36. Vitamins

Vitamins are organic compounds which are necessary for life, and which the organism cannot synthesise by themselves. Many are cofactors in biochemical reactions, or prosthetic groups.

Two vitamins can be synthesised by the body, vitamin D and niacin. Vitamin K is synthesised by bacteria in the gut.

There are two types of vitamins, those which are lipid-soluble and those which are water-soluble.

Learning objectives

• What is the biological function of vitamin B1?
• What is the biological function of vitamin B2?
• What is the biological function of vitamin B3?
• What is the biological function of vitamin B5?
• What is the biological function of vitamin B6?
• What is the biological function of vitamin B7?
• What is the biological function of vitamin B9?
• What is the biological function of vitamin B12?
• What is the biological function of vitamin C?
• What is the biological function of vitamin A?
• What is the biological function of vitamin D?
• What is the biological function of vitamin K?

The B-vitamins

Vitamin B1

Thiamine, or vitamin B1, plays an important role in carbohydrate metabolism. It is phosphorylated to become TPP (thiamine diphosphate), an important cofactor.

TPP is a cofactor for oxidative decarboxylation reactions. It’s needed for transketolase, and the three dehydrogenase complexes.

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

Deficiency of thiamine can cause many diseases, including beriberi and Wernicke-Korsakoff syndrome.

Vitamin B2

Riboflavin, or vitamin B2, is involved in redox reactions. It is converted into two prosthetic groups, FAD and FMN. FAD is needed for:

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

Vitamin B3

Niacin, or B3, can actually be synthesized from tryptophan, so it’s not strictly a vitamin. It’s converted into the prosthetic groups NAD and NADP, which are involved in redox-reactions. It also regulates intracellular calcium concentration. NAD is also the source of ADP-ribose in ADP-ribosylation of proteins.

Niacin deficiency causes pellagra, a photosensitive inflammation of the skin.

Vitamin B5

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

Vitamin B6

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

PLP is also a cofactor for glycogen phosphorylase, but in this enzyme PLP has a completely different function than in the other enzymes.

Vitamin B7

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

Vitamin B9

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

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

Vitamin B12

Cobalamin, or B12, is a cofactor 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 A

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

Vitamin D

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

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

Vitamin K is a cofactor for post-translational carboyxylation of glutamate residues on proteins. Proteins involved in blood clotting are dependent on this modification. Deficiency of vitamin K causes decreased function of these blood clotting proteins.

Summary

• What is the biological function of vitamin B1?
  • Thiamine is the precursor of TPP, a cofactor for:
  • Transketolase
  • Branched chain α-keto acid dehydrogenase complex
  • Pyruvate dehydrogenase complex
  • α-ketoglutarate dehydrogenase complex
• What is the biological function of vitamin B2?
  • Riboflavin is the precursor of FAD and FMN, cofactors for:
  • Acyl-CoA dehydrogenase
  • Pyruvate dehydrogenase complex
  • α-ketoglutarate dehydrogenase complex
  • Succinate dehydrogenase
• What is the biological function of vitamin B3?
  • Niacin is the precursor of NAD and NADP
  • It also reagulates intracelluar calcium concentration
• What is the biological function of vitamin B5?
  • Pantothenate is the precursor of CoA and ACP
• What is the biological function of vitamin B6?
  • Pyridoxine is the precursor of PLP, a cofactor for
  • Glycogen phosphorylase
  • Alanine aminotransferase
  • Aspartate aminotransferase
  • Serine dehydratase
  • Cystathione beta-synthase
  • Tyrosine aminotransferase
  • Glutamate decarboxylase
  • Histidine carboxylase
  • Glycogen phosphorylase
• What is the biological function of vitamin B7?
  • Biotin is a cofactor for:
  • Pyruvate carboxylase
  • Propionyl-CoA carboxylase
  • Acetyl-CoA carboxylase
• What is the biological function of vitamin B9?
  • Folic acid is a cofactor for THF, a cofactor for:
  • Glycine cleavage enzyme/glycine synthase
  • Methionine synthase
  • Serine hydroxymethyltransferase
• What is the biological function of vitamin B12?
  • B12 is a cofactor for:
  • Methylmalonyl-CoA mutase
  • Methionine synthetase
• What is the biological function of vitamin C?
  • Ascorbate is a cofactor for some proline and lysine hydroxylases
  • It’s involved in collagen synthesis and O2-sensing by HIF-1α
• What is the biological function of vitamin A?
  • Vitamin A is involved in vision and cell differentiation
• What is the biological function of vitamin D?
  • Vitamin D is involved in calcium homeostasis
• What is the biological function of vitamin K?
  • Vitamin K is a cofactor for post-translational carboyxylation of glutamate residues on proteins
  • This is especially important in proteins involved in blood clotting