Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. The primary structure is the sequence of amino acids in the polypeptide chain. Secondary structure involves hydrogen bonding that forms alpha helices and beta pleats. Tertiary structure is the folding of the polypeptide chain, influenced by properties of amino acids, to form domains. Quaternary structure occurs when multiple polypeptide subunits combine to form the functional protein structure. Protein structure determines its function, as denaturation can change structure and function.

1. PROTEINS

2. PRIMARY STRUCTURE
 Sequence of amino acids
 Polypeptide chain
Protein

3. SECONDARY STRUCTURE
The folding of the N-C-
C backbone of the
polypeptide chain
using weak hydrogen
bonds
Protein

4. SECONDARY STRUCTURE
 This produces the alpha helix and beta pleating
 The length of the helix or pleat is determined by certain
amino acids that will not participate in these structures
(e.g. proline)
Protein

5. TERTIARY STRUCTURE
The folding of the polypeptide into domains
whose chemical properties are determined
by the amino acids in the chain
Protein

6. TERTIARY STRUCTURE
 This folding is sometimes held together by
strong covalent bonds
(e.g. cysteine-cysteine disulphide bridge)
 Bending of the chain takes place at certain
amino acids
(e.g. proline)
 Hydrophobic amino acids tend to arrange
themselves inside the molecule
 Hydrophilic amino acids arrange themselves
on the outside
Protein

7. QUATERNARY STRUCTURE
Some proteins are
made of several
polypeptide subunits
(e.g. haemoglobin has
four)
Protein Kinase C
Protein

8. QUATERNARY STRUCTURE
 These subunits fit together to form the
functional protein
 Therefore, the sequence of the amino
acids in the primary structure will influence
the protein's structure at two, three or
more levels
Protein

9. Protein

10. PROTEIN FUNCTIONS
 Protein structure determines protein
function
 Denaturation or inhibition which may
change protein structure will change its
function
 Coenzymes and cofactors in general may
enhance the protein's structure
Protein

11. Fibrous proteins
 Involved in structure: tendons ligaments
blood clots
(e.g. collagen and keratin)
 Contractile proteins in movement: muscle,
microtubules
(cytoskelton, mitotic spindle, cilia, flagella)
Protein

12. Globular proteins
 most proteins which move around (e.g.
albumen, casein in milk)
 Proteins with binding sites:
enzymes, haemoglobin, immunoglobulins,
membrane receptor sites
Protein

13. Proteins classified by function
 CATALYTIC: enzymes
 STORAGE: ovalbumen (in eggs), casein (in milk), zein
(in maize)
 TRANSPORT: haemoglobin
 COMMUNICATION: hormones (eg insulin) and
neurotransmitters
 CONTRACTILE: actin, myosin, dynein (in microtubules)
 PROTECTIVE: Immunoglobulin, fibrinogen, blood
clotting factors
 TOXINS: snake venom
 STRUCTURAL: cell membrane proteins, keratin (hair),
collagen
Protein

14.  Conjugated Proteins Yield amino acids and nonprotein
products upon hydrolysis
 Glyco- or mucoproteins : Proteins plus carbohydrates; e.g. mucin
of saliva
 Lipoproteins : Proteins plus a lipid; e.g. liporutellin of egg yolk
 Chromoproteins : Proteins plus a pigmented prosthetic group e.g.
hemoglobin, myoglobin
 Metalloproteins : Proteins plus a metal element such as iron,
magnesium, copper, or zinc; e.g. ferritin (Fe), tyrosine oxidase (Cu),
alcohol dehydrogenase (Zn).
 Nucleoproteins : Proteins plus nucleic acid; e.g. nucleohistone
 Phosphoproteins : Phosphoric acid is the prosthetic group; e.g.
Casein (milk) Protein

15.  Derived Proteins: Derivatives of proteins due to action of heat,
enzymes, or chemical reagents.
 Primary Derived
I. Coagulated Proteins
II. Proteans
III. Metaproteins
 Secondary Derived
Protein

16.  Complete protein:
A complete protein contains essential amino acids to
maintain body tissues and to promote a normal rate of growth and is
referred as having a high biological value. Examples are egg, milk and meat
(including poultry and fish) proteins wheat germ and dried yeast have a
biological value approaching that of animal source.
 Partially complete proteins:
They will maintain life, but lack sufficient amounts of some of
the essential amino acids necessary for growth. Adults under no
physiological stress can maintain satisfactory nutrition for indefinite period
when consuming sufficient amount of protein from certain cereals or
legumes- gladden of wheat.
 Totally incomplete proteins:
These type of proteins are incapable of replacing or building
new tissues and hence cannot support life and promote growth. Zein in corn
and gelatin are examples of this type.
Nutritional classification of Proteins
Protein