Blood plasma contains 8% solids, which has 7% albumin. The different plasma proteins are albumins, globulins, and fibrinogen. Usually, total plasma proteins are 6 to 8 gms / 100 ml with albumin, and the globin ratio is 1.7:1.

what are plasma proteins

The proteins present in human blood plasma are a mixture of simple proteins, glycoproteins, lipoproteins, and other conjugated proteins called “Plasma Protein. “Salt precipitation, immunological techniques, and electrophoresis may separate these.

Types of Plasma Proteins

The three significant plasma protein fractions are albumin, globulin, and fibrinogen. For a more satisfactory resolution by electrophoresis, these fractions are separated as follows:

  • Albumin – 55.2%
  • α1-Globulin – 5.3% (α1-Antitrypsin, TBG, Transcortin, etc.)
  • α2-Globulin – 8.6% (Haptoglobulin, ceruloplasmin, α2- macroglobulin, etc.)
  • β-Globulin – 13.4% (β1-transferrin, β-lipoprotein, etc)
  • ¥-Globulin – 11.0% (Antibodies, etc.)
  • Fibrinogen – 6.5%
plasma proteins seperation in electrophoresis

1. Albumin

It is the most abundant class of plasma protein (2.8 to 4.5 gm/100 ml) with the highest electrophoretic mobility. It is soluble in water and is precipitated by fully saturated ammonium sulfate. Albumin is synthesized in the liver and consists of a single polypeptide chain of 610 amino acids with a molecular weight of 69,000. The serum albumin consists of essential amino acids such as lysine, leucine, valine, phenylalanine, threonine, arginine, and histidine. The acidic amino acids like aspartic acid and glutamic acid are also concentrated in albumin.

albumin - type of plasma proteins

These residues make the molecule highly charged with a positive and negative charge. Besides having a nutritional role, albumin acts as a transport carrier for various biomolecules such as fatty acids, trace elements, and drugs. Another important function of albumin is maintaining osmotic pressure and fluid distribution between the blood and tissues.

2. Globulins

By electrophoresis, plasma globulins are separated into α1, α2, β, and ¥-globulins. These proteins are synthesized in the liver, whereas ¥-globulins are formed in the cells of the reticuloendothelial system. The average normal serum globulin (total) concentration is 2.5 gm/100 ml (Howe method) or 3.53 gm/100 ml by electrophoresis.

a. α1-Globulin

This fraction includes several complex proteins containing carbohydrates and lipids. These are orosomucoid, α1-glycoprotein, and α-lipoproteins. The normal serum level of α1-globulin is 0.42 gm/100 ml. Orosomucoid is rich in carbohydrates, is water-soluble, heat-stable, and has a molecular weight of 44,000. It serves to transport hexosamine complexes to tissues. Lipoproteins are soluble complexes that contain non-covalently bound lipids. These proteins act mainly as transport carriers for different lipids in the body.

b. α2-Globulins

This fraction also contains complex proteins such as α2-glycoproteins, plasminogen, prothrombin, haptoglobin, ceruloplasmin (transports Cu), and α2-macroglobulin. The normal serum value of this fraction is 0.67 gm/100 ml. Plasminogen and prothrombin are the inactive precursors of plasmin and thrombin, respectively, and both of these proteins play an important role in blood clotting.

Haptoglobulins are also glycoproteins with a molecular weight of 85,000. These are synthesized in the liver and can bind with any free hemoglobin that may arise in the plasma due to erythrocytes’ lysis, thus preventing the excretion of Hb and iron associated with it. Ceruloplasmin is a glycoprotein synthesized in the liver and is an essential component of copper metabolism in the body. Nearly 95% of plasma copper is bound to this protein.

c. β-Globulins

This fraction of plasma protein contains these different β-lipoproteins, which are very rich in lipid content. It also has transferrin (siderophilin), which transports non-heme iron in plasma. The normal serum value of β-globulins is 0.91 gm/100 ml. Transferrin is an iron transport protein, and it can be saturated with iron up to 33% in plasma. It has a low content of carbohydrates.

d. ¥-Globulins

These are also called Immunoglobulins and have antibody activity. Based on their electrophoretic mobility, they are classified as IgG, IgA, and IgM. Immunoglobulins are clinically significant components of globulins and are concerned with “immunological reactions.” Lymphocytes from these. Two different types of lymphocytes involved with immunoglobulin formation are “T-cells of Thymus” and “B-Cells” that originate from “bone marrow.”

