structural communications
Carbamate kinase catalyzes the reversible conversion of carbamoyl phosphate and ADP to ATP and ammonium carbamate, which is hydrolyzed to ammonia and carbonate. The three-dimensional structure of carbamate kinase from the human parasite Giardia lamblia (glCK) has been determined at 3 Å resolution. The crystals belonged to the monoclinic space group P21, with unit-cell parameters a = 69.77, b = 85.41, c = 102.1 Å, β = 106.8°. The structure was refined to a final R factor of 0.227. The essentiality of glCK together with its absence in humans makes the enzyme an attractive candidate for anti-Giardia drug development. Steady-state kinetic rate constants have been determined. The kcat for ATP formation is 319 ± 9 s−1. The Km values for carbamoyl phosphate and ADP are 85 ± 6 and 70 ± 5 µM, respectively. The structure suggests that three invariant lysine residues (Lys131, Lys216 and Lys278) may be involved in the binding of substrates and phosphoryl transfer. The structure of glCK reveals that a glycerol molecule binds in the likely carbamoyl phosphate-binding site.