Chemical Composition of the Body
35
hydrogen and ionic bonds.
Table 2–6
provides a summary of
the types of bonding forces that contribute to the conforma-
tion of polypeptide chains. These same bonds are also involved
in other intermolecular interactions, which will be described
in later chapters.
Thus, a protein can fold over on itself in a variety of ways,
resulting in a three-dimensional shape
(tertiary structure)
characteristic of that protein (see Figure 2–16). This three-
dimensional shape allows one protein to interact with other
molecules, including other proteins.
Quaternary Protein Structure
When proteins are composed of more than one polypeptide
chain, they are said to have
quaternary structure
and are
known as
multimeric proteins
(“many parts”). The same
factors that infl uence the conformation of a single polypeptide
also determine the interactions between the polypeptides in
a multimeric protein. Thus, the chains can be held together
by interactions between various ionized, polar, and nonpolar
side chains, as well as by disulfi de covalent bonds between the
chains.
The polypeptide chains in a multimeric protein may be
identical or different. For example, hemoglobin, the protein
that transports oxygen in the blood, is a multimeric protein
with four polypeptide chains, two of one kind and two of
another (
Figure 2–21
).
The primary structures (amino acid sequences) of a large
number of proteins are known, but three-dimensional con-
formations have been determined for only a small number.
Because of the multiple factors that can infl uence the folding
of a polypeptide chain, it is not yet possible to predict accu-
rately the conformation of a protein from its primary amino
acid sequence. However, it should be clear that a change in
the primary structure of a protein may alter its secondary, ter-
tiary, and quaternary structures. Such an alteration in primary
structure is called a
mutation.
Even a single amino acid change resulting from a muta-
tion may have devastating consequences, as occurs when a
molecule of valine replaces a molecule of glutamic acid in the
β
chains of hemoglobin. The result of this change is a seri-
ous disease called
sickle cell anemia
.
When red blood cells
in a person with this disease are exposed to low oxygen lev-
els, their hemoglobin precipitates. This contorts the red blood
cells into a crescent shape, which makes the cells fragile and
unable to function normally.
Nucleic Acids
Nucleic acids
account for only 2 percent of the body’s weight,
yet these molecules are extremely important because they are
responsible for the storage, expression, and transmission of
genetic information. It is the expression of genetic informa-
tion in the form of specifi c proteins that determines whether
one is a human or a mouse, or whether a cell is a muscle cell
or a nerve cell.
There are two classes of nucleic acids,
deoxyribo-
nucleic acid (DNA)
and
ribonucleic acid (RNA).
DNA
molecules store genetic information coded in the sequence of
Beta sheet
Loop conformation
Alpha helix
Figure 2–19
A ribbon diagram illustrating the pathway followed by the backbone
of a single polypeptide chain. Helical regions (blue) are coiled, beta
sheets (red) of parallel chains are shown as relatively straight arrows,
and loop conformations (yellow) connect the various helical and
beta sheet regions. Beginning at the end of the chain labeled “Beta
sheet,” a continuous chain of amino acids passes through various
conformations.
H
O
S
N
C
H
2
Cysteine
Polypeptide chain
Disulfide bond
+2
H
X
C
H
S
H
O
S
NC
CH
2
C
H
H
Cysteine
H
O
S
NC
CH
2
C
H
H
O
C
C
N
CH
2
H
H
+X
C
Figure 2–20
Formation of a disulfi de bond between the side chains of two cysteine amino acids links two regions of the polypeptide together. The hydrogen
atoms on the sulfhydryl groups of the cysteines are transferred to another molecule, X, during the formation of the disulfi de bond.
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