3. Fibrinogen

It is a fibrous protein with a molecular weight of 340,000. It has six polypeptide chains which are held together by disulfide linkages. Thrombin converts fibrinogen into fibrin, which plays a vital role in the clothing of the blood. In addition to the proteins mentioned above, the plasma contains many enzymes, such as acid phosphatase and alkaline phosphatase, which have significant diagnostic value.

Functions of Plasma Proteins

Plasma protein is vital in our body. Here are the primary functions.

Protein Nutrition

It acts as a source of protein for the tissues whenever the need arises.

Osmotic pressure and water balance

  • It exerts an osmotic pressure of about 25 mm Hg and plays an essential role in maintaining a proper water balance between the tissues and blood. 
  • Plasma albumin is mainly responsible for this function due to its low molecular weight and quantitative dominance over other proteins. 
  • During protein loss from the body, as in kidney diseases, an excessive amount of water moves into the tissues, producing edema.

Buffering action

Plasma proteins help maintain the body’s pH by acting as ampholytes. At normal blood pH, they act as acids and accept captions.

Transport of Lipids

  • One of the essential functions of plasma proteins is to transport lipids and lipid-soluble substances throughout the body.
  • Fatty acids and bilirubin are transported mainly by Albumin, whereas cholesterol and phospholipids are carried by the lipoproteins present in β-globulins also transport fat-soluble vitamins (A, D, K, and E)

Transport of other substances

  • In addition to lipids, plasma proteins transport several metals and other substances. α2-Globulins transport copper (Ceruloplasmin), bound hemoglobin (haptoglobin), and thyroxine (glycoprotein), and non-heme iron is transported by transferrin present in the β-globulin fraction.
  • Calcium, magnesium, some drugs and dyes, and several cations and anions are transported by plasma albumin.

Blood Coagulation

Prothrombin is present in the α2-globulin fraction, and fibrinogen participates in blood clotting.

Prothrombin -> Thrombin (Presence–Thromboplastin)

Fibrinogen -> Fibrin (Clot) (Presence–Thrombin)

Frequently Asked Questions (FAQs)

  1. How Are Plasma Proteins Separated?

    The proteins are separated by using electrophoresis (mainly SDS-PAGE).

  2. Describe How Blood Proteins Maintain Osmotic Balance?

    The osmotic pressure of blood proteins=25mmHg (80% of the effect caused by albumin), so it is needed to maintain cell and interstitial fluid volume.

  3. How Do Plasma Proteins Act As Buffers?

    It can function as a buffer because it is responsible for 15% of the blood buffering potential (rest due to bicarbonate) and acts as a zwitterions dependent on pH.

  4. Which Of The Following Plasma Proteins Is Necessary For Blood Clotting To Occur?

    Fibrinogen is very important in the blood coagulation process.

  5. What Plasma Protein Do In The Blood, And What Percent Do They Make Up?

    They mediate the transport of lipids, hormones, vitamins, and metals and engage in immune surveillance, protease inhibitors, complement, and clotting proteins. It makes up 7% of the plasma.

What plasma protein is essential in body defense?

Blood ProteinsNormal level%Function
Albumins3.5-5.0 g/dl55%create and maintain oncotic pressure; transport insoluble molecules
Globulins2.0-2.5 g/dl38%participate in the immune system
Fibrinogen0.2-0.45 g/dl7%Blood coagulation
Regulatory proteins <1%Regulation of gene expression
Clotting factors <1%Conversion of fibrinogen into Fibrin

How much plasma protein in blood?

The protein level in Blood plasma is given below

Albumin3,500 – 4,500
Prealbumin (Transthyretin)10 – 40
α1-Globulins300 – 600
α1 Acid Glycoprotein (orosomucoid)55 – 140
α-Fetoproteinapprox. 0.001
Retinol Binding Protein3 – 6
Thyroxine Binding Globulin1 – 2
Transcortin3 – 3.5
α2 Globulins (excluding lipoproteins)400 – 900
Ceruloplasmin15 – 60
Haptoglobin Type 1-1100 – 220
Haptoglobin Type 2-1160 – 300
Haptoglobin Type 2-2120 – 260
α2 Macroglobulin100 – 280
β-Globulins (excluding lipoproteins)600 – 1,100
C-Reactive Protein< 1
Hemopexin50 – 100
β2 Microglobulinapprox. 0.2
Transferrin200 – 320
γ-Globulins700 – 1,500

Source: Serum Protein Fractions

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