Page 371 - Carrahers_Polymer_Chemistry,_Eighth_Edition

334 Carraher’s Polymer Chemistry

also be an important factor in chain folding. Thus, hydrophilic groupings generally are clustered
to the outside of the globular protein allowing them to take advantage of hydrogen bonding and
other polar bonding opportunities with the water-rich environment while hydrophobic clusters
of amino acid units occupy the internal regions of the protein taking advantage of hydrophobic
interactions.
Globular proteins act in maintenance and regulatory roles—functions that often require mobil-
ity and thus some solubility. Included within the globular grouping are enzymes, most hormones,

hemoglobin, and fibrinogen that is changed into an insoluble fibrous protein fibrin that causes blood

clotting.
Denaturation is the irreversible precipitation of proteins caused by heating, such as the coagu-
lation of egg white as an egg is cooked, or by addition of strong acids, bases, or other chemicals.
This denaturation causes permanent changes in the overall structure of the protein, and, because of
the ease with which proteins are denatured, it makes it difficult to study “natural” protein structure.

Nucleic acids also undergo denaturation.
Small changes in the primary structure of proteins can result in large changes in the second-
ary structure. For instance, researchers have interchanged the positions of two adjacent amino
acid residues of a globular protein portion resulting in a beta strand becoming a right-handed
helix.
Molecular recognition is one of the keys to life. Scientists are discovering ways to both modify
molecular recognition sites and to “copy” such sites. One approach to modifying molecular rec-
ognition sites, namely enzymatic protein sites, is through what is referred to as directed evolution.
Arnold and coworkers have employed the combinatorial approach by taking an enzyme with a
desired catalytic activity and encouraging it to undergo mutation; selecting out those mutations
that perform in the desired manner; and repeating this cycle until the new enzymes perform as
desired. Ratner and coworkers have taken another approach whereby templates containing the
desired catalytic sites are made. First, the protein is mounted on a mica support. The target pro-
tein is coated with a sugar monolayer that allows for specific recognition. A fl uoropolymer plasma

film is deposited over the sugar monolayer. The fluoropolymer reverse image is attached to a

support surface using an epoxy resin. Solvents are then added to etch away the mica, sugar, and
original protein leaving behind a “nano-pit” template that conforms to the shape of the original
protein.

10.2.8 FIBROUS PROTEINS

Fibrous proteins are long macromolecules that are attached through either inter or intrahydrogen
bonding of the individual residues within the chain. Solubility, partial or total, occurs when these
hydrogen bonds are broken. In general, they confer stiffness and rigidity to biological systems that
are themselves fl uid.

Fibrous proteins are found in animals. They appear as filaments and are often formed from a lim-
ited number of amino acid units. They are generally insoluble in water with hydrophobic portions
protruding from the central core. Examples are collagen, elastin, and keratin. Actin and tubulin are
globular soluble monomers that polymerize forming long stiff structures that make up the cytoskel-
eton that allows cells to maintain their shape. Structural proteins are used to construct tendons, bone

matrices, connective tissues, and muscle fiber. They can also be used for storage. They are not as
easily denatured as globular proteins.
Some structural proteins, such as kinesin, dynein, and myosin, serve as so-called motor proteins
that can generate mechanical forces as muscles and are involved in allowing the movement of cells
such as the sperm cell involved in sexual reproduction in multicellular organisms.

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