The unique three-dimensional structure of a polypeptide is known as its tertiary structure.
The α-helix and β-pleated sheet structures are found in many globular and fibrous proteins. The pleated segments align parallel to each other, and hydrogen bonds form between the same pairs of atoms on each of the aligned amino acids. The R groups are attached to the carbons, and extend above and below the folds of the pleat. In the β-pleated sheet, the “pleats” are formed by hydrogen bonding between atoms on the backbone of the polypeptide chain. In the alpha helix, the bonds form between every fourth amino acid and cause a twist in the amino acid chain. Both structures are held in shape by hydrogen bonds.
The most common are the alpha (α)-helix and beta (β)-pleated sheet structures. This can lead to a myriad of serious health problems, such as breathlessness, dizziness, headaches, and abdominal pain for those who have this disease.įolding patterns resulting from interactions between the non-R group portions of amino acids give rise to the secondary structure of the protein. (credit: modification of work by Ed Uthman scale-bar data from Matt Russell)īecause of this change of one amino acid in the chain, the normally biconcave, or disc-shaped, red blood cells assume a crescent or “sickle” shape, which clogs arteries. In this blood smear, visualized at 535x magnification using bright field microscopy, sickle cells are crescent shaped, while normal cells are disc-shaped. The structural difference between a normal hemoglobin molecule and a sickle cell molecule-that dramatically decreases life expectancy-is a single amino acid of the 600.įigure 1. The molecule, therefore, has about 600 amino acids. What is most remarkable to consider is that a hemoglobin molecule is made up of two alpha chains and two beta chains that each consist of about 150 amino acids. In sickle cell anemia, the hemoglobin β chain has a single amino acid substitution, causing a change in both the structure and function of the protein. Any change in the gene sequence may lead to a different amino acid being added to the polypeptide chain, causing a change in protein structure and function. The unique sequence for every protein is ultimately determined by the gene that encodes the protein. The unique sequence and number of amino acids in a polypeptide chain is its primary structure. To understand how the protein gets its final shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary (Figure 2).
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