Transcript Document

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)
 Cells build thousands of different proteins by
stringing together amino acids in different orders
Proteins and Amino Acids
 Protein
• An organic compound composed of one or more
chains of amino acids
 Amino acid
• A small organic compound with an amine group
(—NH3+), a carboxyl group (—COO-, the acid),
and one or more variable groups (R group)
Amino Acid Structure
amine
group
carboxyl
group
valine
Fig. 3-15, p. 44
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
Peptide Bond Formation
Fig. 3-16a, p. 44
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. Think about polarity, charge, and
other properties of functional groups that become
neighbors in the growing chain.
Fig. 3-16a, p. 44
Fig. 3-16b, p. 45
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.
Fig. 3-16b, p. 45
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. Think about polarity,
charge, and other properties of functional groups
that become neighbors in the growing chain.
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
Animation: Peptide bond formation
Levels of Protein Structure
 Primary structure
• The unique amino acid sequence of 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
• Example: hemoglobin
Levels of Protein
Structure
3.6 Why Is Protein Structure
So Important?
 When a protein’s structure goes awry, so does
its function
Fig. 3-17a, p. 45
a Protein primary
structure: Amino
acids bonded as a
polypeptide chain.
Fig. 3-17a, p. 45
Fig. 3-17b, p. 45
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)
sheet
Fig. 3-17b, p. 45
Fig. 3-17c, p. 45
c Protein tertiary
structure: A chain’s
coils, sheets, or both
fold and twist into
stable, functional
domains such as
barrels or pockets.
barrel
Fig. 3-17c, p. 45
Fig. 3-17d, p. 45
d Protein quaternary
structure: two or more
polypeptide chains
associated as one molecule.
Fig. 3-17d, p. 45
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 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
Fig. 3-18a, p. 46
alpha globin
heme
A Globin. The secondary
structure of this protein
includes several helices.
The coils fold up to form a
pocket that cradles heme,
a functional group with an
iron atom at its center.
Fig. 3-18a, p. 46
Fig. 3-18b, p. 46
alpha globin
beta globin
alpha globin
beta globin
B Hemoglobin is one of the proteins with quaternary structure. It consists
of four globin molecules held together by hydrogen bonds. To help you
distinguish among them, the two alpha globin chains are shown here in
green, and the two beta globin chains are in brown.
Fig. 3-18b, p. 46
Animation: Globin and hemoglobin
structure
Molecular Basis of Sickle Cell Anemia
Fig. 3-19a, p. 47
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
Fig. 3-19b, 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
Fig. 3-19c, p. 47
C Glutamic acid carries a negative
charge; valine carries no charge. This
difference changes the protein so it
behaves differently. At low oxygen
levels, HbS molecules stick together
and form rod-shaped clumps that
distort normally rounded red blood
cells into sickle shapes. (A sickle
is a farm tool that has a crescentshaped blade.)
sickled cell
normal cell
Fig. 3-19c, p. 47
Fig. 3-19d, p. 47
Clumping of cells in bloodstream
Circulatory problems, damage to
brain, lungs, heart, skeletal muscles,
gut, and kidneys
Heart failure, paralysis, pneumonia,
rheumatism, gut pain, kidney failure
Spleen concentrates sickle cells
Spleen enlargement
Immune system compromised
Rapid destruction of sickle cells
Anemia, causing weakness, fatigue,
impaired development, heart
chamber dilation
Impaired brain function, heart failure
D Melba Moore is a celebrity spokesperson for sickle-cell
anemia organizations. Right, range of symptoms for a
person with two mutated genes for hemoglobin’s beta
chain.
Fig. 3-19d, p. 47
Animation: Sickle-cell anemia
Proteins Undone – Denaturation
 Proteins function only as long as they maintain
their correct three-dimensional shape
 Heat, changes in pH, salts, and detergents can
disrupt the hydrogen bonds that maintain a
protein’s shape
 When a protein loses its shape and no longer
functions, it is denatured
3.5-3.6 Key Concepts:
Proteins
 Structurally and functionally, proteins are the
most diverse molecules of life
 They include enzymes, structural materials, and
transporters
 A protein’s function arises directly from its
structure
3.7 Nucleic Acids
 Some nucleotides are subunits of nucleic acids
such as DNA and RNA
 Some nucleotides have roles in metabolism
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
• A nucleotide with three phosphate groups
• Important in phosphate-group (energy) transfer
ATP
base (adenine)
sugar (ribose)
3 phosphate groups
Fig. 3-20, p. 48
base (adenine)
sugar (ribose)
3 phosphate groups
Stepped Art
Fig. 3-20, p. 48
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 monomers,
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, coded in the order of
nucleotide bases
Four Nucleotides of DNA
3 phosphate
groups
sugar
(deoxyribose)
adenine (A)
base with a
double ring
structure
thymine (T)
base with a
single ring
structure
guanine (G)
base with a
double ring
structure
cytosine (C)
base with a
single ring
structure
Fig. 3-21, p. 48
Animation: Nucleotide subunits of DNA
The DNA Molecule
covalent
bonding in
sugar–
phosphate
backbone
hydrogen bonding
between bases
Fig. 3-22, p. 49
3.7 Key Concepts:
Nucleotides and Nucleic Acids
 Nucleotides have major metabolic roles and are
building blocks of nucleic acids
 Two kinds of nucleic acids, DNA and RNA,
interact as the cell’s system of storing, retrieving,
and translating information about building
proteins
Summary:
Organic Molecules in Living Things
Video: Effects of trans fats