Protein Structure
All proteins share three levels of structure, primary, secondary, and tertiary. The fourth level, quaternary structure
comes about when a protein consists of two or more polypeptide chains. The primary structure of a protein tells you what
amino acids are being used. It is not determined by the random linking of amino acids, but by inherited genetic
information. Most proteins have segments of their polypeptide chains repeatedly coiled or folded in patterns that
contribute to the protein's overall shape. All of those folds and coils are part of the secondary structure which happens
because of hydrogen bonds between the repeating constituents of the polypeptide backbone(not the amino acid side
chains). Because both of the oxygen and nitrogen atoms have partially negative charges, a weakly positive hydrogen
atom from the nitrogen atom has an affinity for the oxygen atom nearby, creating a weak hydrogen bond. There are two
different types of secondary structure, helix and pleated sheet. Superimposed on the patterns of secondary structure
is a protein's tertiary structure, this is the overall shape of a polypeptide resulting from interactions between the side
chains(R groups) of various amino acids. Hydrophobic(nonpolar) interactions occur with amino acid side chains that
usually end up in clusters at the core of the protein, out of contact with water. In the end this reaction is a bit ironic since it
is caused by water. Once the nonpolar amino acid side chains are close together, van de waals interactions hold them
together. The shape of a protein may be reinforced further by covalent bonds called disulfide bridges. Quaternary
structure is the overall protein structure that results from the aggregation of these polypeptide subunits.
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