protein chemistry 2007
TRANSCRIPT
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PROTEIN
CHEMISTRY
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Protein
Complex biological polymers.
Many are water soluble (eg: antibodies,
enzymes & haemoglobin).
Some are insoluble, tough & resistant
structures (eg: collagen & keratin).
Makes up 18% of the mass of averageperson.
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Protein(s) Function Location
Myosin muscle
contraction
muscle tissue
Actin
Chymotrypsin digestive
enzymes
small intestine
Pepsin stomach
Insulin hormone blood
Ferritin storage bone marrow,
liver, spleen
Immunoglobulins antibodies blood
Collagen structural
proteins
skin, tendon
Keratin hair
Haemoglobin transport blood
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Amino Acids The Building Block of Proteins
20 different amino acids.
19 containsCOOH andNH2 group
attached to the same C atom.
except Proline which is a cyclic contains 2amino group.
H H O
N C C
H R O - H
N terminal C terminal
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20 amino acids differ in the nature of the R
group which vary in their complexity. The physical & chemical properties of the side
chain determine the unique characteistics of an
amino acid.
The R groups are categorized into 3 broadcategories:
i. Non polar group - hydrophobic
ii. Polar group - hydrophiliciii. Electrically charged group acidic & basic
due to negative & positive charges.
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Type of R
group
Example Structure
Non-polar Alanine (ala) H2N-CH(CH3)-COOH
H2N-CH(CH(CH3)2)COOHValine (val)
Polar Serine (ser) H2N-CH(CH2OH)-COOH
Electrically
charged(acidic or
basic)
Aspartic acid
(asp)
H2N-CH(CH2COOH)-COOH
Lysine (lys) H2N-CH(C4H8NH2)-COOH
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The Ionisation of Amino Acids
Shows both properties of an acid & base(amphoteric).
H2NCH(R)COOH H2NCH(R)COO- + H+
(acidic behaviour)
H2NCH(R)COOH + H+
+H3NCH(R)COOH (basic behaviour)
At pH 7 the structure of glycine is+H3NCH2COO
- (dipolar ion known aszwitterions)
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Polypeptide Chains
2 amino acids adjacent to each other canundergo condensation synthesis forming acovalent linkage called a peptide bond.
The chain has a polarity with an N-terminal(for the nitrogen group) and a C-terminal(for the carbon group).
The repeating sequence of atoms alongthe chain (-N-C-C-N-C-C-) is referred to asthe polypeptide backbone.
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monomers form a long chain polymer via
condensation reaction (polypeptide chain).
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Protein conformation
A proteins function depends on its unique
conformation, which is a consequence of
the specific linear sequence of the amino
acids that make up the polypeptide chain.
The function of a protein depends on its
ability to recognize and bind to some other
molecule (eg: enzyme recognize andbinds to its substrate).
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The Primary Structure of Proteins
Linear (no fold/straight) polymer of aminoacids.
Each polypeptide has direction and
sequence of amino acid residues in achain known as primary structure of thepolypeptide.
A polypeptide chain is always synthesizedfrom the N-terminal end to the C-terminalend.
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The precise primary structure of a proteinis genetically controlled and crucial indetermining the other levels of structurethat protein can adopt.
A slight change in the primary structure
can affect a proteins conformation andability to function. (eg: sickle-cell anemia isan inherited blood disorder where oneamino acid is substituted with another in asingle position in the primary structure ofhaemoglobin.)
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Frederick Sanger, determined the
sequence of amino acids for hormoneinsulin.
He used various protein-digesting
enzymes that cleaves the polypeptide atspecific places into fragments and then
searched for overlapping regions.
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Secondary Structure of Protein
The polypeptide chain repeatedly coiled orfolded in patterns that contribute to theproteins overall conformation.
These structures are stabilized byhydrogen bonding between the peptidebond regions of the chains backbone.
One such secondary structure is the alpha
() helix, a delicate coil held together byhydrogen bonds between every fourthpeptide bond.
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Lysozyme is a typical globular (roughly
spherical) protein that has stretches of-helix.
Another type of secondary structure is the
beta () pleated sheet in which thepolypeptide chain folds back and forth or
where the two regions of the chain lie
parallel to each other.
Hydrogen bond between the parallel
regions hold the structure together.
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Tertiary Structure of Proteins
The 3D shape or conformation of a proteinchain is maintained by a series of mainly
non-covalent, intramolecular interactions
between the R groups of the amino acidsmaking the chain.
Some of these interactions are relatively
easily distrupted, others not so andinclude:
i. van der waals forces between non-polar
side chains,
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ii. Hydrogen bonding between polar R
groups,
iii. Ionic bonds (salt bridges) between
ionised R-groups (positive & negative
charged groups) andiv. Strong covalent disulphide bridges
formed between 2 cysteine residues at
different location in the primarysequence.