Transcript Proteins 1 - Dr Rob's A

Proteins
What are Proteins?
The most complex biological molecules
Contain C, H, O and N
Sometimes contain S
May form complexes with other molecules
containing P, Fe, Zn or Cu
Macromolecules with relative mol. Masses
of 104 – 106
Consist of one or more unbranched
polypeptide chains built up of amino
acid monomers linked by peptide bonds
Each protein has a characteristic 3-D
shape resulting from folding and
coiling.
It is usual to describe protein
structure by the level of organisation
of the molecule:
– Primary
– Secondary
– Tertiary
– Quaternary Structure
Primary Structure
1’ Structure
– The number, type and sequence of
amino acids
– Specific to each protein
– Coded for by DNA
Amino Acids
H
General formula:
NH2.RCH.COOH
There is a central
carbon atom (called
the "alpha carbon"),
with four different
chemical groups
attached to it:
– a hydrogen atom
– a basic amino
group
– an acidic carboxyl
group
– a variable "R" group
(or side chain)
aa’s are cyrstalline solids and soluble in
water
20 common aa’s found in living organisms
The amino grp has basic properties
The carboxyl grp has acid properties
Acid and basic properties called
amphoteric
In organisms, pH usually neutral so both
grps become ionised (+ve one end, -ve
the other)
– refered to as zwitterion
2 aa’s can join (condensation) to form
dipeptide
Further reactions can occur making
polypeptides
Most important role of aa’s is as monomers for protein synthesis
Green plants can synthesis all they need from photosynthesis and nitrate
from soil
Animals can synthesise some, but need to obtain 8 from their diet. These
are the essential amino acids
Amino acids are also involved in synthesis of other compounds like nucleic
acids and cytochromes.
Secondary Structure
most basic level of protein folding
– consists of a few basic motifs that are found in
all proteins.
The secondary structure is held together
by hydrogen bonds between the carboxyl
groups and the amino groups in the
polypeptide backbone.
The two most common secondary
structure motifs are the α-helix and the βpleated sheet
The α-helix.
The polypeptide chain is wound round to form
a helix.
Held together by hydrogen bonds running
parallel with the long helical axis.
Many hydrogen bonds make it very stable
and strong.
The β-sheet.
The polypeptide chain zigzags back and forward
forming a sheet of antiparallel
strands.
Once again it is held together
by hydrogen bonds.
Tertiary Structure
Most proteins have α-helix regions
and β-pleating
But folding of the polypeptide chain
into a compact, globular shape is
called the tertiary structure
The bending and folding is irregular
Caused by formation of differing
bonds between aa residues.
Bonding in R-groups
e.g. in insulin
* Can only happen
to end aa residues
Other ionic bonds
may occur within
R-groups
Quaternary Structure
Complex proteins may contain more
than one polypeptide chain
If more than one chain it has a
quaternary structure
The polypeptide chains may be all of
the same type or different types
Further Classification of Proteins
Because of proteins abundance and
diversity it is difficult to classify them
in a simple manner.
It is customary to group them
according to either their structure or
function within living organisms.
Classifying according to structure
Fibrous
Globular
Secondary Structure
important; consist
mainly of α-helix or βpleated sheets
Insoluble in water
Tertiary structure
important; bent and
folded into spherical
shapes
Soluble in water
Structural Functions
e.g. keratin, collagen
Enzymes, antibodies,
hormones
e.g. amylase, globulins,
insulin
Haemoglobin
“a conjugated protein”
Made of 4 chains
2 chains contain 141
aa-residues
(α-globins)
2 chains contain 146
aa-residues
(β-globins)
Total of 574
Other conjugated proteins
Glycoproteins
Lipoproteins
Under some circumstances, the 3D shape
of a globular protein can change
– May be temporary or permanent
Doesn’t affect the primary structure
Alteration of structure will affect the
biological role of the protein especially in
enzymes
Denaturation
– Increase in heat, pH change, high salt conc.,
presence of heavy metals and organic
solvents.
Chemical (Biuret) test for protein