B vitamins are a group of water-soluble nutrients that are necessary for life. These must be obtained through dietary sources – they cannot be synthesized independently in the body. Although they are grouped together and often referred to as vitamin B complex, each of the eight B vitamins is characterized by an important, unique role in the body.
All of the B vitamins are cofactors that work with a cognate enzyme, or chemical precursors of a cofactor. Enzymes catalyze biochemical reactions in the body - through their action, reactions that would not normally be energetically favorable can occur. Cofactors are required for the functioning of some enzymes.
Crucial Metabolic Functions
Vitamin B3 (niacin) is a good example of an important cofactor in several enzymatic reactions. In the body, it is converted to nicotinamide adenine diphosphate (NAD), which has a reduced form known as NADH and an oxidized form known as NAD+. The central metabolic reaction of living cells is oxidative phosphorylation. NADH transfers an electron to the first protein in the electron transport chain of the mitochondria, NADH-coenzyme Q oxidoreductase. The electron is transferred along the chain, generating a proton gradient across the mitochondrial membrane. The established protein gradient drives the conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). ATP then becomes the energy source for subsequent metabolic activity in the cell.
Essential to Life
Vitamin B6 (pyridoxine) is another vitamin that serves a cofactor in many important enzymatic reactions.
- Synthesis of five neurotransmitters - serotonin, dopamine, epinephrine, norepinephrine, and gamma-aminobutyric acid (GABA)
- Breaking down amino acids
- Synthesis of histamine
- Synthesis of serine
- Synthesis of methionine and cysteine
- Conversion of selenium to selenomethionine
- Conversion of selenohomocysteine to hydrogen selenide, for incorporation into selenoproteins
- Conversion of tryptophan to niacin
- Transamination reactions producing amino acids for gluconeogenesis
- Lipid metabolism
- Hemoglobin synthesis
Vitamin B6 is so essential to life that a deficiency leads to a syndrome affecting widespread systems of the body. Fortunately, food sources of pyridoxine are plentiful and B6 deficiency is rare, even in developing countries.
Vitamin B2 (riboflavin) is a distinctive yellow-colored compound that serves as a coenzyme of a family of enzymes known as flavoproteins. Flavoproteins carry out oxidation and reduction reactions, including parts of the electron transport chain in the mitochondria.
B2 is also a cofactor in some reactions responsible for activating other vitamins. One of the active forms of riboflavin, flavin adenine dinucleotide (FAD), is required to convert pyridoxine (vitamin B6) to pyridoxic acid. The coenzyme form of Vitamin B6, pyridoxal phosphate, is also dependent on another active form of riboflavin, flavin mononucleotide (FMN), for its activity.
Forms of riboflavin are used in the conversion of retinol (vitamin A) to retinoic acid, in the synthesis of an active form of folate, and the conversion of tryptophan to niacin.
Two of the B complex vitamins, B9 and B12, work closely together in the body. Vitamin B12 (cobalamin) in one of its active forms, methylcobalamin, enables the function of methionine synthase, a vitamin B9 (folate)-dependent enzyme. Methionine is a critical amino acid in methylation of DNA, RNA, and proteins, an essential function for life. Folate is necessary for synthesis of DNA. Deficiencies of B9 and B12 can lead to DNA damage and cancer, in addition to many other symptoms of B complex vitamin deficiency.
Reviewed by Afsaneh Khetrapal, BSc (Hons)