Lecture 4 - Lecture 4 Chapter 4 Chapter 4 Tertiary...

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Lecture 4 – Chapter 4 Chapter 4 - Tertiary structure – overall 3-D arrangement of all atoms in a protein – whereas “secondary structure” refers to the spatial arrangement of amino acid residues that are adjacent in a segment of a polypeptide, tertiary structure includes longer-range aspects of amino acid sequence. Amino acids that are far apart in the polypeptide sequence and are in different types of secondary structures may interact within the completely folded structure of a protein. Some proteins contain 2 or more separate polypeptide chains, or subunits, which may be identical or different. The arrangement of these protein subunits in 3-D complexes constitutes quaternary structure. In considering these higher levels of structure, it is useful to classify proteins into 2 major groups: fibrous proteins, w polypeptide chains arranged in long strands or sheets, and globular proteins, w polypeptide chains folded into a spherical or globular shape. Fibrous proteins usually consists largely of a single type of secondary structure, and their tertiary structure is relatively simple. Globular proteins often contain several types of secondary structure. - α keratin, collagen, and silk fibroin are fibrous proteins that share properties that give strength and/or flexibility to the structure in which they occur. o α-keratins – evolved for strength – found only in mammals, these proteins constitute almost the entire dry weight of hair, wool, nails, claws, and much of the outer layer of skin. α-keratin helix is a right- handed α-helix 2 strands of α-keratin, oriented in parallel (w their amino termini at the same end), are wrapped about each other to form a supertwisted coiled coil – supertwisting amplifies the strength of the overall structure, just as strands are twisted to make a strong rope. The helical path of the supertwists is left-handed, opposite in sense to the α helix. The intertwining of the 2 α-helical polypeptides is an example of quaternary structure. o When hair is exposed to moist heat, it can be stretched. At the molecular level, the α helices in the α- keratin of hair are stretched out until they arrive at the fully extended beta conformation. On cooling they spontaneously revert the α-helical conformation. The characteristic “stretchability” of α-keratins, and their numerous disulfide cross-linkages, are the basis of permanent waving. – creates new disulfide cross-linkages that exert some torsion or twist on the bundles of α-helical coils in the hair fibers. o
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This note was uploaded on 10/28/2010 for the course BIO/CHEM 2314 taught by Professor Ghiara during the Fall '10 term at UCSD.

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Lecture 4 - Lecture 4 Chapter 4 Chapter 4 Tertiary...

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