Crystal structures of carbamate kinase from Giardia lamblia bound with citric acid and AMP-PNP

PLoS One. 2013 May 20;8(5):e64004. doi: 10.1371/journal.pone.0064004. Print 2013.


The parasite Giardia lamblia utilizes the L-arginine dihydrolase pathway to generate ATP from L-arginine. Carbamate kinase (CK) catalyzes the last step in this pathway, converting ADP and carbamoyl phosphate to ATP and ammonium carbamate. Because the L-arginine pathway is essential for G. lamblia survival and absent in high eukaryotes including humans, the enzyme is a potential target for drug development. We have determined two crystal structures of G. lamblia CK (glCK) with bound ligands. One structure, in complex with a nonhydrolyzable ATP analog, adenosine 5'-adenylyl-β,γ-imidodiphosphate (AMP-PNP), was determined at 2.6 Å resolution. The second structure, in complex with citric acid bound in the postulated carbamoyl phosphate binding site, was determined in two slightly different states at 2.1 and 2.4 Å resolution. These structures reveal conformational flexibility of an auxiliary domain (amino acid residues 123-170), which exhibits open or closed conformations or structural disorder, depending on the bound ligand. The structures also reveal a smaller conformational change in a region associated the AMP-PNP adenine binding site. The protein residues involved in binding, together with a model of the transition state, suggest that catalysis follows an in-line, predominantly dissociative, phosphotransfer reaction mechanism, and that closure of the flexible auxiliary domain is required to protect the transition state from bulk solvent.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Adenylyl Imidodiphosphate / chemistry*
  • Amino Acid Sequence
  • Catalytic Domain
  • Citric Acid / chemistry*
  • Crystallography, X-Ray
  • Giardia lamblia / enzymology*
  • Hydrogen Bonding
  • Models, Chemical
  • Molecular Sequence Data
  • Phosphotransferases (Carboxyl Group Acceptor) / chemistry*
  • Protein Binding
  • Protein Structure, Secondary
  • Protozoan Proteins / chemistry*


  • Protozoan Proteins
  • Adenylyl Imidodiphosphate
  • Citric Acid
  • Phosphotransferases (Carboxyl Group Acceptor)
  • carbamate kinase

Associated data

  • PDB/4JZ7
  • PDB/4JZ8
  • PDB/4JZ9