ABSTRACT
The type II transmembrane serine protease dipeptidyl peptidase IV (DPPIV), also known as CD26 or adenosine deaminase binding protein, is a major regulator of various physiological processes, including immune, inflammatory, nervous, and endocrine functions. It has been generally accepted that glycosylation of DPPIV and of other transmembrane dipeptidyl peptidases is a prerequisite for enzyme activity and correct protein folding. Crystallographic studies on DPPIV reveal clear N-linked glycosylation of nine Asn residues in DPPIV. However, the importance of each glycosylation site on physiologically relevant reactions such as dipeptide cleavage, dimer formation, and adenosine deaminase (ADA) binding remains obscure. Individual Asn-->Ala point mutants were introduced at the nine glycosylation sites in the extracellular domain of DPPIV (residues 39-766). Crystallographic and biochemical data demonstrate that N-linked glycosylation of DPPIV does not contribute significantly to its peptidase activity. The kinetic parameters of dipeptidyl peptidase cleavage of wild-type DPPIV and the N-glycosylation site mutants were determined by using Ala-Pro-AFC and Gly-Pro-pNA as substrates and varied by <50%. DPPIV is active as a homodimer. Size-exclusion chromatographic analysis showed that the glycosylation site mutants do not affect dimerization. ADA binds to the highly glycosylated beta-propeller domain of DPPIV, but the impact of glycosylation on binding had not previously been determined. Our studies indicate that glycosylation of DPPIV is not required for ADA binding. Taken together, these data indicate that in contrast to the generally accepted view, glycosylation of DPPIV is not a prerequisite for catalysis, dimerization, or ADA binding.
Subject(s)
Adenosine Deaminase/metabolism , Dipeptidyl Peptidase 4/metabolism , Alanine/metabolism , Amino Acid Sequence , Amino Acid Substitution , Binding Sites , Catalysis , Chromatography, Gel , Crystallography, X-Ray , Dimerization , Dipeptidyl Peptidase 4/chemistry , Dipeptidyl Peptidase 4/genetics , Dipeptidyl Peptidase 4/isolation & purification , Disulfides , Glycosylation , Humans , Kinetics , Models, Molecular , Molecular Sequence Data , Point Mutation , Protein Folding , Protein Structure, Tertiary , Protein Transport , Sequence Homology, Amino Acid , Substrate SpecificityABSTRACT
UDP-N-acetylmuramic acid:L-alanine ligase (MurC) catalyzes the addition of the first amino acid to the cytoplasmic precursor of the bacterial cell wall peptidoglycan. The crystal structures of Haemophilus influenzae MurC in complex with its substrate UDP-N-acetylmuramic acid (UNAM) and Mg(2+) and of a fully assembled MurC complex with its product UDP-N-acetylmuramoyl-L-alanine (UMA), the nonhydrolyzable ATP analogue AMPPNP, and Mn(2+) have been determined to 1.85- and 1.7-A resolution, respectively. These structures reveal a conserved, three-domain architecture with the binding sites for UNAM and ATP formed at the domain interfaces: the N-terminal domain binds the UDP portion of UNAM, and the central and C-terminal domains form the ATP-binding site, while the C-terminal domain also positions the alanine. An active enzyme structure is thus assembled at the common domain interfaces when all three substrates are bound. The MurC active site clearly shows that the gamma-phosphate of AMPPNP is positioned between two bound metal ions, one of which also binds the reactive UNAM carboxylate, and that the alanine is oriented by interactions with the positively charged side chains of two MurC arginine residues and the negatively charged alanine carboxyl group. These results indicate that significant diversity exists in binding of the UDP moiety of the substrate by MurC and the subsequent ligases in the bacterial cell wall biosynthesis pathway and that alterations in the domain packing and tertiary structure allow the Mur ligases to bind sequentially larger UNAM peptide substrates.
