The B12 Vitamin is a cofactor for 2 enzymes: Methionine synthase and Methylmalonyl mutase.
Methionine synthase is an important enzyme in 1-carbon metabolism that uses methylcobalamin as its cofactor and converts homocysteine to methionine by adding a methyl group. Methionine then is converted to other compounds that serve as methyl donors, as shown below1.
Figure 11.211: One-carbon metabolism
These methyl donors can donate methyl groups for DNA, an epigenetic modification1.
This enzyme uses as its cofactor, and is important in the breakdown of odd chain fatty acids (5 carbons etc.). As you know, odd chain fatty acids are less common than even chain fatty acids, but this enzyme is required to properly handle these less common fatty acids1.
In addition to its role as a cofactor for enzymes, vitamin is also important for preventing degradation of the myelin sheath that surrounds neurons, as shown below.
Figure 11.212: Vitamin B12 is needed to maintain the myelin sheath that surrounds neurons. Image used with permission (CC BY-SA 3.0;).
The mechanism through which vitamin prevents demyelination is not known3.