http://www.ck12.org Chapter 26. Biochemistry
Proteins
Longer chains of amino acids are referred to as polypeptides or proteins, depending on their approximate size. In
general, apolypeptideis a sequence of ten or more amino acids, while aproteinis a polypeptide with a molecular
weight of more than about 10,000 g/mol. This corresponds to a polypeptide that is longer than about 80-100 amino
acids in length, depending on the exact identities of the amino acids used. However, the boundary between these two
categories is not definite, and many polypeptides in the 40-100 amino acid range will often be referred to as proteins
if they perform functions that are similar to those of other larger proteins.
Proteins are very prevalent in living organisms. Hair, skin, nails, muscles, and the hemoglobin in red blood cells
are some of the important parts of your body that are made of different proteins. The wide array of chemical,
physiological, and structural properties exhibited by different proteins is a function of their amino acid sequences.
Because proteins are so large, the number of possible amino acid sequences is virtually limitless. For example, even
a "small" protein that is only 90 amino acids long could have 20^90 = 1.24× 10117 possible sequences. To put this
number in perspective, the entire universe is estimated to contain about 10^80 -10^82 atoms.
The three-dimensional structure of a protein is very critical to its function. This structure can be broken down into
four levels. Theprimary structureis the amino acid sequence of the protein. The amino acid sequence of a given
protein is unique and defines the function of that protein. Thesecondary structureis a highly regular sub-structure
of the protein. The two most common types of protein secondary structure are the alpha helix and the beta sheet (
Figure26.11).
FIGURE 26.11
The two most common secondary struc-
tures of a protein are the alpha helix and
the beta sheet. An alpha helix consists of
amino acids that adopt a spiral shape. A
beta sheet consists of two or more rows of
amino acids that line up in a side-by-side
fashion. In both cases, the secondary
structures are stabilized by extensive hy-
drogen bonding between the polyamide
backbone.The interactions between various nearby amino acids, particularly the hydrogen bonding between the amide groups
that connect amino acids together, leads to the adoption of a particular secondary structure. Thetertiary structure
is the overall three-dimensional structure of the protein. A typical protein consists of several localized secondary
structures (alpha helices or beta sheets) along with other areas in which the structure is less regular. These areas
combine to produce the tertiary structure.
Some functional protein complexes consist of more than one protein molecule. For example, hemoglobin is a
very large protein found in red blood cells. Its function is to bind and carry oxygen throughout the bloodstream.
Hemoglobin consists of four subunits—twoαsubunits and twoβsubunits—that come together in a specific way
through interactions between the side chains (Figure26.12). Each subunit is a single protein strand that is
linked together by covalent bonds, while the overall complex is held together by large numbers of intermolecular
interactions.
Thequaternary structureof a protein refers to the interactions and orientations of the subunits in that protein.
Some proteins consist of only one subunit and thus do not have a quaternary structure. TheFigure26.13 diagrams