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Lecture 5:"Protein Function"

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Western University
Biochemistry 2280A
Mel Usselman

Biochemistry Lecture No. 5: Protein Function th Friday September 14 , 2012 Quaternary Structure: -Many proteins consist of more than one polypeptide and are known for possessing a quaternary structure as a result. The forces/bonds involved in quaternary structures are the same as for tertiary structures (Hydrophobic, H-bonds, van der Waals, ionic, disulfide bonds). Subunits are the individual polypeptide chains within a quaternary structure. For example, the P53 tetramer, mutated in about 50% of cancers, is comprised of 4 subunits that form a quaternary structure. Coiled Coil: -A type of quaternary structure is the coiled coil, whose repeating sequence plays a very important role in its structure. With a repeating pattern every 7 resides, helices wrap around one another to minimize exposure of hydrophobic residues to the aqueous environment, leaving what is called a hydrophobic stripe on one side of the helix. Examples of proteins containing the coiled coil structure are EB1, which binds to microtubule ends and GCN4 which is a transcription factor. Indication Of Protein Structure In Biochemistry: -Biochemists indicate protein structure using three distinct models: wire, ribbon and space-filling. In the wire visualization, side chains and their respective proximity are represented in order to predict amino acids involved in function. In the ribbon model, secondary structures are primarily represented. In the space-filling representation, one can visualize the specific shape of the protein, which amino acids are on the surface, as well as predict possible interactions with water or proteins. Post-Translational Modifications: -After a protein has been translated into its linear sequence and has achieved its 3D conformation, it can experience one of the following post-translational modifications: the binding of lipids (to form a lipoprotein), the binding of metal ions (to form a metalloprotein), and the binding of a heme group (to form a hemeprotein). Another possible modification is the phosphorylation of a protein, which affects its ultimate function. Protein Folding: -Although this is area of molecular biology is greatly mysterious, it is now widely believed that molecular chaperones, proteins that help other proteins achieve their 3D conformation (native state), are primarily responsible for driving protein folding in cells. Conformational Flexibility & Changes: -Proteins are not completely rigid structures and various types of motion are possible depending on their structure. It is often domains that can move, connected by flexible linkers. These adjustments in the protein’s make-up are known as conformational changes and many such changes a
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