Amino acids I
The discovery of proteins
In the 1830s, the Dutch chemist Gerardus Johannes Mulder (1802–1880) recognized that, after
extraction of soluble sugars, organic acids, salts, and fats from organic matter, there was always
an insoluble residue. He showed that this residue contained—in order of decreasing
amount—carbon, oxygen, nitrogen, and hydrogen (with small amounts of sulfur and phosphorus).
In 1838 the Swedish chemist Jöns Jacob Berzelius (1779–1848) suggested to Mulder that these
substances be called proteins, from the Greek word proteios (prwteios), meaning “of primary
importance”. This name was prophetic, as we now know that proteins are absolutely vital for the
function of all living cells. While some cells can function quite well in the absence of DNA (e.g.,
human erythrocytes), no cell can survive without its armament of proteins.
Functions of proteins
Each living cell contains many thousands of proteins. In fact, a typical cell synthesizes ~15,000
proteins of which ~2000 are abundant (50,000 copies each), with the rest present in only low
numbers. Proteins perform a diverse range of functions, including the following:
1. Enzymes: These protein molecules act as biological catalysts. They can enhance reaction rates
by factors of 100,000 (105) to 1 billion (109). Enzymes catalyze the vast majority of reactions that
occur in living organisms.
2. Storage proteins: Various ions and small molecules are stored by being complexed with specific
proteins. For example, iron is stored by ferritin in the liver, and can be transported between
tissues by complexation with transferrin. Hemoglobin in human red blood cells is used to transport
O2 and CO2 around the body.
3. Transport proteins: Some proteins are able to transport ions and small molecules from one side
of a cell membrane to the other side (the membrane could be the plasma membrane or the
membrane surrounding an organelle such as the mitochondrion). A good example is the glucose
transporter which is used to facilitate the entry of glucose into cells.
4. Mechanical work: Specialized assemblies of proteins can do mechanical work, such as the
contraction of muscle and the separation of chromosomes at mitosis.
5. Antibodies: Our immune system consist of a vast repertoire of these proteins, which bind to
and signal the elimination of specific foreign particles such as viruses and bacteria.
6. Structural proteins: These proteins provide mechanical support and shape to cells and hence to
tissues and organisms. The “skeleton” of a cell is comprised of a complicated network of interacting
proteins which are attached to the plasma membrane and physically support it. An excellent
example is the cytoskeleton of the human erythrocyte, which allows the cell to be elastically
deformed in a reversible manner as it passes through capillaries of smaller diameter than the cell
itself. Another good example is collagen, which comprises a quarter (by mass) of mammalian
protein.
7. Regulatory proteins: These include hormones, such as insulin and glucagon, which regulate
biochemical activities only in cells which contain specific receptors for the hormone molecule.
Various nuclear proteins also act as regulators by determining which part of the genetic information
(chromosomal DNA) is read at a particular point in time.
