Classification of proteins:

I- According to Shape:

i- Fibrous: in which polypeptide chains are arranged along a long axis forming to give fibers or sheets. The fibrous proteins are tough and water insoluble. e.g. collagen, keratin, elastin and actin of muscles. They usually play a protective or supportive role (structural proteins). They are usually used to construct connective tissue, tendons, bones and muscle fibers.

ii- Globular: polypeptide chains tightly folded into compact spherical or globular shape. Most are soluble in water. e.g. hemoglobin and plasma proteins (proteins present in blood plasma) such as albumin, different types of globulins. Enzymes and hormones are globular proteins.

Classification of proteins

I- Plasma proteins:

including albumin, all types of globulins (α, β and γ)

All the plasma proteins are synthesized in liver except for antibodies (γ-

globulins). So determination of plasma proteins especially albumin is

used as test for liver function. A low level is indicative for poorly

functional liver (liver disease).

NB: Other liver function test also includes determination of liver

enzymes.

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.

- α1- antitrypsin: See later

- thyroxin-binding globulin which carry thyroid hormones in blood α2 globulin: e.g. - hepatoglobin: protein that binds hemoglobin to prevent its excretion by the kidney

- 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, ….)

They are five classes G, M, A, E and D.

Antibodies (Immunoglobulins (Igs)A- Definition:

• Immunoglobulins are a group of proteins (gamma globulins)

produced by the body from B lymphocytes and plasma cells in

response to presence of foreign bodies (antigens).

• There are 5 basic types: G, M, A, E and D.

• All of immunoglobulins have a similar basic structure.

B- Basic structure of immunoglobulins:

• The basic unit of all Igs molecules consists of 4 polypeptide

chains linked by disulfide bonds:

a) 2 polypeptide chains of low molecular weight called light chains (L)

b) 2 polypeptide chains of high molecular weight called heavy chains

(H)

Types of antibodies:

Ig G: Ig G is the most important antibodies in fighting bacteria and

virus

a) Ig G is the major Ig in serum:70-75% of serum Ig is IgG

b) Ig G is composed of one basic unit (monomer), i.e of low Mol.

weight

c) Placental transfer: IgG is the only class of Ig that crosses the

placenta. In addition, Ig G is also transferred from mother to baby in

breast milk. Transplacental and breast milk-derived Ig G offer the

baby a passive immunologic protection until the endogenous Ig G is

produced. The half life of Ig G is about 30 days and with prolonged

breast feeding the infant can get additional protection.

IgA1. Structure - IgA is a monomer in serum but found in secretions as a dimer as presented in the Figure. In diameric IgA, the molecules are joined by a small polypeptide chain called J chain linked to Fc regions.

- When IgA is found in secretions:, it also has another protein associated with it called the secretory piece. The secretory piece helps IgA to be transported across mucosa and also protects it from degradation in the secretions.

2. Propertiesa) IgA is the 2nd most common serum Ig (10-15%).b) IgA is the major class of Ig in secretions - tears, saliva, colostrum, mucus, secretions of GITIt protects against infection of mucus membranes that lining mouth, GIT and air ways. c) can’t pass placenta

Ig M

1.StructureIgM normally exists as a pentamer. So it has the highest molecular weight. The five units are linked by small polypeptide chain called J chain2. Propertiesa) IgM is the third most common serum Ig (5-10% of total Ig).b) IgM is the first Ig be made by a virgin B cells when it is stimulated by antigen. Then other classes are synthesized.c) It can’t cross placenta

Ig E1. Structure: IgE exists as a monomer. 2. Properties:a) is the least common Ig in serum

b) It is largely responsible for immunity against parasites . Since

serum IgE levels rise in parasitic diseases, measuring IgE levels is

helpful in diagnosing parasitic infections.

c) It plays an important role in allergic response to allergens (antigen

causes allergy or hypersensitivity such as dust, cat hair or pollens). Ig

E, in the presence of alleregen, binds to mast cells and basophils

causing release of histamine and other substances from mast cells.

These substances result in allergic manifestations.