Download AS Biology - Everything Protein

Everything You Need to Know About....
Biochemistry of Proteins
Proteins are POLYMERS that are also known as POLYPEPTIDES; they exclusively
contain Carbon, Hydrogen, Oxygen and Nitrogen; some proteins contain Sulphur.
The MONOMERS that repeat to make up the polymer are called AMINO
ACIDS. Amino acids have a general structure as shown below:
H
H
N
C
C
H
R
The Amino Group
O
OH
The Acid Group
The R Group
The R Group is different for each amino acid (there are about 20 varieties).
The R Group is ALWAYS opposite a lone Hydrogen off the central carbon.
Amino acids join together through a chemical reaction known as CONDENSATION
where a molecule of WATER IS REMOVED.
When two amino acids are joined a DIPEPTIDE is formed, the bond between the Carbon
and the Nitrogen is known as the PEPTIDE BOND. The reaction looks like this:
+ H2O
1
The CONDENSATION reaction is said to be REVERSIBLE. The reverse reaction is
known as HYDROLYSIS.
Protein molecules have very complex and intricate structures that let them perform
their specific roles.
PRIMARY STRUCTURE is the AMINO ACID SEQUENCE; peptide bonds are present
in this level of structure.
SECONDARY STRUCTURE is how the primary structure folds for the first time. The
two most common secondary structure folds are shown below:
ALPHA HELIX - a corkscrew-like arrangement.
BETA PLEATED SHEET - a uniform zigzag arrangement.
Note that the secondary structure is held in place by
HYDROGEN BONDS which are WEAK attractions
between Oxygen and Hydrogen atoms in the primary
structure.
TERTIARY STRUCTURE is the 3D SHAPE that the secondary structure folds into.
This shape is DETERMINED BY THE AMINO ACID SEQUENCE.
TERTIARY STRUCTURE IS UNIQUE FOR EACH DIFFERENT PROTEIN.
3 TYPES OF BOND ARE PRESENT IN TERTIARY STRUCTURE:
HYDROGEN BONDS - Weak (easily broken by heat or pH)
IONIC BONDS - Quite strong
(form between R-Groups)
!DISULPHIDE BRIDGES (COVALENT) - Very Strong
(form between Sulphur atoms
in the R-Group of Cysteine)
Exam Question Advice
Examiners love to ask about enzymes (which are proteins). In particular, the effect of heat or pH on
enzymes. The key points to get across if you are asked “why does an enzyme stop working after it has been
heated” are:
!- Enzyme has been denatured.
!- Weak Hydrogen bonds have been broken.
!- 3D Tertiary structure is unravelled; loss of shape.
- Active site is no longer complementary to the substrate.
2
QUATERNARY STRUCTURE is only found in proteins containing MORE THAN
ONE POLYPEPTIDE chain.
HAEMOGLOBIN features a
quaternary structure; it has four
polypeptide chains associated
together.
A PROSTHETIC GROUP is a NON-POLYPEPTIDE that has been incorporated into
a proteins structure. An example of this is Haem (iron group) in Haemoglobin. Not all
proteins include prosthetic groups.
The final 3D structure of a protein can be classified as either GLOBULAR or FIBROUS.
GLOBULAR PROTEINS are mostly used for METABOLIC processes.
FIBROUS PROTEINS are mostly used for STRUCTURAL processes.
Testing for Protein
We can use a simple biochemical test to determine whether a sample contains protein. This
test is known as THE BIURET TEST for protein. The procedure is described below:
1.
Add an equal volume of a strong base (such as KOH) to the sample.
2.
3.
4.
Add a few drops of Copper (II) Sulphate solution.
Mix gently.
Colour change from blue to lilac indicates presence of protein
3