Download 103 Lecture Ch20b

Tertiary Structure of Proteins
• The tertiary structure defines the specific overall 3-D
shape of the protein
• Tertiary structure is based on various types of interactions
between the side-chains of the peptide chain
Stabilizing Interactions of Tertiary Structures
Globular Proteins
• Globular proteins fold up
into compact, spherical
shapes
• Their functions include
biosynthesis, transport and
metabolism
• For example, myoglobin is
a globular protein that
stores oxygen in the
muscles
- myglobin is a single
peptide chain that is mostly
-helix
- the O2 binding pocket is
formed by a heme group
and specific amino acid
side-chains that are brought
into position by the tertiary
structure
Fibrous Proteins
• Fibrous proteins consist of long fibers and are mainly
structural proteins
• For example, -keratins are fibrous proteins that make
hair, fur, nails and skin
- hair is made of twined fibrils, which are braids of three
-helices (similar to the triple helix structure of collagen)
- the -helices are held together by disulfide bonds
• -keratins are fibrous proteins found in feathers and scales
that are made up mostly of -pleated sheets
Quaternary Structure of Proteins
• Quaternary structure describes the joining of two or
more polypeptide subunits
• The subunits each have their own tertiary structure and are
held together by the same forces involved in tertiary
structure
• For example, hemoglobin is a globular protein that consists
of four subunits, of two different types
- each subunit contains a heme group for O2 binding
Summary of Protein Structural Levels
Hydrolysis of Peptides and Proteins
• Peptide bonds are amide bonds and are resistant to
hydrolysis
• However, they can be hydrolyzed with enzymes or with
strong acid or base and heat
• Proteins are hydrolyzed in the stomach with both acid (HCl)
and enzymes (such as pepsin)
- the amino acids are then absorbed in the intestines and used
to synthesize new proteins
• Below is the acid hydrolysis of the dipeptide Ala-Ser to form
the amino acids alanine and serine
OH
CH3 O
+
H3N CH C N
heat,
+
H2O, H
CH2 O
CH C OH
OH
H
+
H3N
CH3 O
CH COH
CH2 O
+
+ H3N CH C OH
Denaturation of Proteins
• Denaturation causes proteins to lose their 3-D structure
and so they lose their function
• Denaturation involves the disruption of cross-linking in the
secondary, tertiary and quaternary protein structures
• Heat and organic compounds disrupt H-bonding and
hydrophobic interactions
• Acids and bases disrupt H-bonding between polar R
groups and break ionic bonds
• Heavy metal ions break S-S bonds by reacting with the
sulfur
• Agitation such as whipping stretches chains, disrupting all
types of cross-linking
Applications of Denaturation
• Denaturation of protein occurs when:
- an egg is cooked
- the skin is wiped with alcohol
- heat is used to cauterize blood vessels
- instruments are sterilized in autoclaves