Does BHMT convert homocysteine to methionine using betaine and zinc?

Betaine-homocysteine S-methyltransferase (BHMT) serves as a major alternative, folate-independent pathway for the remethylation of homocysteine, operating primarily within hepatic and renal tissues.

Biochemical mechanism and substrate utilization

• Methyl donation: Unlike the vitamin B12- and folate-dependent methionine synthase pathway, BHMT utilizes betaine directly as its methyl donor.
• Reaction dynamics: The enzyme functions via an ordered bi-bi mechanism. Homocysteine binds first to the active site, inducing conformational changes that form the betaine-binding pocket.
• End products: The resulting transfer of the methyl group successfully converts toxic homocysteine into methionine while concurrently producing dimethylglycine (DMG).

Catalytic role of zinc as a cofactor

• Zinc coordination: BHMT is a zinc-dependent cytosolic metalloenzyme. Its catalytic $Zn^{2+}$ site is structurally coordinated by three key cysteine residues (Cys217, Cys299, and Cys300) and a tyrosine residue (Tyr77).
• Activation mechanism: Upon entering the active site, homocysteine coordinates directly with the zinc ion, displacing Tyr77. This interaction lowers the pKa of the homocysteine thiol group, generating a highly reactive thiolate nucleophile that attacks the methyl group of betaine. Loss or oxidation of these zinc-binding cysteines entirely inactivates the enzyme.

Bottom line

• Strong biochemical evidence validates that BHMT operates as a zinc-dependent, folate-independent enzyme that utilizes betaine to convert homocysteine to methionine and dimethylglycine.