Vitamin B-12 (Cobalamin): Basic Information on Molecular Structure and Metabolic Function

Vitamin B-12, also known as cobalamin, actually is a family of chemical compounds with a similar structure. The molecular structure common to all cobalamin compounds is a type of heterocyclic ring known as a corrin ring, which is similar to the porphyrin ring that is the basis of hemoglobin. But whereas most forms of the porphyrin ring bind an iron ion in the center, the corrin binds a cobalt ion (an atom of cobalt with an unbalanced electrical charge).

Forms of cobalamin include methylcobalamin, cyanocobalamin, adenosylcobalamin, and hydroxocobalamin. When an atom ion is attached to the corrin ring, four bonds are formed, each with a nitrogen atom on the ring. This leaves a fifth binding site on the cobalt. The type of cobalamin compound is determined by what chemical group is attached to the fifth binding site of the cobalt atom.

Methylcobalamin has a methyl group (CH3) attached to the fifth binding site of the cobalt atom. Cyanocobalamin has a cyanide group (CH), adenosylcobalamin has an adenosine (C10H13N5O4), and hydroxocobalamin has an hydroxide (OH) group attached to the fifth binding site of the cobalt.

Cabalamins all are water soluble. Methylcobalamin and adenosylcobalamin occur naturally and are produced by bacteria, including those in the human gut. Hydroxocobalamin also is produced by bacteria, but not inside the human body. However, such bacteria are used often to manufacture vitamin B-12. Cyanocobalamin is a synthetic form of vitamin B-12 that is included in many vitamin supplements and also is given therapeutically in higher doses for certain disorders.

When ingested into the body, cyanocobalamin and hydroxocobalamin are converted to methylcobalamin and adenosylcobalamin. Along with the vitamin B-12 produced in the gut by bacteria, cobalimins must bind with intrinsic factor which is produced by specialized cells of the stomach.

Cobalamins are involved in various metabolic reactions throughout the body, particularly in connection with the synthesis of DNA, which also is highly dependent on another compound, folic acid. Since the synthesis of blood cells in bone marrow requires an extremely high rate of DNA synthesis, one condition that can develop as when the body is deficient either in cobalamins or folic acid (or both) is megaloblastic anemia.

Megaloblastic anemia can be caused by a lack of vitamin B-12 or folic acid in the diet, but also may be caused by certain stomach ailments which interfere with the production of intrinsic factor. In the latter case, cobalimins cannot be absorbed through the intestines, resulting in a type of megaloblastic anemia known as pernicious anemia.