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Chapter 14.9-14.12
Proteins: Secondary,
Tertiary, Quaternary, and
Denaturation
14.9- What is the secondary
structure of a protein?
Regular and repeating structural patterns
 2 kinds of repeating patterns proposed by Linus
Pauling and Robert Corey in 1940’s:
1. α-helix
2. β-pleated sheet
 Hydrogen bonds hold polypeptide chain in
place
 Hydrogen bond connects carbonyl oxygen
with amide hydrogen atom of another(–C=O--H-N-)

α-helix
α-helix
single protein chain coiled in a spiral with
a right-handed twist
 held together by hydrogen bonds parallel
to the axis of the coil

β-pleated sheet
β-pleated sheet

Backbone of two protein chains is held
together by hydrogen bonds
Secondary Proteins can be classified
as:
1. Fibrous proteins
 2. Globular proteins

Fibrous proteins:

Tough, insoluble proteins in which chains
form long fibers or sheets
◦ Wool, hair, and fingernails made of αkeratins(fibrous protein)
◦ α-keratins are composed of α-helixes
◦ Natural silk and spider webs are made of
fibroin, proteins mainly composed of βpleated sheets
Globular proteins:
Water-proteins whose chains are folded
into compact, globelike shapes
 Presence of hydrophilic side chains on
outer surfaces account for water
solubility –allowing them to travel
through blood and other body fluids to
sites where activity is needed

14.10- What is the tertiary
structure of a protein?
Three-dimensional shape
 Unlike secondary, it depends on
interactions of amino acid side chains

Tertiary Structures are stabilized
five ways:
1.
 2.
 3.
 4.
 5.

Covalent Bonds
Hydrogen Bonding
Salt Bridges
Hydrophobic Interactions
Metal Ion Coordination
14.11 What is the Quaternary
Structure of a protein?

The way in which 2 or more protein
chains form a single three-dimensional
unit
2 important quaternary proteins:
1. Hemoglobin
 2. Collagen

Hemoglobin:
Composed of 4 polypeptide chains
 2α chains (141 amino acids) and 2 β
chains(146 amino acids)
 Held together by interaction hydrophobic
groups and heme groups (iron in center
of heterocyclic ring)
 Oxygen carrier in red blood cells

Collagen:
Most abundant of all proteins in mammals
 Makes up 30% or more of the total
 Major constituent of skin, tendons, bones,
blood vessels, and other connective
tissues

How are proteins denatured?
Denaturation- the loss of the secondary,
tertiary, and quaternary structures of a
protein by chemical or physical agent that
leaves the primary structure intact
 Enzymes lose their catalytic activity and
other proteins can’t carry out their
biological functions when denatured

Denatured by:
Heat
 Denaturing chemicals
 pH change
 Alcohol

Denaturation
Does not affect primary structures
 Most denaturation is irreversible

Dietary Protein:
Protein structure must be destroyed before
it can provide the nutrition for the body
 Digestion involves denaturation and
hydrolysis
 Stomach acids denature proteins
 Proteolytic enzymes in stomach and small
intestine hydrolyze proteins to smaller
fragments until free amino acids are formed
and can be absorbed through intestinal
membranes into the blood stream
