Protein structure.

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Proteins have a complex three dimensional structure which is important because the function of a protein is closely tied to its three dimensional structure. 

This nature of this structure is usually explained in terms of a structural hierarchy, from primary to quaternary. 
 

 

Primary Structure

The primary structure of a polypeptide or protein is the sequence of amino acids in the protein. In the case of insulin shown here there are two polypeptide chains in the primary structure.Human Insulin Primary structure. Each three letter abbreviation stands for one of the twenty basic amino acids found in living things: 

Chain 1 GLY- ILE -VAL- GLU -GLN -CYS -CYS -THR- SER -ILE -CYS- SER -LEU - TYR -GLN -LEU -GLU -ASN -TYR -CYS -ASN  

Chain 2 PHE -VAL -ASN-GLN -HIS -LEU -CYS- GLY- ASP -HIS -LEU- VAL- GLU- ALA -LEU- TYR -LEU- VAL- CYS- GLY- GLU- ARG -GLY- PHE -PHE -TYR - THR -PRO -LYS -THR  

Data from the Brookhaven Protein Data Base as are the accompanying pictures.

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 Secondary Structure  

Secondary structure refers to the folding of the chain of amino acids into a helix or a pleated sheet. 

This structure is a pleated sheet formed by parallel chains of amino acids. These sheets are important in many structural proteins. Many proteins have sheets and helices. Secondary structure arises from the geometry of the bond angle between amino acids as well as hydrogen bonds between near by amino acids.

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 Tertiary Structure Tertiary structure refers to a higher level of folding in which the helices and sheets of the secondary structure fold upon themselves. This higher level folding arises for several reasons. First, different regions of the amino acid chain are hydrophilic or hydrophobic and arrange themselves accordingly in water. Second different regions of the chain bond with each other via hydrogen bonding or disulfide linkages. 

This kind of structure is most important in globular proteins such as this insulin molecule, shown in cartoon form by RasMol to indicate the folding of helices.

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Quaternary structure Quaternary structure arises when polypeptide chains are bound together usually by hydrogen bonds. For example hemoglobin the oxygen carrying protein in blood has four subunits hydrogen bonded together. Most proteins with a molecular weight of 50,000 or more are made of such units. 

Sometimes quaternary structure maybe very complex. For example, beef glutamate dehydrogenase is an enzyme with a molecular weight of 2,200,000. Each enzyme molecule consists of eight large subunits. In turn, each of these consists of numerous smaller units. 

The interesting thing about proteins made of polypeptide subunits is that given the right solution, they self assemble into a complete and functional protein! 

The cell takes full advantage of this property to rapidly generate the cytoskeleton much of which consists of very long chains or helices, or tubes of protein sub-units as in the example below. 

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 This is just a small section of a long double helix made out of thousands of small protein sub units, and illustrates the size of structures the cell can build using protein subunits.

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 For a case study in protein structure go here! 

 
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pgd. revised 7/15//99