what is the Methylcobalamin ?

Methylcobalamin is the form of vitamin B12 active in the central nervous system. It is an active coenzyme of the vitamin B12 analogs, that is essential for cell growth and replication. The liver may not convert cyanocobalamin, the common supplemental form of vitamin B12, into adequate amounts of methylcobalamin the body may need for proper neuronal functioning. Methylcobalamin may exert its neuroprotective effects through enhanced methylation, acceleration of nerve cell growth, or its ability to promote healthy homocysteine levels. For methylcobalamin to be available to the brain, it should be allowed to dissolve under the tongue.

This vitamer is one of two active coenzymes used by vitamin B12-dependent enzymes and is the specific vitamin B12 form used by 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), also known as methionine synthase.
Methylcobalamin participates in the Wood-Ljungdahl pathway, which is a pathway by which some organisms utilize carbon dioxide as their source of organic compounds. In this pathway, methylcobalamin provides the methyl group that couples to carbon monoxide (derived from CO2) to afford acetyl-CoA. Acetyl-CoA is a derivative of acetic acid that is converted to more complex molecules as required by the organism.
Methylcobalamin is produced by some bacteria. It plays an important role in the environment. In the environment, it is responsible for the biomethylation of certain heavy metals.