13.13 Folic Acid

Tetrahydrofolate (THF) is the coenzyme required by enzymes which catalyze the transfer of a group having a single carbon to their substrates. The one carbon may be methyl (CH₃), methylene (CH₂) or formyl (CH) group.  The coenzyme THF is required for the synthesis of bases in RNA and DNA and synthesis of some aromatic amino acids. Folic acid is the vitamin used for the synthesis of the coenzyme THF. For example, thymidylate synthase is the enzyme that catalyzes the synthesis of T’s from U’s. This transformation requires N⁵,N¹⁰-methylene-THF as a coenzyme (Scheme 6).

Scheme 6

13.14 Vitamin K

• Vitamin K is synthesized by intestinal bacteria.
• Vitamin K is required for proper clotting of blood.  In order for blood to clot, blood-clotting proteins must bind to Ca²⁺.
• Vitamin K is synthesized by intestinal bacteria. Thus, deficiency of vitamin K is rare.
• Vitamin KH₂ is the coenzyme that is derived from vitamin K.
• The coenzyme vitamin KH₂ is required by enzyme that catalyzes the carboxylation of the γ-carbon of glutamate side chains in proteins, forming γ-carboxyglutamates (Scheme 7).
• The process uses CO₂ for the carboxyl group that is introduced in to the glutamate chains.
• The protein involved in the blood clotting contains several glutamates near the N-terminal ends.
• γ-Carboxyglutamate makes stronger complex with Ca²⁺ compared to that of glutamate.

Scheme 7