Transcript 5 Protein Structure Tertiary Quaternary GOB Structures

19.5 Protein Structure: Tertiary and
Quaternary Levels
The ribbon model
represents the tertiary
structure of the
polypeptide chain that
forms myoglobin, a
globular protein.
Learning Goal Describe the tertiary and quaternary structures
of a protein.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Tertiary Structure
The tertiary structure of a
protein is an overall threedimensional shape formed
by the interactions and
repulsions of amino acid
residues in different parts
of the chain.
The fibrous proteins of a-keratin
wrap together to form fibrils of
hair and wool.
Core Chemistry Skill Identifying the Primary, Secondary,
Tertiary, and Quaternary Structures of Proteins
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Interactions, Tertiary Structures
The stabilizing interactions of
tertiary structures are detailed
as follows:
1. Hydrophilic interactions
occur between the external
aqueous environment and
the polar amino acid
residues pulling them to the
outer surface of a protein.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Interactions, Tertiary Structures
2. Hydrophobic interactions occur between
nonpolar amino acid residues forming a
nonpolar center at the interior of the protein.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Interactions, Tertiary Structures
3. Salt bridges are ionic attractions between the
charges of the acidic and basic residues of
amino acid residues.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Interactions, Tertiary Structures
4. Hydrogen bonds form between the H of a polar
residue and the O or N of a second polar amino
acid residue.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Interactions, Tertiary Structures
5. Disulfide bonds (— S — S —) are covalent
bonds that form when the — SH groups of two
cysteine residues are oxidized (H removed).
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Tertiary Structure
Interactions between amino acid residues fold a
polypeptide into a specific three-dimensional shape
called its tertiary structure.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Indicate the type of protein structure.
primary
α-helix
β-pleated sheet
triple helix
A.
B.
C.
D.
polypeptide chains held side by side by H bonds
sequence of amino acids in a polypeptide chain
corkscrew shape with H bonds between amino acids
three peptide chains woven like a rope
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
A. polypeptide chains held side by side by H bonds
β-pleated sheet
B. sequence of amino acids in a polypeptide chain
primary
C. corkscrew shape with H bonds between amino acids
α-helix
D. three peptide chains woven like a rope
triple helix
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Globular Proteins, Myoglobin
Globular proteins
• have compact, spherical shapes formed when sections of
the polypeptide chain fold over on top of each other as a
result of the interactions between amino acid residues.
• carry out the work of the cells, such as synthesis, transport,
and metabolism.
Myoglobin, a globular protein that stores oxygen in skeletal
muscle, contains 153 amino acids in a polypeptide chain, with
about three-fourths of the chain in the α-helix secondary
structure.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Fibrous Proteins, α- and β-Keratins
Fibrous proteins consist of long, thin, fiber-like shapes,
involved in the structure of cells and tissues.
Two types of fibrous protein include
• α-keratins, which make up hair, wool, skin, and nails
and contain three α-helices linked by disulfide — S —
S — linkages that coil together the peptide chain like
a braid.
• β-keratins that are found in the feathers of birds and
scales of reptiles and contain large amounts of a
β-pleated sheet structure.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Quaternary Structure: Hemoglobin
Biologically active proteins
with two or more polypeptide
chains or subunits have a
quaternary structure.
Hemoglobin, a globular
protein that transports
oxygen in blood, consists of
four polypeptide chains: two
α-chains with 141 amino
acids and two β-chains with
146 amino acids.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Quaternary Structure: Hemoglobin
In the quaternary structure of hemoglobin,
• the subunits are held together by the same
stabilizing interactions in tertiary structures.
• each subunit of hemoglobin is a globular protein
with an embedded heme group containing an iron
center that can bind an oxygen molecule.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Hemoglobin and Myoglobin
Hemoglobin and myoglobin
• have similar biological functions.
• carry oxygen in different parts of the body: hemoglobin
carries oxygen in blood, and myoglobin carries oxygen in
muscle.
• have a different molar mass: myoglobin has a molar mass
of 17 000, and hemoglobin has a molar mass of 67 000.
• have similar tertiary structures.
• carry different amounts of oxygen: myoglobin carries one
oxygen molecule, and hemoglobin carries four oxygen
molecules.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Quaternary Structures
Proteins consist of primary, secondary, tertiary, and often
quaternary structural levels.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Sickle Cell
Anemia
Sickle cell anemia is caused by an abnormality in the shape of one
of the subunits of the hemoglobin protein.
• The sixth amino acid in the β-chain, polar acidic glutamic acid, is
replaced by valine, a nonpolar amino acid.
• The nonpolar R group on valine is attracted to the nonpolar
regions within the beta hemoglobin chains.
• The red blood cells change from a rounded shape to a crescent
shape, like that of a sickle, which interferes with their ability to
transport enough oxygen.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Chemistry Link to Health: Sickle Cell
Anemia
Hydrophobic interactions also
cause sickle cell hemoglobin
molecules to stick together.
They form insoluble fibers of
sickle cell hemoglobin that
• clog capillaries.
• cause inflammation, pain,
and organ damage.
• cause low oxygen levels in
the affected tissues.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.