What are the different levels of protein structures?

Answer 1

The different levels of protein structure are primary, secondary, tertiary, and quaternary.

Primary structure is its unique sequence of amino acids. Both the length of the chain and the order of amino acids are decisive in the sequence.

Secondary structure is the localized, repetitive coiling or folding of the polypeptide due to hydrogen bond formation between the amino acids. The two secondary structure are the alpha helix and beta pleated sheet. The alpha helix is a coil formed by the hydrogen bonding between every fourth amino acid. Some fibrous proteins are composed mostly of alpha helixes, like alpha keratin, the structural protein of hair. Likewise, some globular proteins may also be made up of alpha helixes, like hemoglobin. The beta pleated sheet occurs when two or more regions of the polypeptide lying side by side are connected by hydrogen bonds between parts of the two parallel polypeptide portions. Beta pleated sheets make up the core of many globular proteins, like transthyretin. They can also make up some fibrous proteins like the silk proteins of Spiders.

Tertiary structure is the culmination of irregular contortions due to the interactions of the side chains which may have hydrophobic interaction, ionic bonds, hydrogen bonds, and disulfide bridges. As the polypeptide begins to fold into its functional shape, the hydrophobic (nonpolar) side chains of certain amino acids, proline or methionine for example, end up clustering in the center of the protein, away from the polar water. Because they are so close and clustered together, van der Waals interactions fortify their clumping. Oppositely, the hydrogen bonds between polar side chains and ionic bonds between positively and negatively charged side chains aid in stabilizing the tertiary structure.The formation of disulfide bridges from pairs of cysteine monomers (amino acids with sulfhydryl groups) also serve to reinforce the structure.

Quaternary structure is the overall protein structure after two or more polypeptides join together. They can take on many various shapes, each of which is extremely specific to serve their function. For example, collagen is a fibrous protein which serve as girders of connective tissue in skin, bones, tendons, ligaments, and other body parts. It must be pretty strong - and it is. It's made up of helical subunits intertwined into a larger triple helix, giving the long fibers strength and durability.

Quaternary structure might not always be applicable. In the cases where one polypeptide is a protein by itself, quaternary structure is not necessary.