protein structure

7. Proteins
Proteins are polymers of amino acids
Each has a unique 3D shape
Amino acid sequences vary
Proteins are major component of cell parts
The provide:
support and structural components
Several types of proteins are identified:
receptor, contractile, defense, enzymes,
structural

8. Building Blocks: Amino Acids
There are 20
different types
of amino acids
All have this
general formula
The R group is a
variable group

10. Polar vs. Non-polar Amino Acids
 Some amino acids are polar while some are non-polar

11. Amino acids may be either polar or non-polar depending on the composition of their
side chain
Polar amino acids have hydrophilic R groups, while non-polar amino acids have
hydrophobic R groups

12. The localization of polar and non-polar amino acids will be determined by the
type of protein and its function:
Water soluble proteins:
•Non-polar amino acids tend to be found in
the center of the molecule (stabilize the
structure)
•Polar amino acids tend to be located on the
protein surface (capable of interacting with
water molecules)

14. Membrane-bound proteins:
•Non-polar amino acids tend to be
located on the regions of the surface in
contact with the membrane
•Polar amino acids will generally line
interior pores (to create hydrophilic
channels)

15. Significance of polar & non-polar amino acids
polar amino acids non-polar amino acids
 hydrophilic
 can make hydrogen bonds
 found in hydrophilic
channels & parts of proteins
projecting from membranes
 found on surface of water-
soluble proteins
 hydrophobic
 forms van der Waals
(hydrophobic interactions)
with other hydrophobic amino
acids
 found in proteins in interior of
membranes
 found in interior of water-
soluble proteins

16. Peptide Bonds
 Proteins are formed
by condensation
 A peptide bond is
formed

17. Four levels of protein structure

18. Primary Structure
 Primary structure is a
chain of amino acids
 number & unique
sequence of amino acids
determine the properties
of primary structure
 each position is occupied
by one of 20 different
amino acids
 sequence of amino acids
is determined by DNA
sequence in genes
 linked by peptide bonds

19. Secondary structure
 formed by interaction
between amino and carboxyl
i.e. -NH and -C=O groups
 weak hydrogen bonds are
formed between – H & = O
 there are two types: a-helix
and b-sheet
 α- helix formed / polypeptide
coils up e.g. sheep wool
 β- pleated sheet formed e.g.
silk in spider web
 regular repeated folding of
amino acid chain
 secondary structure is
stabilized by hydrogen bonds

20. Tertiary structure
 These are globular proteins
with irregular conformation
 tertiary structure is the
folding up of the polypeptide
chain, secondary structure or
alpha helix
 it gives three dimensional
globular shape i.e. shape of
active site
 the structure is stabilized by
disulphide bridges, hydrogen,
ionic& hydrophobic bonds
 tertiary structure used as
enzymes to catalyze
biochemical reactions

21. Quaternary structure
 made of several
polypeptide subunits
joined together
 they maybe conjugated
proteins i.e. proteins
which combine with a
prosthetic group (non-
protein molecules)
 prosthetic groups
includes: metals e.g. iron
in haemoglobin, nucleic
acids as in ribosomes ,
carbohydrates as in
glycoprotein or lipids as
in glycolipids

22. Denaturation of Proteins
 change in protein’s
usual regular
structure due to:
 High temp
 Change in pH
 Addition of organic
solvent (alcohol,
acetone)
 These factors break
the bonds that
stabilize the
structure

23. Protein Functions
Function Examples
 Structure
 Transport
 Enzymes
 Movement
 Hormones
 Antibodies
 storage
 – collagen/keratin/fibrin
 – myoglobin/hemoglobin, bind &
transport oxygen
 – lysozyme, speeding up metabolic
reactions
 – actin (and myosin tropomyosin (and
troponin)
 – insulin, regulate blood glucose
 – immunoglobulin
 – albumin in egg, casein in milk

24. Difference between Fibrous and
Globular Proteins
 fibrous proteins are long and narrow strands or sheets whereas
globular proteins are rounded, spherical or ball shaped
 fibrous protein made of repetitive amino-acid sequences whereas
globular proteins are made of irregular amino acid sequences
 fibrous proteins are usually insoluble in water whereas globular
proteins usually soluble in water
 globular protein more sensitive to changes in pH, temperature & salt
than fibrous proteins
 fibrous proteins have structural or support functions (roles) whereas
globular proteins have metabolic functions such as catalysis &
transport function
 Examples of fibrous proteins:
keratin/fibrin/collagen/actin/myosin/silk protein
 Examples of globular protein:
insulin/immunoglobulin/hemoglobin/amylase

25. Revision Questions
 Explain how polar and non-polar amino acids help channel
proteins and enzymes carry out their functions. Distinguish
between fibrous and globular proteins with reference to
one example of each protein type.
 Explain primary structures and tertiary structures of a
protein.
 Draw the structure of a generalized dipeptide.
 Draw the basic structure of an amino acid, and label the
groups that are used in peptide bond formation.
 List four functions of membrane proteins.
 Giving a named example of each, state four functions of
proteins