Human ornithine transcarbamylase: crystallographic insights into substraterecognition and conformational changes

Two crystal structures of human ornithine transcarbamylase (OTCase) complex ed with the substrate carbamoyl phosphate (CP) have been solved. One struct ure, whose crystals were prepared by substituting N-phosphonacetyl-L-ornith ine (PALO) liganded crystals with CP, has been refined at 2.3 Angstrom (1 A ngstrom = 0.1 nm) resolution to a crystallographic R factor of 18.4%. The s econd structure, whose crystals were prepared by co-crystallization with CP , has been refined at 2.6 Angstrom resolution to a crystallographic R facto r of 20.2%. These structures provide important new insights into substrate recognition and ligand-induced conformational changes. Comparison of these structures with the structures of OTCase complexed with the bisubstrate ana logue PALO or CP and L-norvaline reveals that binding of the first substrat e, CP, induces a global conformational change involving relative domain mov ement, whereas the binding of the second substrate brings the flexible SMG loop, which is equivalent to the 240s loop in aspartate transcarbamylase, i nto the active site. The model reveals structural features that define the substrate specificity of the enzyme and that regulate the order of binding and release of products.