This document discusses the structure of proteins at multiple levels. It begins by defining proteins and their functions in the body. It then describes the primary, tertiary, and quaternary structure of proteins. The primary structure is the linear sequence of amino acids. Tertiary structure results from interactions between amino acid side chains that give the overall 3D shape. Quaternary structure occurs when multiple protein subunits combine, as seen in hemoglobin. Non-covalent bonds like hydrogen bonding contribute to tertiary and quaternary structure formation.

1. Protein Structure
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2. Protein
• Proteins are large, complex molecules that play many critical roles in the body.
• The Swedish chemists J. J. Berzelius in 1938 coined the name ’protein’ from the
Greek word proteios which means “of the first rank”.
• They function as catalysts, they transport and store other molecules such as
oxygen, they provide mechanical support and immune protection, they generate
movement, they transmit nerve impulses, and they control growth and
differentiation.

3. Primary Structure
• The primary structure of a protein is defined as a linear sequence of amino acids and the location of disulfide
(-S-S-) bonds, if any.

4. Tertiary structure
• The overall three-dimensional
structure of a polypeptide is called
its tertiary structure.
• The tertiary structure is primarily due
to interactions between the R groups of
the amino acids that make up the protein.

5. Tertiary structure
• R group interactions that contribute to tertiary structure include hydrogen bonding, ionic
bonding, dipole-dipole interactions, and London dispersion forces – basically, the whole gamut of
non-covalent bonds.
• For example, R groups with like charges repel one another, while those with opposite charges
can form an ionic bond.
• Similarly, polar R groups can form hydrogen bonds and other dipole-dipole interactions. Also
important to tertiary structure are hydrophobic interactions, in which amino acids with nonpolar,
hydrophobic R groups cluster together on the inside of the protein, leaving hydrophilic amino acids
on the outside to interact with surrounding water molecules.

6. Tertiary structure
• Finally, there’s one special type of covalent bond that can contribute to tertiary structure: the
disulfide bond.
• Disulfide bonds, covalent linkages between the sulfur-containing side chains of cysteines, are
much stronger than the other types of bonds that contribute to tertiary structure.
• They act like molecular "safety pins," keeping parts of the polypeptide firmly attached to one
another.

7. Quaternary structure
• Many proteins are made up of a single
polypeptide chain and have only three levels of
structure (the ones we’ve just discussed).
• However, some proteins are made up of
multiple polypeptide chains, also known as
subunits. When these subunits come together,
they give the protein its quaternary structure.

8. Quaternary structure
• Example of a protein with quaternary structure:
• hemoglobin. As mentioned earlier, hemoglobin carries oxygen in the blood
and is made up of four subunits, two each of the α and β types.
• Another example is DNA polymerase, an enzyme that synthesizes new
strands of DNA and is composed of ten subunits^55start superscript, 5, end
superscript.

9. Quaternary structure
• In general, the same types of interactions that contribute to
tertiary structure (mostly weak interactions, such as hydrogen
bonding and London dispersion forces) also hold the subunits
together to give quaternary structure.

10. Reference
1. Khan, R. H., Siddiqi, M. K., & Salahuddin, P. (2017). Protein structure and function. Basic
biochemistry, 5, 1-39.
2. Berg, J. M., Tymoczko, J. L., and Stryer, L. (2002). Tertiary structure: Water-soluble proteins fold
Into compact structures with nonpolar cores. In Biochemistry (5th ed., section 3.4). New York,
NY: W. H. Freeman. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK22375/.
3. 3. Berg JM, Tymoczko JL, Stryer L (2002). "Section 3.5Quaternary Structure: Polypeptide Chains
Can Assemble Into Multisubunit Structures". Biochemistry (5. ed., 4. print. ed.). New York, NY
[u.a.]: W. H. Freeman. ISBN 0-7167-3051-0.