In enzymology, a hydrogenobyrinic acid a,c-diamide synthase (glutamine-hydrolysing) (EC 6.3.5.9) is an enzyme that catalyzes the chemical reaction

2 ATP + hydrogenobyrinic acid + 2 L-glutamine + 2 H₂O ${\displaystyle \rightleftharpoons }$ 2 ADP + 2 phosphate + hydrogenobyrinic acid a,c-diamide + 2 L-glutamate

The four substrates of this enzyme are ATP, hydrogenobyrinic acid, L-glutamine, and H₂O; 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 B₁₂) in aerobic bacteria.

• Cobalamin biosynthesis

• Debussche L, Thibaut D, Cameron B, Crouzet J, Blanche F (1990). "Purification and characterization of cobyrinic acid a,c-diamide synthase from Pseudomonas denitrificans". J. Bacteriol. 172 (11): 6239–44. PMC 526805. PMID 2172209.
• Warren MJ, Raux E, Schubert HL, Escalante-Semerena JC (2002). "The biosynthesis of adenosylcobalamin (vitamin B12)". Nat. Prod. Rep. 19 (4): 390–412. doi:10.1039/b108967f. PMID 12195810.