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.
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%
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.
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.
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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.
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.
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.
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.
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.”
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.
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.
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.
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)
How Are Plasma Proteins Separated?
The proteins are separated by using electrophoresis (mainly SDS-PAGE).
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.
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.
Which Of The Following Plasma Proteins Is Necessary For Blood Clotting To Occur?
Fibrinogen is very important in the blood coagulation process.
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 Proteins||Normal level||%||Function|
|Albumins||3.5-5.0 g/dl||55%||create and maintain oncotic pressure; transport insoluble molecules|
|Globulins||2.0-2.5 g/dl||38%||participate in the immune system|
|Fibrinogen||0.2-0.45 g/dl||7%||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
|PLASMA PROTEIN||NORMAL RANGE (mg/dL)|
|Albumin||3,500 – 4,500|
|Prealbumin (Transthyretin)||10 – 40|
|α1-Globulins||300 – 600|
|α1 Acid Glycoprotein (orosomucoid)||55 – 140|
|Retinol Binding Protein||3 – 6|
|Thyroxine Binding Globulin||1 – 2|
|Transcortin||3 – 3.5|
|α2 Globulins (excluding lipoproteins)||400 – 900|
|Ceruloplasmin||15 – 60|
|Haptoglobin Type 1-1||100 – 220|
|Haptoglobin Type 2-1||160 – 300|
|Haptoglobin Type 2-2||120 – 260|
|α2 Macroglobulin||100 – 280|
|β-Globulins (excluding lipoproteins)||600 – 1,100|
|C-Reactive Protein||< 1|
|Hemopexin||50 – 100|
|β2 Microglobulin||approx. 0.2|
|Transferrin||200 – 320|
|γ-Globulins||700 – 1,500|
Source: Serum Protein Fractions