Figure 23.13
Computer graphic representation of
the quaternary structure of
hemoglobin. The orange and pink
subunits are identical, as are the
green and purple subunits. The
cylindrical tubes represent the
polypeptide chains, while the beads
represent the iron-containing
porphyrin rings (Section 21.11).
Section 23.15 Quaternary Structure of Proteins 993
>Figure 23.12
The three-dimensional structure of
carboxypeptidase A.
PROBLEM 35
How would a protein that resides in the interior of a membrane fold, compared with the
water-soluble protein just discussed?
23.15 Quaternary Structure of Proteins
Proteins that have more than one peptide chain are called oligomers. The individual
chains are called subunits. A protein with a single subunit is called a monomer, one
with two subunits is called a dimer; one with three subunits is called a trimer, and
one with four subunits is called a tetramer. Hemoglobin is an example of a tetramer.
It has two different kinds of subunits and two of each kind. The quaternary structure
of hemoglobin is shown in Figure 23.13.
The subunits are held together by the same kinds of interactions that hold the indi-
vidual protein chains in a particular three-dimensional conformation: hydrophobic in-
teractions, hydrogen bonding, and electrostatic attractions. The quaternary structure of
a protein describes the way the subunits are arranged in space. Some of the possible
arrangements of the six subunits of a hexamer are shown here:
PROBLEM 36
a. Which of the following water-soluble proteins would have the greatest percentage of
polar amino acids—a spherical protein, a cigar-shaped protein, or a subunit of a
hexamer?
b. Which of these would have the smallest percentage of polar amino acids?
possible quaternary structures for a hexamer
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