In enzymology, a precorrin-8X methylmutase (EC 5.4.99.61) is an enzyme that catalyzes the chemical reaction Hence, this enzyme has one substrate, precorrin 8X, and one product, hydrogenobyrinate.

This enzyme belongs to the family of isomerases, specifically those intramolecular transferases transferring other groups.

Other names in common use include precorrin isomerase, hydrogenobyrinic acid-binding protein and CobH.

This enzyme is part of the biosynthetic pathway to cobalamin (vitamin B12) in aerobic bacteria.

As of late 2007, 6 structures have been solved for this class of enzymes, with PDB accession codes 1F2V, 1I1H, 1OU0, 1V9C, 2AFR, and 2AFV.