The primary structure of a protein is its amino acid sequence. It has an N-terminal end with a free NH3+ group and a C-terminal end with a free COO- group. The secondary structure involves hydrogen bonding between amino acids and forms alpha helices, beta pleated sheets, or triple helices. The tertiary structure is the overall 3D shape from interactions between amino acid side chains far apart in the sequence. Quaternary structure refers to the association of multiple polypeptide chains in an oligomeric protein.

1. Primary structure of а protein
• The primary structure of а protein is the sequence of amino
acids present in its peptide chain or chains.
• The end with the free H3
N+
group is called the N-terminal
end, and the end with the free СОО-
group is called the С-
terminal end.

2. Primary structure

3. Secondary structure of а protein
• The secondary structure of а protein is the arrangement in
space of the atoms in the backbone of the protein. Three
major types of protein secondary structure are known; the
alpha helix, the beta pleated sheet, and the triple helix. The
major force responsible for all three types of secondary
structure is hydrogen bonding between а carbonyl oxygen
atom of а peptide linkage and the hydrogen atom of an amino
group (-NH) of another peptide linkage farther along the
backbone.

4. Secondary structure

5. Alpha Helix
• The Alpha Helix The alpha helix (α-helix) structure resembles а
coiled helical spring, with the coil configuration maintained by
hydrogen bonds between N – Н and С= О groups of every fourth
amino acid

6. Beta pleated sheet
• The beta pleated sheet (β-pleated sheet) secondary
structure involves amino acid chains that are almost
completely extended.

7. Tertiary structure
The tertiary structure of а protein is the
overall three-dimensional shape that results
from the attractive forces between amino
acid side chains (R groups) that are widely
separated from each other within the chain.
There are four types of bonding interactions
between "side chains" including: hydrogen
bonding, salt bridges, disulfide bonds, and
non-polar hydrophobic interactions.

8. Tertiary structure

10. Quaternary structure
• Quaternary structure is the highest level of
protein organization. It is found only in proteins
that have structures involving two or more
polypeptide chains that are independent of each
other — that is, are not covalently bonded to each
other. These multichain proteins are often called
oligomeric proteins. The quaternary structure of а
protein involves the associations among the
separate chains in an oligomeric protein.

11. Hemoglobin