Proteins
Topic 7.5
7.5 Proteins
• 7.5.1 Explain the four levels of protein structure,
indicating the significance of each level.
• 7.5.2 Outline the difference between fibrous
and globular proteins, with reference to two
examples of each protein type.
• 7.5.3 Explain the significance of polar and nonpolar amino acids.
• 7.5.4 State four functions of proteins, giving a
named example of each.
Four levels of protein structure
1. Primary organization
•
•
•
•
Chain of amino acids held together by polypeptide
bonds
20 amino acids may be arranged in any order and
is determined by DNA
Primary structure determines the next three levels
of protein organization
(Changing one amino acid here may completely
alter the structure and function of a protein, as in
sickle cell disease)
2. Secondary
organization
•
•
Created by the
formation of hydrogen
bonds between the
oxygen from the
carboxyl group of one
amino acid and the
hydrogen from the
amino group of another
Most common
configurations
•
•
α-helix
β-pleated sheet
3. Tertiary organization
•
•
•
Polypeptide chain
bends and folds over
itself because of
interactions among Rgroups and the peptide
backbone
Important in determining
the specificity of the
proteins known as
enzymes
Interactions:
•
•
•
•
Disulfide bonds
Hydrogen bonds
Hydrophobic/hydrophillic
Ionic bonds
4. Quaternary
organization
•
•
Involves multiple
polypeptide chains
which combine to form a
single structure
Some include prosthetic
or non-polypeptide
groups called
conjugated proteins
•
Ex. Hemoglobin contains
four polypeptide chains,
each of which contains a
non-polypeptide group
called a heme (contains
an iron atom that binds to
oxygen)
Animations
• http://www.johnkyrk.com/aminoacid.html
Fibrous and globular proteins
• Fibrous proteins
• Composed of many polypeptide chains in a long, narrow
shape
• Insoluble in water
• Examples:
• Collagen – structure of connective tissue in humans
• Actin – component of human muscle
• Globular proteins
• More 3-D in their shape
• Water soluble
• Examples:
• Hemoglobin – delivers oxygen to body tissues
• Insulin – regulates blood glucose level in humans
Polar and non-polar amino acids
Refers to property of R group
• Polar
• Hydrophilic
• Found in regions exposed
to water
• Membrane proteins
toward interior and
exterior of membrane
• Create hydrophilic
channels in proteins
through which polar
substances can move
• Non-polar
• Hydrophobic
• Found in regions of
proteins that are linked to
the hydrophobic area of
the cell membrane
Significance of polar and non-polar
amino acids
• Controls position of proteins in membranes,
creating hydrophilic channels through
membranes
• Determinacy of specificity of an enzyme
• Each enzyme has a region called the active site
• Only specific substrates can combine with a
particular active site
• Combination is possible when ‘fitting’ occurs which
involves general shape and polar properties of the
substrate and the amino acids exposed at the active
site
Function
Protein
Description
Transport
Hemoglobin
Contains iron that
transports oxygen from
the lungs to all parts of
the body in vertebrates
Movement
Actin and myosin
Interact to bring about
muscle contraction in
animals
Hormone
Insulin
Secreted by pancreas
that aids in maintaining
blood glucose level in
vertebrates
Defense
Immunoglobulins
Group that act as
antibodies to fight
bacteria and viruses