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Naturally Occurring Polymers—Animals 335

10.2.9 MEMBRANE PROTEINS

Membrane proteins are attached to or associated with the membrane of a cell. More than half of the
proteins interact with these membranes. Membrane proteins are generally divided according to their
attachment to a membrane. Transmembrane proteins span the entire membrane. Integral proteins
are permanently attached to only one side of membranes. Peripheral membranes proteins are tem-
porarily attached to integral proteins or lipid bilayers through combinations of noncovalent bonding
such as hydrophobic and electrostatic bonding. These membranes often act as receptors or provide
channels for charged or polar molecules to pass through them.

10.2.10 QUATERNARY STRUCTURE

The term quaternary structure is employed to describe the overall shape of groups of chains of
proteins, or other molecular arrangements. For instance, hemoglobin is composed of four distinct
but different myoglobin units, each with its own tertiary structure that comes together giving the
hemoglobin structure. Silk, spider webs, and wool, already described briefly, possess their special

properties because of the quaternary structure of their particular structural proteins.
Both synthetic and natural polymers have superstructures that influence/dictate the properties

of the material. Many of these primary, secondary, tertiary, and quaternary structures are infl u-
enced in a similar manner. Thus, primary structure is a driving force for secondary structure.

Allowed and preferred primary and secondary bonding influence structure. For most natural
and synthetic polymers, hydrophobic and hydrophilic domains tend to cluster. Thus, most heli-
cal structures will have either a somewhat hydrophobic/hydrophilic inner core and the opposite
outer core resulting from a balance between secondary and primary bonding factors and steric
and bond angle constraints. Nature has used these differences in domain character to create the
world about us.
As noted before, some proteins are linear with inner and intrachain associations largely
occurring because of hydrogen bonding. Influences on globular protein structures are more

complex, but again, the same forces and features are at work. Globular proteins have irregular
three-dimensional structures that are compact but which when brought together form quaternary
structures that approach being spherical. While the overall structure is spherical, the surface
is irregular, with the irregularity allowing the proteins to perform different specifi c biological
functions.
The preferred folding confirmation is again influenced by the same factors of bonding type,

polarity, size, flexibility, and preferred bond angles. The folded conformations are possible

because of the flexibility of the primary bonding present within proteins. Thus, polar portions,

namely the amine and carbonyl moieties, are more fixed, but the carbon between them is more

flexible. Again, the folding characteristic conformations are driven by secondary bonding. Some
folding is chemically “fi xed” through use of cross-links. In hair, these cross-links are often dis-
ulfi des, –S–S–.
As previously noted, the fl exibility of proteins allows them to carry out a wide variety of tasks.
Our cells often build about 60,000 different kinds of proteins. A bacterial cell will synthesize only
a little more than 1,000 different kinds of proteins.
When a protein contains roughly more than about 200 amino acid groups, it often assumes two
or more somewhat spherical tertiary structural units. These units are often referred to as domains.
Thus, hemoglobin is a combination of four myoglobin units with each of the four units infl uenced
by the other three, and where each unit contains a site to interact with oxygen.
Enzymes act to lower the activation energy through a combination of holding the reactants in
the correct geometry and making the number of “hits” or connections needed for reaction to be
greatly decreased. The increases in reaction rate are generally huge. In the case of the enzyme

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