Page 61 - Biology_F5

Biology for Advanced Level Secondary Schools

R groups are hydrophobic and they repel are mostly found in animals, and are
from water. In a long polypeptide chain, usually insoluble in water as well as in
there are many such non-polar amino other aqueous media. Fibrous proteins

acids which may occur adjacent to each aid in protection and structural support.
other. In an aqueous environment, such Examples of fibrous proteins include
as inside the cell, the linear polypeptide collagen (tendons, bones, and other
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chain will fold into a particular shape that connective tissues), myosin (found in
hydrophobic amino acids come in contact muscles) and keratin (found in nails,
with each other, while excluding water horns, hairs, fur, and feathers).
due to its hydrophobicity.

Van der Waals interactions
(dispersion forces) act between
hydrophobic side branches

Hydrogen Polypeptide backbone
bond

Disulphide
link

Ionic bond
Figure 1.35: Structure of fibrous proteins
(α-keratin)
Figure1.34: Bonds in a polypeptide molecule
Globular proteins: These proteins have
Categories of proteins tertiary structure in which the polypeptide
Due to their complexity, it is difficult to chains are tightly folded to form the
classify protein molecules into a single, spherical shape (Figure 1.36). They
well defined category. They can be classified are soluble in water. Examples of such
according to their structure, composition, proteins include enzymes, antibodies, and
level of organisation, and functions. some hormones, such as insulin.

(a) Classification of proteins based on
their structure
Based on their structure, proteins are
categorised as fibrous, globular, and
intermediate proteins.

Fibrous proteins: These proteins form
long polypeptide chains, cross linked at
intervals, forming long fibres or sheets
(Figure 1.35). Their shapes resemble
long ribbons or fibres. Fibrous proteins Figure 1.36: Structure of globular proteins

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