Download Proteins synthesisand expression

Proteins
by Sakvinder S Khalsa
• WE SHALL LOOK AT PROTEIN
SYNTHESIS.
• CONSIDER PROTEIN STRUCTURE AT THE
MOLECULAR LEVEL.
• DISCUSS DIFERENT USES OF PROTEINS.
• BREIFLY LOOK AT ENZYMES.
Protein synthesis
• Protein synthesis is the making of proteins,
using the information that is found in DNA
CELL
(Chromosomes).
chro
Proteins
Nucleus
The cell
Chromos
omes
• Proteins are very
important molecules for
a cell.
• Proteins are used to
build cell structures and
are used as enzymes.
Proteins
• Proteins are long chains of small molecules called
amino acids.
• Different proteins are made using different sequences
of amino acids.
• The pieces of information in DNA are called genes.
• Genes describe how to make proteins by putting the
correct amino acids into a long chain in the correct
order.
Nucleus
The cell
Piece of
DNA
Selected
For
study
chromosomes
Nucleus
Piece of
DNA
Selected for
study
Let’s zoom in on
This short segment of
DNA to see how its
information
Is used.
DNA inside the nucleus
D
N
A
• Protein synthesis begins
with the stored genetic
information of a DNA
molecule.
• The DNA of this gene
will ‘unzip’ like DNA
does during replication.
Unu
d
D
N
A
The new strand is an RNA
molecule. Note that there is one
difference in the subunits: RNA
contains yellow Uricil instead of
purple Thymine.
RNA
The RNA now has copied the subunit
sequence of the gene.
The DNA is no longer needed in the
process of protein synthesis.
D
N
A
RNA
D
N
A
RNA
RNA
A]
ion will
rotein
At the ribosome
mRNA
At the
ribosome
At the ribosome
• The genetic information is interpreted and used
to assemble a protein.
• We should remember, the mRNA is a sequence
of subunits (like a chain) that tells how to build
a protein
• A protein is a sequence of subunits – a chain of
amino acids.
• The mRNA contains information in
sets of three subunits.
• Each set of three is the code for a
particular amino acid.
m
R
N
A
The information of the messenger RNA (mRNA)
describes which amino acids should be in the
protein chain.
A molecule of transfer RNA (tRNA) will carry in
the proper amino acid, one at a time.
.
m
R
N
A
Amino acid
m
R
N
A
A different set of three mRNA subunits means a different
tRNA molecule. That means a different amino acid will be
Carried in.
Two different
Amino acids
Two different tRNA molecules
m
R
N
A
The next tRNA will
Carry in the proper
amino acid
and the process will
continue.
The chain of amino acids is called
a ‘polypeptide’
And when it is very long it is
called a protein.
polypeptide
A polypeptide chain
• Even this is a very, very short polypeptide chain. Most
have hundreds or thousands of amino acids.
A very short polypeptide chain, or part of a protein
The end of protein synthesis at the
ribosome.
PROTEIN STRUCTURE
• We will look at the main
elements found in
proteins.
• Recall how proteins are
constructed.
• Look at the structure of
proteins.
• Overview the major
functions of proteins.
The building blocks of proteins
• Like carbohydrate and
lipid molecules proteins
contain the elements :
Oxygen(O),
Carbon(C),and
Hydrogen(H)
• In addition they always
contain the element
Nitrogen(N).
• Before we can
understand how proteins
are constructed, the
structure of amino acids
needs to be considered.
R represents groups such as CH3 or C2H5
R
O
H
Amine group
N
C
C
H
H
AN AMINO ACID
Carboxylic acid
group
OH
How are proteins constructed
• First the Amino acids
bond together.
• They are joined together
by what is known as a
peptide bond.
Formation of a peptide bond via condensation.
H
R
N
H
Amino acid
C
H
O
C
H
+
OH
R
N
H
C
H
Amino acid
O
C
OH
A peptide bond between two amino
acids.
H
N
H
R
C
H
O H
C N
H20 [WATER]
H
C
R
O
C
OH
A condensation reaction
Protein construction
• When two amino acids join together they
form a dipeptide.
• When many amino acids are joined together
a long-chain polypeptide is formed.
• Organisms join amino acids in different
linear sequences to form a variety of
polypeptides in to complex molecules, the
proteins.
Amino acid
Peptide bond
Primary protein structure
primary
This is the linear sequence of amino acids
structure
Secondary protein structure
Polypeptides become twisted or coiled.
