Structural studies of duck delta2 crystallin mutants provide insight into the role of Thr161 and the 280s loop in catalysis.
Delta crystallin, a taxon-specific crystallin present in avian eye lenses, is homologous to the urea cycle enzyme ASL (argininosuccinate lyase). Although there are two delta crystallin isoforms in duck lenses, ddeltac1 (duck delta1 crystallin) and ddeltac2 (duck delta2 crystallin), only ddeltac2 is catalytically active. Previous structural studies have suggested that residues Ser283 and His162 in the multi-subunit active site of ddeltac2/ASL are the putative catalytic acid/base, while the highly conserved, positively charged Lys289 is thought to help stabilize the carbanion intermediate. The strict conservation of a small hydroxy-containing residue (Thr or Ser) at position 161 adjacent to the putative catalytic base, as well as its proximity to the substrate in the S283A ddeltac2 enzyme-substrate complex, prompted us to investigate further the role this residue. Structures of the active T161S and inactive T161D ddeltac2 mutants, as well as T161D complexed with argininosuccinate, have been determined to 2.0 A resolution. The structures suggest that a hydroxy group is required at position 161 to help correctly position the side chain of Lys289 and the fumarate moiety of the substrate. Threonine is probably favoured over serine, because the interaction of its methyl group with Leu206 would restrict its conformational flexibility. Residues larger than Thr or Ser interfere with substrate binding, supporting previous suggestions that correct positioning of the substrate's fumarate moiety is essential for catalysis to occur. The presence of the 280s loop (i.e. a loop formed by residues 270-290) in the 'open' conformation suggests that loop closure, thought to be essential for sequestration of the substrate, may be triggered by the formation of the carbanion or aci-carboxylate intermediates, whose charge distribution more closely mimics that of the sulphate ion found in the active-site region of the inactive ddeltac1. The 280s loop in ddeltac1 is in the closed conformation.
Structural Biology and Biochemistry, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8.
Liliana M
LM
Sampaleanu
Penelope W
PW
Codding
Yuri D
YD
Lobsanov
May
M
Tsai
G David
GD
Smith
Cathy
C
Horvatin
P Lynne
PL
Howell
D000595:Amino Acid Sequence / Q000235:genetics; D000818:Animals; D001125:Argininosuccinic Acid / Q000378:metabolism; D030161:Avian Proteins / Q000737:chemistry / Q000235:genetics; D001665:Binding Sites / Q000235:genetics; D003460:Crystallization / Q000379:methods; D018360:Crystallography, X-Ray / Q000379:methods; D004372:Ducks; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D016297:Mutagenesis, Site-Directed / Q000235:genetics; D009154:Mutation / Q000235:genetics; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics; D038207:delta-Crystallins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D030161:Avian Proteins; D020033:Protein Isoforms; D038207:delta-Crystallins; D001125:Argininosuccinic Acid