What are four levels of protein structure, and explain how a protein's shape determines its function? Explain denaturation.

Proteins are bluiding blocks of an organisms structure.  These seve various functions such as formation of structure in form of muscles, provide immunity in form of antibodies, catalyze reactions in form of enzymes etc,  

Chemically, proteins are biomolecules containing carbon, nitrogen, hydrogen, oxygen and sulfur atoms in their structure. These are polymers formed of amino acids as their monomers. Amino acids contain both -COOH as well as -NH₂ groups in their structure. These groups of two amino acids form condense with each other and form peptide bonds.

Structurally, protiens are classified in four levels. 

1. First level is primary structure which is  straight chain structure composed on amino acids bonded to each other by peptide bonds. The first amino acid with free -NH₂ group is called N-terminal amino acid while one with free -COOH group is called C-terminal amino acid. This level has protein which is freshly translated from mRNA.  Protein is termed as simple polypeptide in this level.
2. Second level is the secondary structure of protein in which primary structure forms hydrogen bonds with itself. Two types of secondary structures are alpha helix and beta sheets. 

Alpha helix is formed when primary structure forms helical shape, it is stabalized by hydrogen bonds formed between -NH and -CO of first and fourth amino acid. 

Beta sheets are formed when primary structure lines up and forms pleats. The amino acids, which occupy opposite position in these pleats, form hydrogen bonds between their CO and NH groups. 

So, secondary structure is stabalized by hydrogen bonds.

3. Third level or tertiary structure is that in which the polypeptide is coiled on itself to form 3-dimensional structure. Distant amino acids of polypeptide come closer in this structure. It is stabalized by four types of interactions i.e. hydrogen bonds, ionic interactions, disulfide linkages, hydrophobic interactions. 

4. Fourth level or quarternary structure is that in which protein is composed of two or more subunits. Each subunit is an idependent polypeptide. These subunits are coiled with each other and together form a functional protein. Example, hemoglobin has four polypeptides.  

Shape of protein directly governs its function. Shape itself depends on the primary structure or more specifically to the amino acids present in protein. These amino acids decide which type of secondary or tertiary structure is possible for a protein and hence ...