cobyrinate a,c-diamide synthase;
cobyrinic acid a,c-diamide synthetase;
CbiA (gene name)

Ligases;
Forming carbon-nitrogen bonds;
Carbon-nitrogen ligases with glutamine as amido-N-donor

2 ATP + cobyrinate + 2 L-glutamine + 2 H2O = 2 ADP + 2 phosphate + cobyrinate a,c-diamide + 2 L-glutamate (overall reaction) [RN:R05815];
(1a) ATP + cobyrinate + L-glutamine + H2O = ADP + phosphate + cobyrinate c-monamide + L-glutamate [RN:R09598];
(1b) ATP + cobyrinate c-monamide + L-glutamine + H2O = ADP + phosphate + cobyrinate a,c-diamide + L-glutamate [RN:R09599]

R05815 R09598 R09599

ATP [CPD:C00002];
cobyrinate [CPD:C05773];
L-glutamine [CPD:C00064];
H2O [CPD:C00001];
cobyrinate c-monamide [CPD:C19724]

This enzyme is the first glutamine amidotransferase that participates in the anaerobic (early cobalt insertion) biosynthetic pathway of adenosylcobalamin, and catalyses the ATP-dependent synthesis of cobyrinate a,c-diamide from cobyrinate using either L-glutamine or ammonia as the nitrogen source. It is proposed that the enzyme first catalyses the amidation of the c-carboxylate, and then the intermediate is released into solution and binds to the same catalytic site for the amidation of the a-carboxylate.
The Km for ammonia is substantially higher than that for L-glutamine.

1  [PMID:15311923]

Fresquet V, Williams L, Raushel FM

Mechanism of cobyrinic acid a,c-diamide synthetase from Salmonella typhimurium LT2.

Biochemistry. 43 (2004) 10619-27.

Salmonella typhimurium LT2 [GN:stm]