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 acidse.g. albumin and globulins

ii- complex (conjugated) proteins: protein combined with non-protein 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 parte.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 theprotein (Disrubtion of secondary, tertiary and/ or quaternarystructure). 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 bondsDenaturation can be:Reversible If the denatured protein returns to its native state afterthe denaturing agent is removed, the process is calledrenaturationIrreversible: 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