Bifunctional enzyme with a methyltransferase domain that catalyzes the ring contraction and methylation of C-17 in cobalt-factor III to form cobalt-factor IV, and an isomerase domain that catalyzes the conversion of cobalt-precorrin-8 to cobyrinate. Co(II)-factor III + H(+) + S-adenosyl-L-methionine = Co(II)-factor IV + S-adenosyl-L-homocysteine Co-precorrin-8X = cob(II)yrinate Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 3/10. Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 9/10. In the N-terminal section; belongs to the precorrin methyltransferase family. In the C-terminal section; belongs to the CobH family.