His-163 is a stereospecific proton donor in the mechanism of d-glucosaminate-6-phosphate ammonia-lyase.

d-Glucosaminate-6-phosphate ammonia-lyase (DGL) catalyzes the conversion of d-glucosaminate-6-phosphate to 2-keto-3-deoxyglutarate-6-phosphate, with stereospecific protonation of C-3 of the product. The crystal structure of DGL showed that His-163 could serve as the proton donor. H163A mutant DGL is fully active in the steady-state reaction, and the pre-steady-state kinetics are very similar to those of wild-type DGL. However, H163A DGL accumulates a transient intermediate with λmax at 293 nm during the reaction that is not seen with wild-type DGL. Furthermore, NMR analysis of the reaction of H163A DGL in D2 O shows that the product is a mixture of deuterated diastereomers at C-3. These results establish that His-163 is the proton donor in the reaction mechanism of DGL.