Download F212 2.1.1 Biological Molecules Proteins

Module 2 2.1.2 Biological
Molecules
Proteins
By Ms Cullen
Proteins
• Proteins are a group of organic compounds.
• Our bodies need essential proteins, from our
diet and non-essential proteins which the
body can synthesise. (Despite the names the body
needs all of them to function!)
• Proteins have many functions -
Proteins
• The test for protein is the biuret test.
• An equal volume of biuret reagent is added to
crushed food.
• Biuret is made up of sodium hydroxide and
copper sulphate, when these are added to
protein they will turn lilac in colour.
• Which solution
contains proteins?
Amino Acids
• Amino acids are the monomers of all proteins.
Q: What is a monomer?
• Amino acids can not be stored by the body so
they are deaminated (amino group removed) in
the liver to form urea.
• All amino acids contain the elements Carbon,
Hydrogen, Oxygen and Nitrogen (some may
contain sulphur as well).
• Amino acids condense together to form a
polypeptide chain.
• Protein molecules consist of 1 or more of these
polypeptide chains.
Amino acid structure
Week 14
Amino acids have:
• A central Carbon atom
• A carboxyl group COOH
• A Hydrogen atom
• An amino group- NH2
• An ‘R’ group.
Basic amino acid structure - glycine
Q: What is the R group
in glycine?
A:
Dipeptides and Polypeptides
• Two amino acids bond together to form a dipeptide.
• This occurs by a condensation reaction, where water is
lost.
• To do this the hydroxide from the carboxyl group of one
amino acid reacts with the hydrogen from the amine group
of the other amino acid to form water.
• The water is removed.
• The new bond is known as a peptide bond, a type of
covalent bond.
• http://www.biotopics.co.uk/as/aminocon.html
• More and more amino acids can join together in this way to
form polypeptide chains, the ‘backbones’ of all protein
molecules.
Condensation and hydrolysis
Hydrolysis
• Polypeptide changes can be split back into
amino acids by the addition of water.
• This is known as an hydrolysis reaction –
‘splitting by water’.
• http://www.biotopics.co.uk/as/dipeptidehydr
olysis.html
Primary structure of Proteins
• This is the order of
specific amino acids
found in a polypeptide
chain.
• Only peptide bonds are
involved.
• Frederick Sanger was the
first Scientist to work out
the primary structure of a
protein, insulin, it took
him 10 years! (19451955)
Insulin consists of 2 chains
linked together by
disulphide bridges.
Secondary structure of Proteins
• This is where the
polypeptide folds to
form either an α-helix
or a β-pleated sheet.
• Hydrogen bonds hold
the coil in place.
• Although H bonds are
weak there are many of
them.
Tertiary structure of Proteins
The tertiary structure is
stabilised by 4 types of
bonds:
1.
2.
3.
4.
Quaternary structure of Proteins
• The quaternary structure
of a protein is the final 3D
structure.
• It is formed by all the
polypeptide chains
making up the protein
molecule.
• The protein haemoglobin
is formed by 4
polypeptide chains.
(square parts represent haem – the
non-protein part)
Copy and complete the table below:
Level
Primary
Secondary
Tertiary
Quaternary
Description
Bonds involved
Which proteins are
they found in?
Haemoglobin – a conjugated protein
• Haemoglobin consists of 4
polypeptide subunits.
• 2 are known as α chains, and 2
are β chains.
• The haemoglobin molecule is
a water-soluble globular
protein.
• The haem group, which
contains an iron (Fe2+ ) ion, is
the part that binds to oxygen.
• It is not an amino acid and is
known as a prosthetic group.
• Haemoglobin is a conjugated
protein as it is a globular
protein with a prosthetic
group attached.
Catalase – an enzyme
• Catalase is a globular protein, it’s
quaternary structure contains 4•haem
prosthetic groups.
• The presence of iron II prosthetic
groups allows the enzyme to speed up
the break down hydrogen peroxide.
• Hydrogen peroxide is often a waste
product of metabolic reactions, it is
harmful to cells and therefore the
enzyme catalase is necessary to
prevent it accumulating in cells.
• Catalase is found in particularly high
concentrations in liver cells –
hepatocytes.
Carbonic anhydrase – an enzyme
• Carbonic anhydrase is a globular
protein; an enzyme that catalases
the conversion of carbon dioxide
into hydrocarbonate ions in a cell. It
is a reversible reaction.
CO2 + H20 ⇌ HCO3- + H+
• There are a whole family of carbonic
anhydrases all based around
different proteins, but all of them
have a zinc ion bound up in the
active site, the prosthetic group.
Collagen – a fibrous protein
• Collagen is made up of 3 polypeptide chains wound
around each other.
• Each of the chains is a coil composed of about 1000
amino acids. The most abundant of these is glycine.
• Hydrogen bonds form between chains to give
strength.
• Collagen molecules are linked together by covalent
bonds between molecules, adding to the strength.
• We call this a collagen fibril.
• Many fibrils from a collagen fibre.
Collagen structure
The functions of Collagen
Essentially to provide mechanical strength:
• in artery walls
• tendons which connect muscle to bone
• bones – with the addition of minerals
• cartilage and connective tissue
Collagen is now being used for cosmetic
purposes. Injecting into the lips gives
a fuller appearance.
Other fibrous Proteins
Keratin
• Found in hair, nails and skin.
• This fibrous protein contains a lot of the amino acid
cysteine, which contains sulfur.
• The large number of disulfide bridges in it’s structure
makes keratin insoluble, inflexible and strong.
• Hair has less cysteine, and therefore less disulfide
bridges than nails making it more flexible.
Other fibrous Proteins
Elastin
• Found in elastic fibres.
• Elastic fibres are found in blood vessels and walls of alveoli.
• Elastin is flexible and allows these structures to expand, but
also to recoil to their original shape and size.
• Composed of many stretchy molecules called tropoelastin.
• This gives a cross-linked, stable structure to elastin.
• Elastin gives strength and elasticity to the skin and other
tissues.
Activity
Dissect chicken leg and identify
areas where collagen, elastin and
haemoglobin would be found.
Fibrous v Globular Proteins
Complete the table below:
Fibrous Proteins
Structure
Shape and chain
length
Solubility
Stability
Function
Globular Proteins
Compare and contrast the
structure and function of
haemoglobin and collagen.