Vitamin K2 synthesized by intestinal bacteria

Vitamin K occurs naturally in two forms.
*Phylloquinone or vitamin K1 (2-methyl-3-phytyl-1,4-naphtoquinone) is synthesized by plants.
*Menaquinones or vitamin K2 (multi-isoprenyl-quinones, several species) are primarily produced by bacteria.

Bacteria synthesize a range of vitamin K forms (but not vitamin K1) using repeating isoprene (5-carbon) units in the side chain of the molecule (vide infra). These forms of vitamin K are designated menaquinone-n (MK-n), where n stands for the number of 5-carbon units in the structure. Menaquinones (MK-n) are collectively referred to as vitamin K2.

Many bacteria that populate the microbial ecosystem of the human intestine synthesize menaquinones, which they utilize as redox reagents in electron transport and oxidative phosphorylation.

Vitamin K is taken in the diet or synthesized by the intestinal bacteria. Its absorption takes place along with fat (chylomicrons) and is dependent on bile salt. Vitamin K is transported along with LDL and is stored mainly in liver and, to a lesser extent, in other tissues.

Menaquinones play important roles in electron transport, oxidative phosphorylation, active transport, and endospore formation in bacteria. In addition to these functions, the variations in the inherent structures of menaquinones and their uneven distributions among bacteria are considered important in bacterial taxonomy.
Vitamin K2 synthesized by intestinal bacteria