Proteins are a large class of biomolecules and macromolecules consisting of chains of amino acids called polypeptides. While simple proteins consist of a single polypeptide, many proteins consists of multiple polypeptide subunits.
Examples of Proteins
Proteins are common in everyday life. Most of the structure of organs and tissues consist of proteins. Here are some examples:
- Keratin
- Actin
- Myosin
- Hemoglobin
- Collagen
- Elastin
- Albumin
- Fibrin
- Insulin
- Immunoglobulins (antibodies)
- Catalase (and all enzymes)
Functions of Proteins
Proteins perform important functions in organisms. In fact, this class of molecules occurs in every cell and is essential for life. Here are some examples of functions performed by proteins:
- Form the scaffolding that maintains cell shape
- Catalyze metabolic reactions
- Essential in animal diets as a source of certain amino acids
- Transport molecules within cells and throughout organism
- Essential for replicating DNA
- Act in the immune response
Protein Structure
A protein may consist of a single polypeptide or multiple polypeptide subunits. Some proteins include non-peptide groups, called cofactors. A cofactor may be an organic group (e.g., coenzyme, prosthetic group) or an inorganic group (e.g., metal ion or iron-sulfur cluster).
Each polypeptide is a linear molecule made from a string of amino acids that are bonded into a chain by peptide bonds. An organism’s DNA or RNA codes for the sequence of amino acids that form proteins. Building each amino acid chain from the genetic code is termed translation. Once a polypeptide has been translated, it usually undergoes additional chemical changes, called post-translation modification.
Levels of Organization
Proteins typically fold into three-dimensional structures. Biochemists describe proteins according to four levels or organization:
- Primary structure: This is the amino acid sequence.
- Secondary structure: Refers to regularly repeating structures that are stabilized by hydrogen bonds. Examples include turns, α-helix, and β-sheet.
- Tertiary structure: This is the overall spatial shape or folding, which determines the basic function of the molecule.
- Quaternary structure: For proteins consisting of multiple subunits (e.g., hemoglobin), this is the final protein shape or protein complex.
History
Antoine Fourcroy and other scientists first recognized proteins in the 1700s. At that time, the class of molecules had the term “albumins.” Jöns Jacob Berzelius coined the term “proteins” in 1838. The term derives from the Greek word proteios (“primary” and –in (in front). Dutch chemist Gerardus Johannes Mulder performed an elemental analysis of proteins and discovered that most had the empirical formula C400H620N100O120P1S1.
In the 19th century, scientists believed “flesh makes flesh” and that protein was the key nutrient for forming the body’s structure. While obtaining protein from eggs, skin, hair, and blood was possible, purifying individual proteins did not really happen until the 1950s. The first amino acid sequence for a protein was obtained by Frederick Sanger in 1949 for insulin. This earned him the Nobel Prize in 1958. Christian Anfinsen’s research into the folding of proteins (specifically ribonuclease A) gained him the Nobel Prize in 1972.
References
- Kent, S.B. (2009). “Total chemical synthesis of proteins”. Chemical Society Reviews. 38 (2): 338–351. doi:10.1039/b700141j
- Radzicka, A.; Wolfenden, R. (1995). “A proficient enzyme”. Science. 267 (5194): 90–93. doi:10.1126/science.7809611
- Reynolds, J.A.; Tanford, C. (2003). Nature’s Robots: A History of Proteins (Oxford Paperbacks). New York, New York: Oxford University Press. ISBN 978-0-19-860694-9.
- Sleator, R.D. (2012). “Prediction of Protein Functions”. Functional Genomics. Methods in Molecular Biology. 815: 15–24. doi:10.1007/978-1-61779-424-7_2. ISBN 978-1-61779-423-0.
- Van Holde, K.E.; Mathews, C.K. (1996). Biochemistry. Menlo Park, California: Benjamin/Cummings Pub. Co., Inc. ISBN 978-0-8053-3931-4.