15. Biosynthetic pathways using tetrahydrofolate and vitamin B12

Cobalamin-dependent methylation reactions
15.4.1 Structure of methylcobalamin Up
Structure of methylcobalamin

Cobalamin is a coenzyme with a complex structure. Its corrin ring coordinates a single cobalt ion, which is central to the ability of cobalamin to facilitate the cleavage and formation of carbon bonds by way of radical intermediates.

The two major forms of cobalamin are adenosylcobalamin and methylcobalamin. Shown here is methylcobalamin, which is formed as an intermediate in the synthesis of methionine from homocysteine (see below). The methyl group is obtained from N5-methyl-THF. In this chapter, we only deal with methylcobalamin-dependent reactions. Adenosylcobalamin is needed in the utilization of propionyl-CoA (see slide 10.3.6); it is not considered in detail in these notes.

15.4.2 The S-adenosylmethionine (SAM) cycle requires vitamin B12 Up
The S-adenosylmethionine (SAM) cycle requires vitamin B12

In the SAM cycle, methionine is first activated to S-adenosylmethionine (SAM), which then serves as a methyl group donor in various biosynthetic reactions. Methyl group donation leaves behind S-adenosylhomocysteine, which is then cleaved to adenosine and homocysteine. The regeneration of methionine from homocysteine requires vitamin B12 in the form of methylcobalamin.

15.4.3 Structures of S-adenosylmethionine and S-adenosylhomocysteine Up
Structures of S-adenosylmethionine and S-adenosylhomocysteine

The key feature of SAM is the methyl group attached to a sulfonium ion (S+). The inherent instability of the sulfonium group makes this compound a good methyl group donor.

S-adenosylhomocysteine (SAH) is not only the byproduct of SAM-dependent methylation reactions but also a competitive inhibitor. It must therefore be recycled promptly in order to avoid disruption of the SAM cycle. In vitamin B12 deficiency, the regeneration of SAM from SAH is impeded, and the accumulating SAH interferes with subsequent methylation reactions [1]Author: Weir, D G;Scott, J M
Title: The biochemical basis of the neuropathy in cobalamin deficiency
Journal: Baillieres Clin Haematol
Pages: 479-97
Volume: 8
Year: 1995
ISBN: 0950-3536
.

15.4.4 SAM-dependent methylation reactions Up
  1. methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC)
  2. guanidinoacetate → creatine
  3. norepinephrine → epinephrine
  4. acetylserotonin → melatonin
  5. DNA methylation
  6. methylation of drugs (e.g. mercaptopurine)

Among these pathways, the biosynthesis of PC and of creatine are quantitatively the most important ones.

15.4.5 Phosphatidylethanolamine methylation Up
Phosphatidylethanolamine methylation

Phosphatidylcholine (PC) is a major constituent of mammalian cell membranes. It is formed in three successive SAM-dependent methylation steps from phosphatidylethanolamine (PE).

PC can also be synthesized using dietary choline, and conceivably this pathway might supply enough PC for myelin synthesis. However, in animal experiments, depletion of vitamin B12 has been shown to decrease PC and to increase PE in the brain [2]Author: van der Westhuyzen, J;Cantrill, R C;Fernandes-Costa, F;Metz, J
Title: Effect of a vitamin B-12-deficient diet on lipid and fatty acid composition of spinal cord myelin in the fruit bat
Journal: J Nutr
Pages: 531-7
Volume: 113
Year: 1983
ISBN: 0022-3166
, suggesting that PE methylation is important for sufficient PC synthesis.

15.4.6 Sphingomyelin acquires its phosphocholine headgroup from PC Up
Sphingomyelin acquires its phosphocholine headgroup from PC

Sphingomyelin, another important membrane phospholipid, is synthesized from its precursor ceramide through phosphocholine transfer from PC; therefore, its synthesis indirectly also depends on the SAM cycle and vitamin B12.

15.4.7 Major nerve fibers are myelinated Up
Major nerve fibers are myelinated

Myelin sheaths consist of multiple layers of cell membranes, which contain more phospholipids and less protein than most other cell membranes. These elaborate structures enable the saltatory conduction of action potentials, which is much faster than the pedestrian non-saltatory conduction that occurs along non-myelinated fibers. The picture shows a cross section through a single nerve fiber; the thick, dark zone surrounding the axon is the myelin, which consists of multiple stacked membrane bilayers.*The number of stacked bilayers surrounding the axon in this picture is approximately 20, but it can be substantially higher in the fastest-conducting axons.

Cobalamin deficiency causes demyelination or nerve fibers in the central and peripheral nervous system. The neurological, and in advanced stages neuropsychiatric, consequences can be severe. In addition to the disruption of PC and sphingomyelin biosynthesis, deficient methylation of myelin-associated proteins has been considered as another pathogenic mechanism [1]Author: Weir, D G;Scott, J M
Title: The biochemical basis of the neuropathy in cobalamin deficiency
Journal: Baillieres Clin Haematol
Pages: 479-97
Volume: 8
Year: 1995
ISBN: 0950-3536
.