In enzymology, an adenosylcobinamide-phosphate guanylyltransferase () is an enzyme that catalyzes the chemical reaction
The two substrates of this enzyme are GTP and adenosylcobinamide phosphate; its two products are diphosphate and adenosylcobinamide-GDP.
This enzyme belongs to the family of transferases, specifically those transferring phosphorus-containing nucleotide groups (nucleotidyltransferases). The systematic name of this enzyme class is GTP:adenosylcobinamide-phosphate guanylyltransferase. Other names in common use include CobU, adenosylcobinamide kinase/adenosylcobinamide-phosphate, guanylyltransferase, and AdoCbi kinase/AdoCbi-phosphate guanylyltransferase. This enzyme is part of the biosynthetic pathway to cobalamin (vitamin B<sub>12</sub>) in bacteria.