Lys Ala Ala Asp Asp Thr Trp Glu Pro Phe Ala Ser Gly Lys Thr Ser Glu Ser Gly Primary structure of transthyretin Glu 55 Leu 70 65 60 His Asp Ile Glu Val Lys Tyr Ile Gly Glu Val Phe Glu Glu Glu Thr Thr Leu Gly Thr 75 Lys 80 85 90 Ser Tyr Trp Lys Ala Leu Gly Ile Ser Pro Phe His Glu His Ala Glu Val Val 95 Phe Thr 115 110 105 100 Ala Tyr Ser Tyr Pro Ser Leu Leu Ala Ala Ile Thr Tyr Arg Arg Pro Gly Ser Asp Asn Ser Thr Thr 120 125 Ala Val Val Thr Asn Pro Lys Glu C O O

Carboxyl end The primary structure of a protein is a linked series of amino acids with a unique sequence. As an example, lets consider transthyretin, a globular blood protein that transports vitamin A and one of the thyroid hormones throughout the body. Transthyretin is made up of four identical polypeptide chains, each composed of 127 amino acids. Shown here is one of these chains unraveled for a closer look at its primary structure. Each of the 127 positions along the chain is occupied by one of the 20 amino acids, indicated here by its three-letter abbreviation. The primary structure is like the order of letters in a very long

If left to chance, there would be 20127 different ways of making a polypeptide chain 127 amino acids long. However, the precise primary structure of a protein is determined not by the random linking of amino acids, but by inherited genetic information. The primary structure in turn dictates secondary and tertiary structure, due to the chemical nature of the backbone and the side chains (R groups) of the amino acids positioned along the chain. 82 U N I T O N E The Chemistry of Life Secondary Structure Regions stabilized by hydrogen bonds between atoms of the polypeptide backbone helix Hydrogen bond pleated sheet strand, shown as a flat arrow pointing toward the carboxyl end

Hydrogen bond Most proteins have segments of their polypeptide chains repeatedly coiled or folded in patterns that contribute to the proteins overall shape. These coils and folds, collectively referred to as secondary structure, are the result of hydrogen bonds between the repeating constituents of the polypeptide backbone (not the amino acid side chains). Within the backbone, the oxygen atoms have a partial negative charge, and the hydrogen atoms attached to the nitrogens have a partial positive charge (see Figure 2.16); therefore, hydrogen bonds can form between these atoms. Individually, these hydrogen bonds are weak, but because they are repeated many times over a relatively long region of the polypeptide chain, they can support a particular shape for that part of the protein. One such secondary structure is the helix, a delicate coil held