092909-BCH311

092909-BCH311 - Tertiary(3° Structure of Proteins • The...

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Unformatted text preview: Tertiary (3°) Structure of Proteins • The three-dimensional arrangement of all the atoms in the molecule - this now includes the R side chains • The conformations of the side chains and the positions of any prosthetic groups ( any non-amino acid group attached to the protein ) are part of the tertiary structure, as is the arrangement of the α-helices and β-sheets with respect to each other • In a Fbrous protein, the overall shape of which is a long rod, the secondary structure also provides much of the information about the tertiary structure • The helical backbone does not fold back upon itself, and the only aspect of the tertiary structure that is not speciFed by the secondary structure is the arrangement of the atoms of the side chains • ¡or a globular protein, considerably more information is needed - how do the α- helices and β-sheets fold back on each other, and how does that inFuence the interactions between side chain R groups? Forces Involved in Tertiary (3°) Structures • The primary structure of proteins - the order of amino acids - depends on the formation of covalent peptide bonds • Higher-order levels of structure, such as the conformation of the backbone (2 ° structure) and the positions of all the atoms in the protein (3 ° structure) usually depends on noncovalent interactions • If a protein consists of several subunits, the interaction of the subunits (4 ° structure) also depends on noncovalent interactions - contributing to the most stable structure for a given protein • Several types of noncovalent interactions can occur; a. backbone hydrogen bonding (2 ° structure), b. hydrogen bonding between the side chains of amino acids c. hydrophobic interactions between nonpolar amino acids d. electrostatic interactions between oppositely charged R side groups e. electrostatic interactions between metal ions and charged R side groups • In addition to these noncovalent interactions, covalent interactions in the form of disul¡de bridges/bonds can occur between cysteine side chains Fig. 4-13, p. 100 Tertiary Structure of Proteins • The three-dimensional conformation of a protein is the result of the interplay of all these stabilizing forces ( both covalent and noncovalent ) Myoglobin • Myoglobin is a classic example of a globular protein, and an excellent case study for the 3D arrangement of a protein • Myoglobin is a relatively small (M.W. = 16.7 kDa) oxygen-binding protein of muscle cells • It functions primarily to store oxygen and also to facilitate oxygen diffusion in rapidly contracting muscle tissue, and is a sensitive marker of muscle injury, e.g. heart attack • Myoglobin was the Frst protein to be completely crystallized ( using X-ray crystallography ) - revealing its 3° structure • The complete myoglobin molecule consists of a single polypeptide chain of 153 amino acid residues and includes a prosthetic group, the heme group, • The myoglobin molecule (including the heme group) has a compact structure, with the interior atoms very close to each other Fig. 4-15, p. 102Fig....
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092909-BCH311 - Tertiary(3° Structure of Proteins • The...

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