Chapter 2
Chemistry Comes Alive
51
2
chaperones. It is now clear that these proteins are vitally important
to cell function in all types of stressful circumstances. Although a
lot of environmental stress can turn your hair gray or cause cardiac
problems, it seems that a little stress can delay aging. As part of
the heat shock response, special protein repair molecules patch up
damaged proteins and refold them correctly, helping to hold off
the grim reaper and prolong life.
Enzymes and Enzyme Activity
Enzymes
are globular proteins that act as biological catalysts.
Catalysts
are substances that regulate and accelerate the rate of
biochemical reactions but are not used up or changed in those
Representative Types of Proteins in the Body
CLASSIFICATION ACCORDING TO
 
OVERALL
STRUCTURE
GENERAL FUNCTION
EXAMPLES FROM THE BODY
Fibrous
 
 
Structural framework/
mechanical support
Collagen
, found in all connective tissues, is the single most abundant protein in the body. It is
responsible for the tensile strength of bones, tendons, and ligaments.
 
 
Keratin
is the structural protein of hair and nails and a water-resistant material of skin.
 
 
Elastin
is found, along with collagen, where durability and flexibility are needed, such as in the
ligaments that bind bones together.
 
 
Spectrin
internally reinforces and stabilizes the surface membrane of some cells, particularly
red blood cells.
Dystrophin
reinforces and stabilizes the surface membrane of muscle cells.
Titin
helps organize the intracellular structure of muscle cells and accounts for the elasticity of
skeletal muscles.
 
Movement
Actin
and
myosin
, contractile proteins, are found in substantial amounts in muscle cells, where
they cause muscle cell shortening (contraction); they also function in cell division in all cell
types. Actin is important in intracellular transport, particularly in nerve cells.
Globular
 
 
Catalysis
Protein enzymes are essential to virtually every biochemical reaction in the body; they
increase the rates of chemical reactions by at least a millionfold. Examples include
salivary
amylase
(in saliva), which catalyzes the breakdown of starch, and
oxidase enzymes
, which act
to oxidize food fuels.
 
Transport
Hemoglobin
transports oxygen in blood, and
lipoproteins
transport lipids and cholesterol.
Other transport proteins in the blood carry iron, hormones, or other substances. Some
globular proteins in plasma membranes are involved in membrane transport (as carriers or
channels).
 
Regulation of pH
Many plasma proteins, such as
albumin
,
function reversibly as acids or bases, thus acting as
buffers to prevent wide swings in blood pH.
 
Regulation of
metabolism
Peptide
and
protein hormones
help to regulate metabolic activity, growth, and
development. For example,
growth hormone
is an anabolic hormone necessary for optimal
growth;
insulin
helps regulate blood sugar levels.
 
Body defense
Antibodies
(immunoglobulins) are specialized proteins released by immune cells that
recognize and inactivate foreign substances (bacteria, toxins, some viruses).
 
 
Complement proteins
,
which circulate in blood, enhance both immune and inflammatory
responses.
 
Protein management
Molecular chaperones
aid folding of new proteins in both healthy and damaged cells and
transport of metal ions into and within the cell. They also promote breakdown of damaged
proteins.
Help to translocate proteins and certain metal ions (copper,
iron, zinc) across cell membranes
Promote the breakdown of damaged or denatured proteins
Interact with immune cells to trigger the immune response
to diseased cells in the body
Te first such proteins discovered were called
heat shock proteins
(
hsp
) because they seemed to protect cells from the destructive ef-
fects of heat. It was later found that these proteins are produced
in response to a variety of traumatizing stimuli—for example, in
the oxygen-deprived cells of a heart attack patient—and the name
stress proteins
replaced hsp for that particular group of molecular
Table 2.3
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