The Chemistry of Life (^25)
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Figure 2- 9 The formation of a dipeptide bond to form a protein.
Proteins function in a number of very important ways
in the human body. Many are structural proteins. Proteins
are part of a cell’s membranous structures: plasma
membrane, nuclear membrane, endoplasmic reticulum, and
mitochondria. In addition, actin and myosin are structural
proteins found in a muscle cell. We could not move, talk,
breathe, digest, or circulate blood without the proteins actin
and myosin. Chemi-cal reactions inside a cell allow a cell
to function prop-erly. These chemical reactions would not
occur in cells without the assistance of enzymes.
Enzymes are protein catalysts, which increase the rate of
a chemical reaction without being affected by the reaction.
In addition, our immune system functions because
antibodies, which are proteins of a high molecular weight,
are formed to combat foreign proteins called antigens that
enter the body. Some examples of foreign proteins are
bacterial cell membranes, virus protein coats, and bacterial
fla-gella. Finally, proteins are also a source of energy that
can be broken down and converted to ATP just like car-
bohydrates and fats.
Proteins are also discussed in terms of their struc-ture
(Figure 2-10). The primary structure of a protein is
determined by its amino acid sequence. The secondary
structure is determined by the hydrogen bonds between
amino acids that cause the protein to coil into helices or
pleated sheets. This shape is crucial to the function-ing of
proteins. If those hydrogen bonds are destroyed, the protein
becomes nonfunctional. Hydrogen bonds
can be broken by high temperatures or increased acid-ity,
resulting in changes in pH. The tertiary structure is a
secondary folding caused by interactions within the pep-
tide bonds and between sulfur atoms of different amino
acids. Changes affecting this structure can also affect the
function of the protein. Finally, the quaternary
structure is determined by the spatial relationships
between indi-vidual units.
Nucleic Acids
Two very important nucleic acids are found in cells.
Deoxyribonucleic- (dee-ock-see-rye-boh-noo-KLEE-ik)
acid (DNA) is the genetic material of cells located in the
nucleus of the cell. It determines all of the func-tions and
characteristics of the cell. Ribonucleic (rye-boh-noo-
KLEE-ik) acid (RNA) is structurally related to DNA.
Two important types of RNA are messenger RNA and
transfer- RNA, which are important mol-ecules necessary
for -protein synthesis (discussed in Chapter 3).
The nucleic acids are very large molecules made of
carbon, oxygen, hydrogen, nitrogen, and phos-phorous
atoms. The basic structure of a nucleic acid is a chain of
nucleotides. The DNA molecule is a dou-ble helical
chain, and the RNA molecules are single chains of
-nucleotides. A nucleotide is a complex com-bination of a
sugar (deoxyribose in DNA and ribose in RNA), a nitrogen
base, and a phosphate group