cytidine 5'-{[hydroxy(2-hydroxyethyl)phosphonoyl]phosphate} + methylcob(III)alamin + S-adenosyl-L-methionine = cytidine 5'-({hydroxy[(S)-2-hydroxypropyl]phosphonoyl}phosphate) + cob(III)alamin + 5'-deoxyadenosine + L-methionine — reaction