Name: Perry Peh Bing Xian
Class: 3S2
Assignment: Biology ACE – PPT
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About two-thirds of the total dry mass of a cell is
made up of proteins.
The main difference between proteins and other
biological molecules is the presence of the
element Nitrogen (and sometimes Sulfur and
Phosphorous).
They are formed from amino acids, linked by a
long chain and hence are huge molecules.
Once the chain of amino acids are constructed,
they take a specific shape and it is closely related
to its function.
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Some amino acids have the additional carboxyl
groups, and hence they are given the name of
acidic amino acids.
Some others that have additional amino groups are
known as the basic amino acids.
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When 2 amino acids react with each other, and
water is lost, a dipeptide is formed.
The amino group of one amino acid reacts with the
carboxyl group of the other amino acid and hence a
peptide bond is formed .
Polypeptides are long strings of amino acids that
are linked by peptide bonds.
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The terms ‘polypeptide’ and ‘protein’ are used
interchangeably, but when the polypeptide has
about 50 amino acid molecules long, it is
considered as a protein molecule.
It is used as an indicator of the presence of protein,
and gives purple colouration in the presence of
peptide bonds.
Procedure
1. To a solution thought to contain protein, and equal
volume of Sodium Hydroxide is added and mixed,
and a few drops of 0.5% Copper(II) Sulfate solution is
introduced with gentle mixing.
2. Purple colouration occurs when there are peptide
bonds present.
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The primary structure of a protein, is the arrangement of
the long chain of amino acids in the protein molecule.
Proteins differ in the variety, number and order of their
constituent amino acids.
In the living cell, the sequence of the amino acids in the
polypeptide chain is controlled by coded instructions
stored in DNA of nucleus.
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When one amino acid in the sequence of the
protein is changed, the properties of the protein
may change completely.
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It develops when part of the polypeptide chain take a
particular shape, immediately after the formation at the
ribosomes.
Parts of the chain become folded, and/or twisted, in
many different ways.
The most common shapes are formed by
coiling to produce α-helix
folding to produce β-sheets
These shapes are permanent, and held in place by
Hydrogen bonds.
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Alpha Helix
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Beta Sheets
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It is the precise, compact structure and is unique to
the protein, and arises when the protein molecule
is further folded and held in a particular complex
shape.
This shape is made permanent by 4 different types
of bonding.
These bonds are established between adjacent
parts of the chain.
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Fibrous proteins take up a tertiary structure, in a
form of a long, coiled chain.
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Lysozyme
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Blood protein
involved in clotting
mechanism: Fibrin
Collagen , a
component of bone
and tendons
Keratin, found in
hair, horn and nails
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Some other proteins that take up a tertiary
structure are more spherical and are called
globular proteins
Most of them are enzymes
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It arises when 2 or more proteins become held
together and hence a complex and biologically
active molecule is formed.
An example will be haemoglobin.
Haemoglobin contains 4 polypeptide chains held
around a non-protein haem group that contains a
Ferrous Iron Ion.
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In solution, amino acids will ionize
The amino group and carboxyl group will do so.
The carboxyl group produces Hydrogen ions and
acts like an acid, while the amino group removes
Hydrogen ions from solution, acting as a base.
Neutral solution
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Both the
amino and
carboxyl
groups are
ionized
Acidic solution
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The amino
acids picks up
the Hydrogen
ions and
becomes
positively
charged
Alkaline solution
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The amino
acids donates
the Hydrogen
ions to
solvent,
becoming
negatively
charged
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Hence, as we have seen, the amino acids tend to
stabilize the pH of the solution, because they can
remove the excess Hydrogen/Hydroxide ions, and
forming water in the process.
They are hence, acting as buffers.
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When the amino acids have been built up into
proteins, the buffering capacity is retained.
This is due to the presence of additional amino and
carboxyl groups of the basic and acid residues of
the protein.
Hence, proteins play and important part as the
buffer for cells and organisms.
For instance, the pH of blood is partly buffered by
the proteins present in blood.
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It is the loss of the three-dimensional structure.
This happens when the relatively weak bonds that
maintain the 3D shape of the molecule are
changed.
It may be due to the following as seen on the next
slide.
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Exposure to …
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Triggered by …
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High temperature
Heavy metal ions
Organic solvents
•
Extreme pH (extreme
acidity / alkalinity)
Some other
chemicals
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•
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Like mentioned earlier, the properties of proteins
within cells and organisms depend on their
particular/specific shape
When the shape changes, the protein may cease to
be useful.
As a result, the biochemistry of cells and organisms
are extremely sensitive to such conditions that
might alter their mode of function.
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High temperature and heavy metal ions cause
changes to the proteins which are often
irreversible.
A denatured protein forms long, disorganized
strands which are insoluble in water.
We see this when the egg is cooked.
The egg white is a globular protein, albumen
and when it is exposed to high temperature, it
is denatured, becoming irreversibly opaque
and insoluble.
Roles of Proteins
Structural role
Membranes around
the cell and in the
organelles
Enzymes
Act as biological
catalysts
Pumps
These pumps
transport molecules
across membranes
Clegg, C. J. (2000). Introduction to advanced Biology. United
Kingdom : Hodder Arnold .
 Clegg, C. Lysozyme's primary, secondary and tertiary structures.
Introduction to advanced Biology. Hodder Arnold, United Kingdom.
 Clegg, C. Primary and Secondary structure of a protein. Introduction
to advanced Biology. Hodder Arnold, United Kingdom.
 Wikipedia. (2010, 08 18). Wikipedia, the free encyclopedia.
Retrieved 08 18, 2010, from Proteins - Wikipedia, the free
encyclopedia: http://en.wikipedia.org/wiki/Proteins
 Wikipedia. (2010, 08 18). Wikipedia, the free encyclopedia.
Retrieved 08 18, 2010, from Haemoglobin :
http://en.wikipedia.org/wiki/Haemoglobin
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