Chapter 2
Chemistry Comes Alive
47
2
Trans fats
, common in many margarines and baked prod-
ucts, are oils that have been solidified by addition of H atoms at
sites of carbon double bonds. Tey have recently been branded
as increasing the risk of heart disease even more than the solid
animal fats. Conversely, the
omega-3 fatty acids
, found natu-
rally in cold-water fish, appear to decrease the risk of heart dis-
ease and some inflammatory diseases.
Phospholipids
Phospholipids
are modified triglycerides. Specifically, they are
diglycerides with a phosphorus-containing group and two, rather
than three, fatty acid chains (Figure 2.16b). Te phosphorus-
containing group gives phospholipids their distinctive chemical
properties. Although the hydrocarbon portion (the “tail”) of the
molecule is nonpolar and interacts only with nonpolar molecules,
the phosphorus-containing part (the “head”) is polar and attracts
other polar or charged particles, such as water or ions. Tis unique
characteristic of phospholipids allows them to be used as the chief
material for building cellular membranes. Some biologically im-
portant phospholipids and their functions are listed in ±able 2.2.
Steroids
Structurally, steroids differ quite a bit from fats and oils.
Steroids
are basically flat molecules made of four interlocking hydrocar-
bon rings. Like triglycerides, steroids are fat soluble and contain
little oxygen. Te single most important molecule in our steroid
chemistry is
cholesterol
(ko-les
9
ter-ol) (Figure 2.16c). We ingest
cholesterol in animal products such as eggs, meat, and cheese,
and our liver produces some.
Cholesterol has earned bad press because of its role in
atherosclerosis, but it is essential for human life. Cholesterol is
found in cell membranes and is the raw material for synthesis
of vitamin D, steroid hormones, and bile salts. Although steroid
hormones are present in the body in only small quantities, they
are vital to homeostasis. Without sex hormones, reproduction
would be impossible, and a total lack of the corticosteroids pro-
duced by the adrenal glands is fatal.
Eicosanoids
Te
eicosanoids
(i-ko
9
sah-noyds) are diverse lipids chiefly de-
rived from a 20-carbon fatty acid (arachidonic acid) found in all
cell membranes. Most important of these are the
prostaglandins
and their relatives, which play roles in various body processes
including blood clotting, regulation of blood pressure, inflam-
mation, and labor contractions (±able 2.2). Teir synthesis and
inflammatory actions are blocked by NSAIDs (nonsteroidal
anti-inflammatory drugs) and the newer COX inhibitors.
Check Your Understanding
20.
What are the monomers of carbohydrates called? Which
monomer is blood sugar?
21.
What is the animal form of stored carbohydrate called?
22.
How do triglycerides differ from phospholipids in body
function and location?
23.
What is the result of hydrolysis reactions and how are these
reactions accomplished in the body?
For answers, see Appendix H.
Proteins
Describe the four levels of protein structure.
Indicate the function of molecular chaperones.
Describe enzyme action.
Te full set of proteins made by the body, called the
proteome
,
and the way those proteins network in the body or change with
disease, is a matter of intense biotech research.
Protein
composes 10–30% of cell mass and is the basic
structural material of the body. However, not all proteins are
construction materials. Many play vital roles in cell function.
Proteins, which include enzymes (biological catalysts), hemo-
globin of the blood, and contractile proteins of muscle, have the
most varied functions of any molecules in the body. All proteins
contain carbon, oxygen, hydrogen, and nitrogen, and many
contain sulfur as well.
Amino Acids and Peptide Bonds
Te building blocks of proteins are molecules called
amino acids
,
of which there are 20 common types (see Appendix C). All amino
acids have two important functional groups: a basic group called
an
amine
(ah
9
mēn)
group
(
¬
NH
2
), and an organic
acid group
(
¬
COOH). An amino acid may therefore act either as a base (pro-
ton acceptor) or an acid (proton donor). All amino acids are identi-
cal except for a single group of atoms called their
R group
. Hence, it
is differences in the R group that make each amino acid chemically
unique, as the examples in
Figure 2.17
show.
Proteins are long chains of amino acids joined together
by dehydration synthesis, with the amine end of one amino
acid linked to the acid end of the next. Te resulting bond
produces a characteristic arrangement of linked atoms called
a
peptide bond
(Figure 2.18)
. ±wo united amino acids form
a
dipeptide
, three a
tripeptide
, and ten or more a
polypeptide
.
Although polypeptides containing more than 50 amino
acids are called proteins, most proteins are
macromolecules
,
large, complex molecules containing from 100 to over 10,000
amino acids.
Because each type of amino acid has distinct properties, the
sequence in which they are bound together produces proteins
that vary widely in both structure and function. We can think of
the 20 amino acids as a 20-letter “alphabet” used in specific com-
binations to form “words” (proteins). Just as a change in one let-
ter can produce a word with an entirely different meaning (flour
S
floor) or that is nonsensical (flour
S
flocr), changes in the
kinds or positions of amino acids can yield proteins with differ-
ent functions or proteins that are nonfunctional. Nevertheless,
there are thousands of different proteins in the body, each with
distinct functional properties, and all constructed from different
combinations of the 20 common amino acids.
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