Download Fibrous proteins

Classification of proteins:
I- According to Shape:
i- Fibrous proteins:
- Polypeptide chains are folded into filaments or
sheets (rod or thread-shaped chain).
-The fibrous proteins are water insoluble.
- Fibrous proteins are structural proteins usually
play a protective or supportive role. e.g. collagen,
keratin and elastin. They are usually used to
construct connective tissue, tendons, bones and
muscle fibers. They have unique (specific) structure
and amino acids sequence to be functional.
ii- Globular:
-polypeptide chains tightly folded into
compact spherical or globular shape.
- Most are soluble in water and biologically
active
- Examples on globular proteins are:
-Hemoglobin
-Myoglobin
-plasma proteins (proteins present in
blood plasma) such as albumin, different
types of globulins.
- Enzymes and protein hormones are
globular proteins.
II- Classification of proteins according to composition (chemistry)
i- Simple proteins: composed only of amino acids
e.g. albumin and globulins
ii- complex (conjugated) proteins: protein combined with nonprotein part
i- Simple proteins
e.g. albumin and all types of globulins (α, β and γ) are plasma proteins
i.e proteins present in blood plasma (fluid portion of the blood).
Albumin:
is a single polypeptide chain consists of 585 amino acids. In human
about 60% of plasma protein is albumin.
Functions of albumin:
It is the most abundant plasma protein in humans and other mammals.
Albumin is essential for maintaining the oncotic pressure (osmotic
pressure needed for proper distribution of body fluids between
intravascular compartments (capillaries) and body tissues).
It also acts as a plasma carrier, can transport many hydrophobic
substances such as free fatty acids, thyroid hormones, steroids,
bilirubin and some exogenous drugs and transport them into their target
tissues..
Types of globulins:
α1 globulin: e.g.
- thyroxin-binding globulin which carry thyroid hormones in blood
α2 globulin: e.g.
- Ceruloplasmin: Iron is absorbed in the form of ferrous but is
circulated in blood in the form of ferric. Ceruloplasmin is a plasma
protein that oxidize ferrous into ferric to help its transfer in blood.
β-globulin:
e.g. transferrin: protein that transport iron from blood to
cells
γ-globulins = Immunoglobulins (antibodies): responsible for
immunity.
They are produced from bone marrow by B lymphocytes in
response to the presence of antigen (foreign body such as
bacteria, virus, ….)
ii-Complex (Conjugated) proteins
Protein combined with non-protein part
e.g.
1- Phosphoproteins: These are proteins conjugated with phosphate
group. Phosphate is attached to OH group of serine or threonine.
2- Lipoproteins:
These are proteins conjugated with lipids.
Functions: help lipids to transport in blood
3- Glycoproteins:
proteins conjugated with sugar (carbohydrate)
e.g.
- Some hormones such as erythropoeitin, LH, FSH
- present in cell membrane structure
- blood groups that are present on the surface of RBCs ( A, B,
O)
4- Nucleoproteins: These are basic proteins called histones
conjugated with nucleic acid (DNA or RNA).
e.g. a- chromosomes: are proteins conjugated with DNA
b- Ribosomes: are proteins conjugated with RNA
5- Metalloproteins: These are proteins conjugated with metal like
iron, copper, zinc, ……
a- Iron-containing proteins:
- hemoglobin (Hb),
- myoglobin (protein of skeletal muscles and cardiac muscle),
NB: Hb and myoglobin are hemeproteins that bind O2
- Ferritin: Main store of iron in the body in non toxic form,
because free iron is toxic and oxidize cells (form reactive oxygen
species). Ferritin is present in liver, spleen and bone marrow. The
amount of ferritin stored reflects the amount of iron stored.
Ferritin releases iron to areas where it is required.
- Hemosidrin: another iron store. (report hemosidrosis)
-Transferrin: is the iron carrier protein in plasma.
hemoglobin
Ferritin
b- Copper containing proteins:
e.g. - Ceruloplasmin which oxidizes ferrous ions into ferric ions.
c- Mg containing proteins:
e.g. Kinases and phosphatases.
Kinases are enzymes that add phosphate to a molecule.
Phoshatases are enzymes that remove phosphate from a molecule.
Protein denaturation
Proteins can be denatured by agents that cause unfolding of the
protein (Disrubtion of secondary, tertiary and/ or quaternary
structure). Primary structure is not affected.
Agents:
Heat: affects the hydrogen bonding
PH: affects the ionic bonding
Detergents: affects the Hydrophobic interactions
Reducing agents: affects the disulfide bonds
Denaturation can be:
Reversible If the denatured protein returns to its native state
after
the denaturing agent is removed, the process is called
renaturation
Irreversible: A cooked egg cannot be “uncooked”
Functions of proteins
1.Catalytic function : Enzymes
2. Transport function : e.g.
Hemoglobin transport oxygen from lung to
Tissues
Albumin: general carrier
3. Storage function : e.g.
Myoglobin stores oxygen in muscle cells
4. Defense function : e.g.
Clotting proteins: prevent loss of blood
Immunoglobulins: protect against bacteria
5. Regulatory function : e.g.
Protein hormones and their receptors
6. Contractile function : e.g.
Actin and myosin in muscles
7. Structural function : e.g.
Collagen: present in skin and bones
Elastin: present in lungs, wall of blood vessels
Keratin: present in hair, nails, skin