In enzymology, a hydrogenobyrinic acid a,c-diamide synthase (glutamine-hydrolysing) () is an enzyme that catalyzes the chemical reaction
The four substrates of this enzyme are ATP, hydrogenobyrinic acid, L-glutamine, and H<sub>2</sub>O; its four products are ADP, phosphate, hydrogenobyrinic acid a,c-diamide, and L-glutamate.
This enzyme belongs to the family of ligases, specifically those forming carbon-nitrogen bonds carbon-nitrogen ligases with glutamine as amido-N-donor. The systematic name of this enzyme class is hydrogenobyrinic-acid:L-glutamine amido-ligase (AMP-forming). This enzyme is also called CobB and is part of the biosynthetic pathway to cobalamin (vitamin B<sub>12</sub>) in aerobic bacteria.