Proteins are polymers (long chain of monomers) of amino acids (see left). Each amino acid contains an amino group (-NH2) and a carboxylic acid group (-COOH) bonded to the second carbon. They are called 2-amino acids for that reason.
Aminos are crystalline compounds with high melting points (200 C and up) and greater solubility in water than in nonpolar substances. |
Aminos move in an electric field, which hints that aminos contain charged groups, which are a result of acid-base behavior according to Bronsted-Lowry. In aqueous solutions and crystalline form, aminos exist with both positive and negative charges within the molecules. These are called zwitterions. The charges result from an internal acid-base reaction with the transfer of an H+ from the acidic -COOH group to the alkaline -NH2 group (that becomes -NH3). Because they contain both acidic and alkaline groups, aminos are amphoteric. They will accept and donate H+ according to changes in pH.
The hydrogen attached to the -OH portion of -COOH breaks off and attaches to the -NH2 group to form the zwitterion -NH3. As the pH of the medium changes, it influences the charge of the aminos. In alkaline solutions, the first reaction is favored as the -NH3 group loses its H+ and forms an anion. In acidic solutions, the second reaction is favored as the -COO- group gains an H+ and forms a cation. See below.
Amino acids form peptides through condensation reactions. A molecule of water is removed and a new bond forms between the acid group of one amino and the amino group of another. This is called a peptide bond. Two aminos grouped like this are called dipeptides. When there are a lot of amino acids grouped, the resulting structure is called a polypeptide. Proteins are made from polypeptide chains with at least 50 amino acids, so there is a near unlimited number of amino sequences and each forms a different protein.
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The primary structure of proteins is the amino acid sequence. This refers to the number and sequence of aminos in a polypeptide chain that is held together by peptide bonds (see above). This is the covalent backbone of any protein. Once the primary structure is formed, it allows for the creation of secondary, tertiary and quaternary structures.
Secondary structures are the result of hydrogen bonding that folds the polypeptide chain into either a helix or a pleated sheet. Helix structures are flexible and elastic. Keratin is an example of a secondary structure protein. Pleated structures have extended flexibility, but are inelastic. An example is fibroin.
Tertiary structures are the result of interactions between R groups. This results in further twisting and folding of the polypeptide chain. There are 4 kinds of interactions (see below).
Quaternary structures result from the association between different polypeptide chains.
Secondary structures are the result of hydrogen bonding that folds the polypeptide chain into either a helix or a pleated sheet. Helix structures are flexible and elastic. Keratin is an example of a secondary structure protein. Pleated structures have extended flexibility, but are inelastic. An example is fibroin.
Tertiary structures are the result of interactions between R groups. This results in further twisting and folding of the polypeptide chain. There are 4 kinds of interactions (see below).
Quaternary structures result from the association between different polypeptide chains.
Protein Structures
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