Proteins : Structure & Function
COURSE TITLE: BIOCHEMISTRY 1
COURSE CODE: BCHT 201
PLACEMENT/YEAR/LEVEL: 2nd Year/Level 4, 1st Semester
M.F.Ullah, Ph.D
Showket H.Bhat, PhD
WHAT IS A PROTEIN?
Proteins are polymer (chain) made up of
smaller units called amino acid which are
linked together in a specific sequence by
peptide bonds
Formation of a Dipeptide
Dehydration synthesis
Amino Acid + Amino Acid --> Dipeptide
Amino Acid + Dipeptide --> Tripeptide
A.A. + A.A. + …..+ Tripeptide --> Polypeptide
AMINO ACID: Sequence
•
•
•
•
•
Dipeptide – 2 amino acids
Tripeptide – 3 amino acids
Oligopeptides – 4-10 amino acids
Polypeptide – more than 10 amino acids
Proteins in the body and diet are long
polypeptides (100s of amino acids)
Peptide bond formation in details:
- Each polypeptide chain starts on the left side by free amino group of the first amino acid. It is
termed as amino terminus or N- terminus.
- Each polypeptide chain ends on the right side by free COOH group of the last amino acid. It is
termed as carboxy terminus or C-terminus.
Peptides
• Amino acids linked by amide (peptide) bonds
Gly
H2Nend
Lys
Phe
Peptide bonds
Arg
Ser
-COOH
end
Glycine-lysine-phenylalanine-arginine-serine
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Protein structure:
There are four levels of protein structure (primary,
secondary, tertiary and quaternary)
1. Primary structure:
• The primary structure of a protein is its unique sequence of amino
acids.
2- Secondary structure:
Results from hydrogen bond
formation between hydrogen of –NH
group of peptide bond and the carbonyl
oxygen of another peptide bond.
According to H-bonding there are two
main forms of secondary structure:
α-helix: It is a spiral structure resulting
from hydrogen bonding between one
peptide bond and the fourth one
β-sheets: is another form of secondary
structure in which two or more
polypeptides (or segments of the same
peptide chain) are linked together by
hydrogen bond between H- of NH- of one
chain and carbonyl oxygen of adjacent
chain (or segment).
•
3. Tertiary structure is determined
by a variety of interactions (bond
formation) among R groups and
between R groups and the
polypeptide backbone.
a. The weak interactions include:
 Hydrogen bonds among polar
side chains
 Ionic bonds between
charged R groups ( basic and
acidic amino acids)
 Disulphide bonds between
cysteine residues of the
polypeptide chain
 Hydrophobic
interactions among
hydrophobic ( non polar) R
groups.
•
4. Quaternary structure: results from the aggregation (combination) of two or
more polypeptide subunits held together by non-covalent interaction like Hbonds, ionic or hydrophobic interactions.
• Examples on protein having quaternary structure:
– Collagen is a fibrous protein of three polypeptides (trimeric) that are
supercoiled like a rope.
•
This provides the structural strength for their role in connective tissue.
– Hemoglobin is a globular protein with four polypeptide chains (tetrameric)
– Insulin : two polypeptide chains (dimeric)
Structural organization of protein
Types of Proteins
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•
•
•
•
•
•
Type
Structural
Contractile
Transport
Storage
Hormonal
Enzyme
Protection
Examples
tendons, cartilage, hair, nails
muscles
hemoglobin
milk
insulin, growth hormone
catalyzes reactions in cells
immune response
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PROTEINS: Function
1. Structural Functions:
• Collagen – is the most abundant protein
in mammals, and gives bone and skin
their strength
• Keratin – provides structure to hair and
nails
2. Functions as ENZYMES
• Enzymes are proteins that catalyze chemical
reactions without being used up or destroyed in
the process
• Used in – digestion, releasing of energy from
nutrients for fuel, triggering reactions that build
muscle and tissue
3. Functions as HORMONES
• Hormones are chemical messengers that
are made on one part of the body, but act
on cells in other parts of the body
• Example: Insulin & Glucagon (hormones
that maintain blood glucose level).
4. IMMUNE FUNCTION
• The Immune Response is a series of steps
your body takes to mount an attack against
invaders (such as bacteria, viruses &
parasites)
• Antibodies are blood proteins that attack
and inactivate bacteria and viruses
• Once an antibody has been made for a
certain invader, your body can more quickly
respond (Immunization)
How Much Protein Do We
Need?
Adults:
0.8 grams of protein per kilogram of body weight
per day
Endurance Athletes:
1.2 to 1.4 g/kg/day
Heavy Weight Trainers:
1.7 to 1.8 g/kg/day
Protein Sources
Almonds (1 cup)
24 grams
Pinto Beans (1 cup)
15 grams
Cheese (1 oz.)
7 grams
Ham (3 oz.)
18 grams
1 Egg
6 grams
2% Milk (1 cup)
8 grams
Clams (3 oz.)
60 grams
Whole Wheat Bread
3 grams
Lean Hamburger
30 grams
Peanut Butter (1 T)
4 grams
Salmon (3 oz.)
20 grams
Tofu (4 oz.)
9 grams
Yogurt (8 oz.)
10 grams
White rice (1 cup)
4 grams
DENATURING of PROTEINS
• Acid, alkaline, heat, alcohol, and agitation can
disrupt the chemical forces that stabilize
proteins and can cause them to lose their
biological structure & function (activity). This
process is called as DENATURATION.
• Denaturing of proteins happens during food
preparation (cooking, whipping, adding acids)
or digestion (in the stomach with hydrochloric
acid)
Denaturation
Structure and function of hemoglobin
• The main function of red blood cell
• Transport of O2 from lungs to tissue
• Transport of CO2 from tissue to lungs
• Hemoglobin (Hb) is the iron-containing oxygentransport metalloprotein in the red blood cells.
Hemoglobin in the blood carries oxygen from the
respiratory organs (lungs or gills) to the rest of the
body (i.e. the tissues) where it releases the oxygen to
burn nutrients to provide energy to power the
functions of the organism, and it also collects the
resultant carbon dioxide to bring it back to the
respiratory organs to be released out of the body.
• Each red cell has 640 million molecules of Hb
Structure of Haemoglobin
Hemoglobin has a quaternary structure made up of four subunits. Each subunit is composed of
a protein chain tightly associated with a non-protein heme group.
A heme group consists of an iron (Fe) ion (charged atom) held in a heterocyclic ring, known as
a porphyrin.
This porphyrin ring consists of four pyrrole molecules cyclically linked together
(by methene bridges) with the iron ion bound in the center. The iron ion, which is the site of
oxygen binding, coordinates with the four nitrogens in the center of the ring, which all lie in one
plane.
The iron is bound strongly (covalently) to the globular protein and also allows reversible binding
of oxygen with it.
In adult humans, the most common hemoglobin type is a tetramer (which contains 4 subunit
proteins) called hemoglobin A, consisting of two α (141 amino acids) and two β (146 amino
acids) subunits joined together in a quaternary structure by non-covalent interactions such
as salt bridges, hydrogen bonds, and the hydrophobic effect. This is denoted as α2β2.
In human infants, the hemoglobin molecule is made up of 2 α chains and 2 γ chains (α2 γ 2)
called hemoglobin F. The gamma chains are gradually replaced by β chains as the infant
grows.
Structure of Hemoglobin
Hemoglobin consists of four subunits, each of which has a binding site for oxygen.
Therefore, four oxygen molecules can bind to one hemoglobin.