36. Vitamins

Last updated on November 19, 2018 at 17:16

Summary

  • The last topic
  • Vitamins are organic compounds that we require through diet
  • Many of them are converted to important cofactors
  • 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.

Riboflavin, or B2, is converted to FAD and FMN. FAD is needed in many dehydrogenase reactions.

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. It’s also required for glycogen phosphorylase.

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.

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

Cobalamin, or B12, is a coenzyme in one-carbon segment transfer, like folic acid. It’s needed in methylmalonyl-CoA mutase. A vitamin B12 deficiency causes a folic acid deficiency, as B12 is needed for folic acid metabolism.

The other water-soluble vitamin

Vitamin C, or ascorbic acid, is a cofactor for some hydroxylases. Notably, it’s needed in collagen synthesis and O2-sensing by HIF-1α. It’s also an antioxidant, and it increases absorption of iron.

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 enhance 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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