proteins- simple and conjugated, fibrous and globular, collagen, elastin, keratin, myoglobin, hemoglobin etc.

2. Simple and conjugated proteins
Simple proteins Conjugated proteins
Ex. Serum, albumin, contain only
amino acids
Consists of simple proteins combined
with a non protein component
The non protein component is called a
prosthetic group.
A conjugated protein without its
prosthetic group is called apoprotein
Apoprotein combined with its prosthetic
group referred to as holoprotein

3. Conjugated proteins
 These are further classified according to
the nature of their prosthetic groups
 ex. Glycoprotein contains a
carbohydrate component, lipoprotein
contains lipid molecules and
metalloproteins contain metal ions,
phosphoproteins contain phosphate
groups and hemoproteins posses heme
groups

4. Class
Glycoprotein Fibronectin
Cadherin
Lipoproteins Chylomicron
High density lipoprotein (HDL)
Metalloproteins Ferritin (iron)
Alcohol dehydrogenase (zinc)
Cytochrome oxidase (copper and iron )
Nitrogenase (molybdenum and iron)
Hemoproteins Hemoglobin (transport of oxygen in blood)
Myoglobin (storage of oxygen in muscle)
Cytochrome c (involvement in electron
transport chain )
Cytochrome P450 (hydroxylation of
xenobiotics)
Catalase (degradation of hydrogen
peroxide)

5. Fibrous and globular
proteins
Fibrous proteins Globular proteins
Long, rod shaped molecules that
are insoluble in water and
physically tough
ex. Keratins have structural and
protective functions
These proteins usually consists
largely of a single type of
secondary structures
Are compact spherical molecules
that are usually water- soluble.
These proteins often contains
several types of secondary
structure
The non polar residues Val, Leu,
Ile, Met and Phe largely occur in
the interior of a protein, out of
contact with the aqueous solvent .
The charged polar residues Arg,
His, Lys, Asp and Glu are largely
located on the surface of the
protein in the contact with the
aqueous solvent. Uncharged polar
residues Ser, Thr, Asn, Gln and Tyr
are usually present on the protein
surface

6. Functions of proteins
Function Example
Structural Collagen, keratin, fibrin
catalytic Enzymes such as lysosomes,
proteases, polymerase, kinase
Membrane transport Glucose transporter, aquaporin
Cell recognition MHC proteins
Osmotic regulation Serum albumin
Gene regulation Lac repressor
Hormonal Insulin, vasopressin, oxytocin
Transport throughout the body Hemoglobin
Ion binding Ferritin, calmodulin
Contraction Actin, myosin
Defense Antibodies, snake venom

7. Collagen
 Major structural protein in the extracellular matrix
 Most abundant proteins in vertebrates
 A typical collagen molecule is long, inelastic, stiff, triple stranded
helical structure
 Fundamental unit- tropocollagen
 Tropocollagen consists of 3 coiled polypeptides called α-chains and
are left handed polypeptide helices
 α-chains are synthesized on membrane bound ribosomes and enter
into the lumen of the endoplasmic reticulum. In lumen of ER selected
proline and lysine residues are hydroxylated to form hydroxyproline
and hydroxylysine and some of hydroxylysine residues are
glycosylated
 Each α-chains then combines with 2 others to form a hydrogen
bonded, triple- stranded helical molecule known as tropocollagen(or
collagen molecule)
 Tropocollagen is secreted into extracellular space. After secretion the
tropocollagens assemble in the extracellular space to form collagen
fibrils

9. Elastin
 Is highly hydrophobic connective tissue protein that is
responsible for extensibility and elasticity
 Is a 2nd major protein In the extracellular matrix, which is
the main component of elastic fibers found in ligaments,
large arteries and lungs
 After synthesis, a 72kDa molecule of soluble
tropoelastin is secreted into the matrix.
 This protein is rich in nonpolar amino acid and unusually
rich in proline and glycine.
 After secretion, the tropoelastin molecules become
highly cross-linked to one another, generating an
extensive network of elastin fibers. after secretion from
cell, certain lysyl residues of tropoelastin are oxidatively
deaminated to aldehydes by lysyl oxidase. The
condensation of 3 of these lysine- derived aldehydes
with an unmodified lysine results in formation of tetra
functional cross link called desmosine

11. Keratin
 Are fibrous proteins present in
eukaryotes.
 Keratins are classified as either α-
keratin or β-keratin
Proteins α-keratin β-keratin
Characteristics Tough, insoluble Soft, flexible
Conformation Helical Extended chain
Basic unit Protofibril Antiparallel β-pleated
sheet

12. Myoglobin
 A globular protein, contains a single polypeptide chain of 153 amino
acid residues and a single heme group
 The inside of myoglobin consists of almost exclusively of non polar
residues, whereas the outside contains both polar and non polar
residues.
 Heme- globin of Mb binds a single heme group by forming a co-
ordinate bond. The heterocyclic ring system of heme is a porphyrin
derivative. The porphyrin in heme is known as protoporphyrin IX.
 It is made up of 4-pyrrole ring and 4-pyrroles are linked by methine
bridges to form a tetrapyrrole ring. The Fe atom is present either in
Fe2+ or Fe3+ oxidation state in the center of the protoporphyrin IX ring
 Only myoglobin in Fe2+ state can bind O2. the iron atom has 6 co-
ordinate bond, 4 in the plane of flat porphyrin ring and 2 perpendicular
to it. The iron atom of the heme is directly bonded to one of the
histidine called proximal histidine of globin protein. It is 93rd amino acid
residue in helix F. the O2- binding site is present on the other side of
the heme plane, at the 6th coordination position. A 2nd histidine residue ,
termed as distal histidine makes H-bond with oxygen molecules. O2
binds directly to the iron atom of the heme only

14. Hemoglobin, Hb
 Is an oligomeric, allosteric, conjugated protein with 4
polypeptide chains joined by non- covalent bonds. It contains
heme prosthetic group. Heme is iron-containing porphyrin
derivative.
 Hemoglobin is present in all vertebrates except arctic fish and in
many invertebrates including annelids, many arthropods and
some molluscs
 A variety of hemoglobin molecules are produced in humans. All
are tetramers consisting of numerous combinations of 7 distinct
polypeptide chains, each encoded by separate gene. HbA1,
which consists 2 α and 2 β-chains represents about 98%of all
hemoglobin found in adult RBCs. α-chains contain 141 amino
acid residues and β-chains contains 146 amino acid residues.
Thus HbA1 is designated as α2 β2. the remaining 2% consists
of HbA2 a minor adult hemoglobin. This molecule contains 2α
and 2δ chains and is designated as 2α 2δ . The δ-chain is very
similar to β– chain

15. Hemoglobin, Hb
 Role of hemoglobin
 Facilitates O2transport
 Facilitates CO2transport
 Has an important role as a buffer
 Transports NO (the ferrous O2 binding
site in hemoglobin also bind nitric oxide
(NO), and an additional NO binding site
is present on the β–chains)