Download proteins USP

Overview
•Building blocks of proteins
(i) amino acid - composition/functional groups
(ii) different amino acids
• polymer formation of amino acids
proteins
•Function of proteins
•Classification/ structure
(i) primary/ secondary/ tertiary
•Proteins function as an enzyme
Hemoglobin and myglobin
What are Proteins?
relatively large organic compounds made of amino acids
•Perform variety of functions
•The blueprint of cells (genes) defines the a particular protein and
its function.
Amino Acids
• Building block of proteins
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Contains: Amino and carboxyl group –
functional groups
> contributes to the behavior of the
molecule
R group attached to alpha carbon
> determines chemical properties
(hydrophilic, hydrophobic)
300 aa occur in nature, only 20 occur in
proteins
most biochemistry occurs in the
physiological pH range near
neutrality. At this pH most
amino acids exist as Zwitterions
•Amino acids are classified according to
their capacity to interact with water.
> 4 classes
1. Neutral Non-polar
> contain hydrocarbon R group
> neutral - R group does not bear +ve
or –ve charge
> non polar – poorly react with water;
hence have a role in maintaining the
3-D structure
2. Neutral Polar
> hydrophilic- hydroxyl group enables
hydrogen bonding that easily
interacts with water.
3. Acidic Amino Acid (eg. aspartate, glutamate)
> Have side chains with carboxylate groups
which are –vely charged at physiological pH
4. Basic Amino Acids (eg. lysine, arginine,
histidine
> Bears a +ve charge at physiological pH.
Peptide Bond Formation
•Amino acid molecules link > peptides form
- dipeptide, tripeptide ….
•Facilitated by peptide bonds
- occurs b/w amino group of one amino acid
and carbonly carbon of another
•Reaction is called dehydration (a water molecule
removed is removed
•Amino acid polymers longer than 50 aa
> PROTEINS
A peptide bond is a chemical
bond formed between two
molecules :
when the carboxyl group of
one amino acid reacts with
the amino group of the other
amino acid
>>>>>>>releasing a molecule of
water (H2O).
Function of proteins
•Catalysis: acts as enzymes and regulate chemical
reaction in biological systems. Nearly all known
enzymes are proteins
•Transport & Storage:
carriers of molecules or ions across membranes or
between cells. E.g- hemoglobin carries O2 to
tissues from lungs
•Immune Protection- antibodies
•
Coordinated motion
muscle-major component is proteins (myosin and
actin.
•
Structural
forms structural network of various part of the
body. Eg. Collagen (fibrous) bones & keratin in
skin, hair, nail.
•
Regulatory- hormones regulate physiological
process, growth, development & response to
nervous system
Protein Classification
•
2 major groups based on their shape
(i) Fibrous
long rod shaped molecules; insoluble in water
- serve structural roles e.g Keratin- hair, nails, skin
(ii) Globular
compact spherical molecules; water soluble
- function as enzymes
Protein can also be classified as:
Simple- contain only amino acids e.g. keratin and serum albumin
conjugated- simple + non protein component
- Non protein component is called the prosthetic group
Protein Structure
1.
Primary
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The specific sequence of
aa which also determines
the ultimate conformation
•
Sequence not random and
specified by inherited
genetic information
•
Slight change can affect
conformation; hence its
function
2. Secondary
•
Refers to the spatial arrangements of aa that are near one
another in a liner sequence
•
Are repeated twisting and folding of these aa.
•
Occurs because of H-bonding (b/w CO &NH groups) at
regular intervals (along the polypeptide backbone)
•
Weak H-bond repeated several times support/stabilize a
particular shape.
E.g. alpha helix and beta sheet
(i) Alpha helix
•Common 2º encountered in
globular class
•Formation is spontaneous and
stabilized by H-bond b/w amide N
and carbonyl C of peptide bonds
spaced 4 residues apart
•Rod like structure –tightly coiled
polypeptide main chain forms the
inner part of the core and side
chains extended outward like a
helix
•3.6 residues per turn
(ii) ß-sheet
•Is a sheet (not a rod)
•Polypeptide chain is
fully extended
•Stabilized by H-bond
between CO and NH
groups in different
polypeptide chains
Tertiary
•Interaction b/w side chains (R
groups) 2º structure- 3-D
structure (distinct domains)
•H-bond, ionic bonds and van
der Waals interactions stabilize
the 3º structure
-all weak but cumulative effect
•Further strengthening-strong
covalent bond
> Disulphide bridges
Quaternary
•Two or more polypeptides
aggregate into one
functional molecule.
E.g.
•Collagen, a fibrous
protein-triple helix
• Hemoglobin, globular
protein
-4 polypeptides (of 2
kinds)
•Protein folding assisted my proteins called
Chaperonins
> Physical and chemical factors important
to maintain the 3-D structure:
•pH
•Salt concentration
•Temperature
Study Questions
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What are proteins?
Draw and label an amino acid?
What are the 4 classes of proteins classified into?
Distinguish between proteins and peptides?
Illustrate how an amino acid is joined together?
Explain why peptide bond is rigid and planar?
Distinguish between primary, secondary, tertiary and quarternary
structures?
Compare and contrast alpha helix and beta sheet?
Differentiate between the following:
Fibrous and globular?
Simple and conjugate proteins
Apoproteins and holoproteins