36. Vitamins

Last updated on January 11, 2020 at 12:03


  • The last topic
  • Lipid-soluble vitamins can only be absorbed with fat, and are transported in lipoproteins
  • Water-soluble vitamins mainly function as enzyme co-factors
  • Only vitamin D and niacin can be synthesized in the body
  • Flashcards here

The B-vitamins

Thiamine, or B1, is phosphorylated to become TPP, an enzyme important in oxidative decarboxylation. It’s needed for transketolase, and the three dehydrogenase complexes.

  • Branched chain α-keto acid dehydrogenase complex
  • Pyruvate decarboxylase
  • Transketolase
  • Pyruvate dehydrogenase complex
  • α-ketoglutarate dehydrogenase complex

Riboflavin, or B2, is converted to FAD and FMN. FAD is needed for:

  • Acyl-CoA dehydrogenase
  • Pyruvate dehydrogenase complex
  • α-ketoglutarate dehydrogenase complex
  • Succinate dehydrogenase
  • +++

Niacin, or B3, can actually be synthesized from tryptophan, so it’s not really a vitamin. It’s converted to NAD and NADP. It also regulates intracellular calcium regulation.

Pantothenate, or B5, is a part of CoA and ACP.

Pyridoxine, or B6, is converted into PLP, an enzyme important in amino acid metabolism. PLP is needed for

  • Glycogen phosphorylase
  • Alanine aminotransferase
  • Aspartate aminotransferase
  • Serine dehydratase
  • Cystathione beta-synthase
  • Tyrosine aminotransferase
  • Glutamate decarboxylase
  • Histidine carboxylase

Biotin, or B7, is not converted to anything but straight out used as a coenzyme for carboxylase reactions. It’s produced by bacteria in the intestines. Biotin is needed for

  • Pyruvate carboxylase
  • Propionyl-CoA carboxylase
  • Acetyl-CoA carboxylase

Folic acid, or B9, is converted into THF, a co enzyme in reactions that transfer one-carbon segments. THF is needed for:

  • Glycine cleavage enzyme/glycine synthase
  • Methionine synthase
  • Serine hydroxymethyltransferase

Cobalamin, or B12, is a coenzyme in one-carbon segment transfer, like folic acid. A vitamin B12 deficiency causes a folic acid deficiency, as B12 is needed for folic acid metabolism. Vitamin B12 is needed for:

  • Methylmalonyl-CoA mutase
  • Methionine synthetase

Vitamin C

Vitamin C, or ascorbic acid, is a cofactor for some hydroxylases. More specifically, it’s a cofactor for proline hydroxylase and lysine hydroxylase, both involved in collagen synthesis. Proline hydroxylase is also involved in O2-sensing by HIF-1α. It’s also an antioxidant, and it increases absorption of iron. Vitamin C is needed for:

  • Proline hydroxylase
    • HIF-1α
    • Collagen synthesis
  • Lysine hydroxylase
    • Collagen synthesis

The lipid-soluble vitamins

Vitamin D can be synthesized from cholesterol in the skin, upon exposure to UV-light. It is converted into calcitriol by hydroxylation in the liver and the kidney. Calcitriol increases absorption of calcium, reduces excretion of calcium in the kidneys, while mobilizing Ca2+ in the bones.

It also binds to its nuclear receptor to regulate gene expression.

Vitamin E does not have precisely defined functions; we don’t really know what it’s for. It inhibits PKC and acts as an anti-oxidant.

Vitamin K is needed for synthesis of blood clotting proteins. It’s also involved in post-translational processing of proteins.

Vitamin A is essential for vision. It’s also a hormone that binds to nuclear receptors and regulates gene expression.

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