Plasma proteins are the proteins of blood plasma. They differ from serum proteins primarily in that they contain clotting factors. Plasma proteins perform numerous tasks in the organism and can be affected by deficiency symptoms in the context of various diseases.
What are plasma proteins?
By plasma proteins, physicians mean the proteins of the blood plasma, also known as blood protein. Plasma is distinguished from blood serum by its clotting factors, which are also plasma proteins. In total, blood plasma contains about one hundred different proteins and glycoproteins. For every 100 milliliters of blood plasma, the proteins account for about six to eight grams. The term serum protein must be distinguished from plasma proteins. Serum proteins are all blood proteins minus the clotting factor fibrinogen. Plasma proteins can be split into albumins and globulins via electrophoresis. This means that the proteins of the blood plasma divide into albumins and globulins as charged colloidal parts or molecules when they migrate across an electric field. These two groups are present in plasma in an approximate 40 to 60 percent ratio.
Anatomy and structure
Globulins are either α1-, α2-, γ-, or β-globulins. The electrophoretic mobility of these four subgroups is their most important distinguishing feature. In addition to about four percent α1-globulins, plasma contains about eight percent α2-globulins and twelve percent β-globulins. At 16 percent, γ-globulins make up the largest proportion of blood plasma. The biosynthesis of plasma proteins takes place mainly in the liver and lymph. Glycoproteins are translated by posttranslational modification. Glycosyl residues bind in their active form to nucleoside diphosphate. Glycosyl transferases bind them to proteins. Like all proteins, plasma proteins are biological macromolecules of amino acids. Globular proteins are almost spherical in quaternary or tertiary structure. More than 100 amino acids are linked into chains in proteins. The proteins of the blood plasma are also called spheroproteins. They are easily dissolved in water and salt solution.
Function and tasks
Plasma proteins perform versatile tasks in the human body. On the one hand, they maintain colloid osmotic pressure, which in turn plays a role in maintaining plasma volume. The pH value of the blood is also maintained by plasma proteins. Apart from this, blood proteins have a transport function. They therefore transport water-insoluble substances through the body and are therefore also called carrier proteins. The transport of hormones and enzymes also takes place on carrier proteins of the blood plasma. Plasma proteins such as fibrinogen, which help with homeostasis, are particularly irreplaceable for blood clotting. In addition, plasma proteins play important roles in immune system processes, such as inflammation. In this context, there is also talk of immunoglobulins or antibodies, which are formed in response to antigens. Immunoglobulins recognize foreign bodies and bind to these antigens to destroy them. The α1-globulins mainly include transcortin, which is responsible for the transport of steroids. The α1-antitrypsin inhibits protease. The same is true for α1-antichymotrypsin. The plasma protein HDL is a carrier protein for blood lipids. Prothrombin acts as a proenzyme of thrombin, and transcobalamin transports cobalamin through the bloodstream. The α2-globulins include haptoglobin, which binds and transports hemoglobin. α2-Macroglobulin and α2-Antithrombin inhibit blood clotting, while Caeruloplasmin transports copper. The β-globulins include transferrin, which is responsible for the transport of iron. β-Lipoprotein transports blood lipids, while fibrinogen is known as a blood clotting factor. Hemopexin is a final β-globulin and binds free heme. Immunoglobulins belong to the fifth globulin group, whose components are also known as γ-globulins.
Diseases
Dysproteinemias involve shifts in the quantitative ratio of blood proteins. This phenomenon can be either congenital or acquired. Acquired dysproteinemias can be caused by acute infections, for example. In this case, the proportion of albumins decreases and the proportion of globulins increases. This phenomenon can also occur with major blood loss or after surgery.A distinction must be made between these acquired forms of dysproteinemia and a congenital maldistribution, as is the case with alpha-1-antitrypsin deficiency. Due to a genetic defect, too little alpha-1-antitrypsin is produced. A genetically caused deficiency of individual plasma proteins is also referred to as defective proteinemia. Paraproteinemia is to be distinguished from this. In the context of this disease, certain immunoglobulins or chains of immunoglobulins are formed in increased amounts. Such processes occur, for example, in Waldenstrom’s disease, a malignant lymphoma disease in which the lymphoma cells overproduce immunoglobulin M. This is also the case in multiple myeloma. In multiple myeloma, there is also an overconcentration of immunoglobulins. In this cancer of the bone marrow, antibody-producing cells proliferate in the blood plasma. These degenerate plasma cells produce an excess of antibodies or antibody fragments. Moreover, in connection with plasma proteins, both hypoproteinemia and hyperproteinemia may occur. In the former phenomenon, the concentration of plasma proteins falls below 66 grams per liter. In hyperproteinemia, on the other hand, the concentration exceeds 83 grams per liter. The cause of hypoproteinemia may be, for example, liver damage or malnutrition. Hyperproteinemias, on the other hand, are usually associated with inflammatory processes and may occur, for example, in the setting of tuberculosis.
