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Molecules of Life
Chapter 3 Part 2
3.5 Proteins – Diversity
in Structure and Function
 Proteins are the most diverse biological
molecule (structural, nutritious, enzyme,
transport, communication, and defense proteins)
Proteins and Amino Acids
 Protein
• An organic compound composed of one or more
chains of amino acids
Amino Acid Structure
Polypeptides
 Protein synthesis involves the formation of
amino acid chains called polypeptides
 Polypeptide
• A chain of amino acids bonded together by
peptide bonds in a condensation reaction
between the amine group of one amino acid and
the carboxyl group of another amino acid
A DNA encodes the
order of amino acids in a
new polypeptide chain.
Methionine (met) is
typically the first amino
acid.
B In a condensation reaction, a peptide bond
forms between the methionine and the next amino
acid, alanine (ala) in this example.
Leucine (leu) will be next.
Stepped Art
Fig. 3-16a, p. 44
C A peptide bond forms between the
alanine and leucine.
Tryptophan (trp) will be next. The chain
is starting to twist and fold as atoms
swivel around some bonds and attract or
repel their neighbors.
D The sequence of amino acid
subunits in this newly forming peptide
chain is now met–ala–leu–trp. The
process may continue until there are
hundreds or thousands of amino acids
in the chain.
Stepped Art
Fig. 3-16b, p. 45
Levels of Protein Structure
 Primary structure
• The sequence of amino acids in a protein
 Secondary structure
• The polypeptide chain folds and forms hydrogen
bonds between amino acids
Levels of Protein Structure
 Tertiary structure
• A secondary structure is compacted into
structurally stable units called domains
• Forms a functional protein
 Quaternary structure
• Some proteins consist of two or more folded
polypeptide chains in close association
a) Protein primary structure: Amino
acids bonded as a polypeptide chain.
b) Protein secondary structure: A
coiled (helical) or sheetlike array held
in place by hydrogen bonds (dotted
lines) between different parts of the
polypeptide chain.
helix (coil)
c) Protein tertiary structure: A chain’s
coils, sheets, or both fold and twist
into stable, functional domains such
as barrels or pockets.
sheet
barrel
d) Protein quaternary structure: two
or more polypeptide chains
associated as one molecule.
Stepped Art
Fig. 3-17, p. 45
Just One Wrong Amino Acid…
 Hemoglobin contains four globin chains, each
with an iron-containing heme group that binds
oxygen and carries it to body cells
 In sickle cell anemia, a DNA mutation changes a
single amino acid (glutamate to valine) in a beta
chain, which changes the shape of the
hemoglobin molecule, causing it to clump and
deform red blood cells
Globin Chains in
Hemoglobin
valine
histidine
leucine
threonine
proline
glutamic
acid
glutamic
acid
A Normal amino acid sequence at the
start of the hemoglobin beta chain.
Fig. 3-19a, p. 47
valine
histidine
leucine
threonine
proline
valine
glutamic
acid
B One amino acid substitution results in the abnormal
beta chain of HbS molecules. The sixth amino acid in
such chains is valine, not glutamic acid.
Fig. 3-19b, p. 47
Animation: Sickle-cell anemia
Proteins Undone – Denaturation
 Heat, changes in pH, and salts can disrupt the
hydrogen bonds that maintain a protein’s shape
 When a protein loses its shape and no longer
functions, it is denatured
 Ex. Albumin (egg white)
3.7 Nucleic Acids
 Some nucleotides are subunits (monomers) of
nucleic acids (polymers) ; DNA and RNA
Nucleotides
 Nucleotide
• A small organic molecule consisting of a sugar
with a five-carbon ring, a nitrogen-containing
base, and one or more phosphate groups
 ATP (Energy Currency of the Cell)
• A nucleotide with three phosphate groups
• Important in phosphate-group (energy) transfer
Nucleic Acids
 Nucleic acids
• Polymers of nucleotides in which the sugar of one
nucleotide is attached to the phosphate group of
the next
• RNA and DNA are nucleic acids
RNA
 RNA (ribonucleic acid)
• Contains four kinds of nucleotide bases, including
ATP
• Important in protein synthesis
DNA
 DNA (deoxyribonucleic acid)
• Two chains of nucleotides twisted together into a
double helix and held by hydrogen bonds
• Contains all inherited information necessary to
build an organism
Four Nucleotides of DNA
covalent
bonding in
sugar–
phosphate
backbone
hydrogen bonding
between bases
Fig. 3-22, p. 49
Video: Effects of trans fats