Page 60 - Biology_F5

Cytology

Structure of polypeptide molecule plays a considerable role in determining
The structure of polypeptide molecule shape and stability of the polypeptide
is determined by amino acids sequences molecule.
and its configuration. For each particular
type of protein, the chain of amino acid Disulphide bond
makes up a polypeptide molecule of This is formed between side chains of
FOR ONLINE READING ONLY
a specific shape. This shape is very cysteine (amino acid containing sulphur)
important in the functioning of the and it keeps parts of polypeptide intact and
maintains its stability. If two molecules
protein, especially enzymes. The type and of cysteine line up alongside with each
the three-dimensional configuration of other, the neighbouring sulphur can be
a polypeptide molecule, is stabilised by oxidised to form a disulphide bridge.
the interactions of five different types of A disulphide linkage may be formed
bonds (Figure 1.34). between the cysteine residues of the same
polypeptide chain or different polypeptide
Peptide bond chains of a functional protein. Disulphide
This is a bond formed between carboxyl bonds stabilise the tertiary or quaternary
group (-COOH) of one amino acid and structures of the protein.
amino group (-NH ) of another amino
2
acid. A polypeptide molecule has a free Ionic bond
amino group at one end, and a carboxyl This bond occurs between the positively
group at the other end.
and negatively charged side chains of

Hydrogen bond the amino acids that come in contact
Hydrogen bond is a bond that is formed with each other. Normally, ionic bond
in the polypeptide chain between is formed between ions of opposite
amino acid and side (R) groups. The charges from ionised acid (-COO ) and
-
bond is an electrostatic attraction basic (-NH ) groups of the amino acids.
+
3
between the hydrogen atom and another The availability of ionised carboxylic
electronegative atom. Hydrogen bond group (COO ) and amino group (-NH )
-
+
3
forms between the carbonyl (C=O) of one at the side chain of the amino acid, and
amino acid or the amino group (N-H) of at the terminal of the polypeptide chain
another amino acid. Hydrogen present in may form ionic interactions, which help
hydroxyl (-OH) group or amino group to make a polypeptide molecule of a
(-NH ) of amino acids, becomes slightly particular shape.
2
electropositive. Therefore, hydrogen
bond is a partially electrostatic attraction Hydrophobic interaction
between the hydrogen (H) atom which is This is considered to be among the major
covalently bound to a more electronegative driving force for the folding of globular
atom or group, such as nitrogen (N) and proteins in aqueous environment. Some
oxygen (O). Although hydrogen bonds are R groups of amino acids are non-polar as
very weak, the absolute number of bonds they have equal number of charges from
amino and carboxyl groups. The non-polar

Form Five Student’s Book
53