Pp 50 – 51 & 206 - 210
Pp 15 & 66 - 68
Proteins
 Proteins are polymers of amino acids
 Each has a unique 3D shape
 Amino acid sequences vary
 Proteins are major component of cell parts
 The provide:
 support and structural components
 storage of amino acids
 Several types of proteins are identified:
receptor, contractile, defense, enzymes,
structural
Building Blocks: Amino Acids
There are 20
different types of
amino acids
All have this
general formular
The R group is a
variable group
Polar vs. Non-polar Amino Acids
 Some amino acids are
polar while some are
non-polar
 This property causes
proteins to fold into
varied shapes
Significance of polar & non-polar amino acids
polar amino acids
non-polar amino acids
 hydrophilic
 hydrophobic
 can make hydrogen bonds
 forms van der Waals
 found in hydrophilic
channels & parts of proteins
projecting from membranes
 found on surface of watersoluble proteins
(hydrophobic interactions)
with other hydrophobic amino
acids
 found in proteins in interior of
membranes
 found in interior of water-
soluble proteins
Peptide Bonds
 Proteins are formed
by condensation
 A peptide bond is
formed
Four levels of protein structure
Primary Structure
 Primary structure is a
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chain of amino acids
number & unique
sequence of amino acids
determine the properties
of primary structure
each position is occupied
by one of 20 different
amino acids
sequence of amino acids
is determined by DNA
sequence in genes
linked by peptide bonds
Secondary structure
 formed by interaction
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between amino and carboxyl
i.e. -NH and -C=O groups
weak hydrogen bonds are
formed between – H & = O
there are two types: a-helix
and b-sheet
α- helix formed / polypeptide
coils up e.g. sheep wool
β- pleated sheet formed e.g.
silk in spider web
 regular repeated folding of
amino acid chain
 secondary structure is
stabilized by hydrogen bonds
Tertiary structure
 These are globular proteins

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with irregular conformation
tertiary structure is the
folding up of the polypeptide
chain, secondary structure or
alpha helix
it gives three dimensional
globular shape i.e. shape of
active site
the structure is stabilized by
disulphide bridges, hydrogen,
ionic& hydrophobic bonds
tertiary structure used as
enzymes to catalyze
biochemical reactions
Quaternary structure
 made of several
polypeptide subunits
joined together
 they maybe conjugated
proteins i.e. proteins
which combine with a
prosthetic group (nonprotein molecules)
 prosthetic groups
includes: metals e.g. iron
in haemoglobin, nucleic
acids as in ribosomes ,
carbohydrates as in
glycoprotein or lipids as
in glycolipids
Denaturation of Proteins
 change in protein’s
usual regular
structure due to:
 High temp
 Change in pH
 Addition of organic
solvent (alcohol,
acetone)
 These factors break
the bonds that
stabilize the
structure
Protein Functions
Function
Examples
 Structure
 – collagen/keratin/fibrin
 Transport
 – myoglobin/hemoglobin, bind &
 Enzymes
 Movement
 Hormones
 Antibodies
 storage




transport oxygen
– lysozyme, speeding up metabolic
reactions
– actin (and myosin tropomyosin (and
troponin)
– insulin, regulate blood glucose
– immunoglobulin
 – albumin in egg, casein in milk
Difference between Fibrous and
Globular Proteins
 fibrous proteins are long and narrow strands or sheets whereas


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globular proteins are rounded, spherical or ball shaped
fibrous protein made of repetitive amino-acid sequences whereas
globular proteins are made of irregular amino acid sequences
fibrous proteins are usually insoluble in water whereas globular
proteins usually soluble in water
globular protein more sensitive to changes in pH, temperature & salt
than fibrous proteins
fibrous proteins have structural or support functions (roles) whereas
globular proteins have metabolic functions such as catalysis &
transport function
Examples of fibrous proteins:
keratin/fibrin/collagen/actin/myosin/silk protein
Examples of globular protein:
insulin/immunoglobulin/hemoglobin/amylase
Revision Questions
 Explain how polar and non-polar amino acids help channel
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
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proteins and enzymes carry out their functions.
[5]
Distinguish between fibrous and globular proteins with
reference to one example of each protein type.
[6]
Explain primary structures and tertiary structures of a
protein.
[3]
Draw the structure of a generalized dipeptide.
[4]
Draw the basic structure of an amino acid, and label the
groups that are used in peptide bond formation.
[4]
List four functions of membrane proteins.
[4]
Giving a named example of each, state four functions of
proteins.
[8]