These shapes are known as the secondary
Structure.
There are two common secondary structures
The alpha-helix and the beta-pleated sheet.
Secondary protein structure
Alpha-helix
Amino acid
Hydrogen bonds hold shape
together
Secondary Protein structure
[The beta pleated sheet]
Amino acid
Hydrogen bonds
The polypeptides are held in position by hydrogen
bonds.
In both alpha-helices and beta pleated sheets the
C=O of one amino acid bonds to the
H-N of an adjacent amino acid.
As below:
C=O----H-N
Secondary structures
• Both secondary structures give additional
strength to proteins. The alpha-helix helps make
fibres like in your nails, e.g. Keratin.
• The beta pleated-sheet helps make the strength
giving protein in silk, fibroin.
• Many proteins are made from both alpha-helix
and beta-pleated sheet.
Fibrous proteins
• A fibrous protein only achieves a secondary
structure .
• The simple alpha-helix polypeptides do not
undergo further folding.
d
elix
ure
Structure of a fibrous protein
Tertiary protein structure
• This is when a polypeptide is folded into a
precise shape.
• The polypeptide is held in ‘bends’ and ‘tucks’
in a permanent shape by a range of bonds
including:
• Disulphide bridges [sulphur-sulphur bonds]
• Hydrogen bonds
• Ionic bonds.
Tertiary protein structure
Quaternary protein structure
• Some proteins consist of different
polypeptides bonded together to form
extremely intricate shapes.
• A haemoglobin molecule is formed for
separate polypeptide chains.
• It also has a haem group, which contains
iron.
• The inorganic group is known as the
prosthetic group.
• In haemoglobin it aids oxygen transport.
Quaternary protein structure
How useful are proteins?
• Cell membrane proteins: Transport substances
across the membrane for processes such as
facilitated diffusion and active transport.
• Enzymes: Catalyse biochemical reactions, e.g.
pepsin breaks down protein in to polypeptides.
• Hormones: are passed through the blood
and trigger reactions in other parts of the
body e.g. insulin regulates blood sugar.
• Immuno-proteins: e.g. antibodies are made
by lymphocytes and act against antigenic
sites on microbes.
• Structural proteins: give strength to organs,
e.g. collagen makes tendons tough.
• Transport proteins: e.g. haemoglobin
transports oxygen in the blood.
• Contractile proteins: e.g. actin and myosin
help muscles shorten during contraction
• Storage proteins: e.g. aleurone in seeds helps
germination, and casein in milk helps supply
valuable protein to babies.
• Buffer proteins: e.g. blood proteins, due to
their high charge, help maintain the pH of
plasma.
Enzymes
• Living cells carry out many biochemical
reactions.
• These reactions take place rapidly due to
enzymes.
• All enzymes consist of globular
proteins.
Enzymes
• The tertiary folding of polypeptides are
responsible for the special shape of the
‘active’ site.
• Some enzymes require additional nonprotein groups to enable them to work
efficiently. e.g the enzyme
dehydrogenase needs coenzyme NAD to
function.
The lock and key theory
• Substrate
Enzyme
+
Enzyme-substrate complex
oducts
A catabolic reaction
[substrate broken down]
•
•
enzyme-substrate
complex
An anabolic reaction
[substrates used to build a new
molecule]
substrate
Anabolic reaction continued
• Enzyme substrate complex
Metabolic reactions
• Metabolic reactions = anabolic reaction
+ catabolic reaction.
• Metabolism is a summary of build up
and break down reactions.
Induced fit theory
• The active site is a cavity of a particular shape.
• Initially the active site is not the correct shape in
which to fit the substrate.
• As the substrate approaches the active site, the
site changes and this results in it being a perfect
fit.
• After the reaction has taken place, and the
products have gone, the active site returns to its
normal shape.
Induced fit theory
• Enzyme
Sustrate
+
Induced fit continued
In
Enzyme-substrate complex
pr
enzyme
Lowering of activation energy
• Every reaction requires
the input of energy.
• Enzymes reduce the
level of activation
e
energy needed as seen in
n
the graph.
e
r
g
y
Reaction without
enzyme
Reaction
with
enzyme
substrate
products
Progress of reaction
Two minute summary
• Now you have seen the presentation !
• Summarise the most important points of this
presentation.
• What was the ‘muddiest’ point in the
presentation?
• Hand in your paper to the teacher before you
leave the classroom.
END OF PRESENTATION
by S S Khalsa [science PGCE]
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