Properties and mechanism of d-glucosaminate-6-phosphate ammonia-lyase: An aminotransferase family enzyme with d-amino acid specificity

Salmonella enterica serovar Typhimurium utilizes a wide range of growth substrates, some of which are relatively novel. One of these unusual substrates is d-glucosaminate, which is metabolized by the enzymes encoded in the dga operon. d-Glucosaminate is transported and converted to d-glucosaminate-6-phosphate (G6P) by a phosphotransferase system, composed of DgaABCD. The protein product of dgaE, d-glucosaminate-6-phosphate ammonia lyase (DGL), converts G6P to 2-keto-3-deoxygluconate-6-phosphate, which undergoes a retroaldol reaction catalyzed by the DgaF protein to give d-glyceraldehyde-3-phosphate and pyruvate. We have now developed an improved synthesis of G6P which gives a higher yield. The DGL reaction is of mechanistic interest because it is one of only a few enzymes in the pyridoxal-5′-phosphate (PLP) dependent aminotransferase superfamily known to catalyze reaction of a d-amino acid substrate. The pH dependence of DGL shows an optimum at 7.5–8.5, suggesting a requirement for a catalytic base. α-Glycerophosphate and inorganic phosphate are weak competitive inhibitors, with K_i values near 30 mM, and d-serine is neither a substrate nor an inhibitor. We have found in rapid-scanning stopped-flow experiments that DGL reacts rapidly with its substrate to form a quinonoid intermediate with λ_max = 480 nm, within the dead time (ca. 2 msec), which then rapidly decays (k = 279 s^? 1) to an intermediate with absorption between 330 and 350 nm, probably an aminoacrylate complex. We suggest a mechanism for DGL and propose that the unusual stereochemistry of the DGL reaction requires a catalytic base poised on the opposite face of the PLP-substrate complex from the other members of the aminotransferase superfamily.