AS level amino acids and proteins

1. Proteins

2. Proteins are polymers of amino acids
They make up more than 50% dry mass
of cells
Proteins contain C,H,O, and N. Some
contain S
Proteins may form complexes with other
substances

3. Diversity of functions
Proteins are diverse. Dome of their functions
include:
Cell membranes
Antibodies
Haemoglobin
Keratin in hair
Collagen in bone and connective tissue

4. Amino acid structure

5. This is the amino group
It contains nitrogen and is basic
Nitrogen bonds to 2 hydrogen atoms and the
central carbon

6. This is the acid carboxyl group –COOH
Which will lose a H+ into solution

7. This is the residual group
Each amino acid has a different R group
It will determine the type of amino acid

8. Types of amino acids
The R group gives amino acids different
properties
There are 20 essential amino acids used to make
proteins
The smallest amino acid is glycine where R = H

9. Some other R groups
The amino acid is named according to the R group [in red]
Each name has a abbreviation [in blue]

10. This is the a central carbon
If the R group is not H then this is an asymmetrical
carbon
All amino acids can exist as optical isomers but in nature only one form
is found
This is the L -isomer

11. Remember amino acids are 3-D structures
A large R group will make this more complex

12. Amino acids form Zwitterions
the proton is lost here making this
side negatively charged

13. Amino acids form Zwitterions
The N picks up a proton and this
now has a positive charge

14. Amino acids form Zwitterions
The molecule has an overall neutral
charge
This only occurs at a particular pH
This is the isoelectric point
The R group will change the
isoelectric point for different amino
acids
Or:

15. If the amino acid is placed in a more
acidic (more H+) solution:
Protons are
accepted here
Because protons are removed from
solution the solution becomes less acidic
again
The amino acid has acted as a buffer

16. If the solution is becomes more
basic:
H+ is donated from here so the
acidity is restored

17. To recap
Amino acids form zwitterions – with both
basic and acidic properties
At a certain pH – the isoelectric point - the
ions formed are neutral
They are amphoteric
They are able to donate or accept hydrogen
ions to keep the pH the same
So they act as buffers
Soluble proteins in cells and in the blood
are important as buffers

18. Dipeptids
2 amino acids can be joined by the removal of water

19. This is a dipeptide molecule
Held by a peptide bond

20. This is a dipeptide molecule
Held by a peptide bond
Water is formed this is
condensation
A peptide bond can be broken by hydrolysis or addition of water

21. In this way long chains of amino acids can
be formed = polypeptides
Notice that there is still an amino end
and a carboxylic end

22. The order of the amino acids in the chain
is determined by the DNA sequence of
the gene coding for the protein
This is the Primary Protein structure

23. The primary sequence is folded into a highly specific 3D
structure which is held together by various bonds

24. Bonds involved in maintaining
the shape of proteins
 H bonds
 Ionic bonds
 Disulfide bond/ bridges
 Hydrophobic interactions

25. 1. H Bond [hydrogen bonds]
These are weak attractions between an electronegative oxygen
in a carboxylic group and an electropositive H on OH or NH
groups
The large number of these bonds make them significant even
though they are weak

27. 2. Ionic bonds
These only form at the right pH
An electron is donated or accepted between
ionized amine and carboxylic group.
A relatively weak bond broken by a change
in pH
Ionic bonds may also form between residual
groups

30. 3. Disulfide bridge
This forms between 2 cysteine amino acids
(R= CH2
SH)
It is a covalent bond

31. 4. Hydrophobic interactions
Hydrophobic R groups in the polypeptide chain are
shielded by Hydrophilic ones in an aqueous
solution

33. There are 4 levels of Protein
structure
Primary
Secondary
Tertiary
Quaternary

34. There are 4 levels of Protein
structure
Primary
Secondary
Tertiary
Quaternary
All proteins show this
Seen in most proteins
Seen in globular proteins
Seen in some proteins

35. Primary structure
This is the sequence of
amino acids
It is determined by the
DNA code
The amino acids are
held together by peptide
bonds
All proteins will have
primary structure

36. Secondary structure
The polypeptide
chain is twisted into
∝-helix
Or a
β- pleated
sheet
Held together by
H-bonds

37. All or only part
of a polypeptide
chain may coil
into an α-helix

38. β-Pleated sheets are formed by hydrogen
bonds between parallel chains of
polypeptides or a single chain folded back on
itself

39. Proteins with just
secondary structure
form fibrous proteins
which are insoluble

40. Fibrous Proteins
e.g. collagen
A tough protein used as
connective tissue
It is an insoluble, fibrous
proteins
Collagen and has a high
tensile strength
Collagen is made from 3
α-helix molecules twisted like
a rope

41. The most common amino acid
is glycine which is small
because R=H
This allows the molecule to
twist tightly

42. The collagen triple helices are
bundled together into fibrils
These form Collagen fibres
Notice how the joins
of the fibrils are
staggered to prevent
lines of weakness
forming
Fibrils
Fibre

43. Scanning em of
collagen

44. e.g. Keratin
This is the protein in
hair and skin
It is formed from α-
Helix polypeptides

45. e.g. elastin
This protein is found in
connective tissue for
instance in alveoli
It can be stretched
and will recoil to the
original shape

46. Tertiary Structure: globular proteins
This structure
may involve any
of these :
Hydrogen bonds
Disulfide bridges
Ionic bonds
Hydrophobic
interactions

47. There may be sections with
secondary structure
The proteins have an overall
3-D globular shape which is
highly specific
because it is determined by
the bonds forming between
specific amino acids in the
primary sequence

48. Globular proteins
have specific shapes and
are soluble
Enzymes are globular
proteins
They have a specific
active site
The hydrophilic exterior R
groups makes the
molecule soluble in water

49. Quarternary Structure
e.g. Haemoglobin: made of 4
polypeptides
These are formed from
2 or more polypeptide
chains
Held together by
hydrophobic
interactions

50. Summary of levels of Protein structure

51. Conjugated proteins
these contain a non-protein group
For example the
haem group in
haemoglobin
There is one
haem group in
each of the 4
polypeptides

52. myogloin
Myoglobin has only
one polypeptide with a
haem group in it

53. Test for protein – the Biuret test
In a basic solution
CuSO4 will interact
with peptide bonds
The colour will change
to lilac