Transcobalamin is a transport protein that carries vitamin B12. This vitamin plays an important role as a cofactor for various enzymes, especially those that function in the metabolism of amino acids.
What is transcobalamin?
Transcobalamin is a globulin. It is also known as R-binder protein or haptocorrin. Globulins are transport proteins in the human body. They are divided into four groups according to protein size. These are the α1-globulins, the α2-globulins, the β-globulins and the γ-globulins. The transcobalamin describes several globulins, namely the transcobalamin I (TCN I), II (TCN II) and III (TCN III). The most important globulins here are transcobalamin I and II. TCN I is also described as haptocorrin. This is a β-globulin. TCN II, on the other hand, is an α1-globulin. In general, however, the transcobalamines are referred to as α1-globulins. Thyroxine-binding globulin and transcortin also belong to this group. The α2-globulins include hemoglobin-binding globulin and α2-macroglobulin. The β-globulins include globulins that are responsible for the transport of lipids. And γ-globulins include those that are essential components of the body’s immune response.
Function, action, and roles
Transcobalamin I, also called haptocorrin, serves to protect and transport vitamin B12. It is found in saliva, where it directly binds vitamin B12 that has been ingested with food and has already been at least partially dissolved out of food in the mouth. Transcobalamin I binds the vitamin B12 and protects it in the stomach from the aggressive stomach acid. In the duodenum, the first section of the small intestine, transcobalamin I is separated from vitamin B12. The vitamin B12 can then bind to the respective enzymes for which it serves as a cofactor. Transcobalamin II, on the other hand, binds vitamin B12 that has been taken up by enterocytes. Enterocytes are fringing cells that make up part of the small intestinal epithelium. Their function is to absorb certain substances from food during digestion. Vitamin B12 is also known as cobalamin. It serves mainly as a cofactor for two reactions. It plays an important role in the breakdown of the amino acids isoleucine, valine, thymine, threonine and methionine, and in the breakdown of fatty acids with an odd number of carbons. In this process, succinyl-CoA is formed from methylmalonyl-CoA. Vitamin B12 is the cofactor of the enzyme methylmalonyl-CoA mutase in this process. Vitamin B12 also plays a role in folic acid metabolism, hematopoiesis, blood formation, and myelin synthesis. Myelin is a protein that surrounds and protects the axons of nerve cells. In folic acid metabolism it is the cofactor of the enzyme methionine synthase. After transcobalamin binds vitamin B12, so it is transported through the blood to the liver and other organs.
Formation, occurrence, properties, and optimal levels
Transcobalamin I is synthesized by the cephalic gland. It has a molecular mass of 48.2 kDa and consists of 433 amino acids. Transcobalamin II, on the other hand, has a size of 47.5 kDa and consists of 427 amino acids. The gene for transcobalamin I is encoded on chromosome 11, while the gene for transcobalamin II is located on chromosome 22. Both globulins bind and transport vitamin B12 to the various organs. The daily requirement of vitamin B12 is 2 to 3 µg. The liver can store about 2 mg of vitamin B12. The normal value of vitamin B12 is 233 to 1,132 pg/ml.
Diseases and disorders
In the case of transcobalamin II, it is known that mutations of the gene can occur. Then, transcobalamin II is no longer functional and vitamin B12 deficiency occurs. This deficiency leads to anemia, a disorder during the formation of blood. This is also called anemia. There is a decrease in hemoglobin. This hemoglobin is used to transport oxygen in the blood. It is an essential component of red blood cells. These are also known as erythrocytes. A lack of oxygen occurs. The organs are not sufficiently supplied with oxygen. This is accompanied by symptoms of fatigue, reduced ability to exercise such as during sports activities, and headaches. In addition, vitamin B12 deficiency can also lead to neurological damage. One example of this is funicular myelosis.Degeneration of part of the white matter of the spinal cord and lymphatic tissue. These structures are also called the posterior cord and the lateral cord. The reason for vitamin B12 deficiency is usually not nutritional. It is mainly due to a disturbance in the absorption capacity of vitamin B12. This can be caused by disorders of the gastric mucosa. In the case of deficient vitamin B12, methylmalonylaziduria can also occur. In this case, there is not enough vitamin B12 as a cofactor for the enzyme methylmalonyl-CoA mutase. This leads to a malfunction of amino acid metabolism and the breakdown of fatty acids with an odd number of carbons. Methylmalonyl-CoA is not converted to succinyl-CoA. Accumulation of methylmalonyl-CoA in the blood occurs. It is then released through the urine. Another symptom of vitamin B12 deficiency is an impaired immune response. This occurs because of a lowering of immunoglobulin.
