X-RAY CRYSTALLOGRAPHIC STUDY OF PYRIDOXAL 5'-PHOSPHATE-TYPE ASPARTATEAMINOTRANSFERASES FROM ESCHERICHIA-COLI IN OPEN AND CLOSED-FORM

We determined the three-dimensional structures of aspartate aminotrans ferase (AspAT) from Escherichia coli and its complex with inhibitor (2 -methyl-L-aspartate) at 1.8 Angstrom resolution. This enzyme reversibl y catalyzes the transamination reaction and is a dimer of two identica l subunits. Each subunit has 396 amino acid residues and one pyridoxal 5'-phosphate as a cofactor, and is divided into two domains, one larg e and the other small. Upon binding of the inhibitor, the small domain rotates by 5 degrees toward the large domain to close the active site . This domain movement is caused mainly by small but important main-ch ain conformational changes in the residues located over the domain int erface of the small domain. In chicken mitochondrial AspAT, the domain movement was larger, with a rotational angle of 13 degrees. By compar ison of these two structures, the difference in the rotational angles was found to be caused by the larger opening of the domain in the open form of chicken mitochondrial AspAT. Although the overall structures of these two enzymes were almost identical, the surface area of the do main interface in the E. coli enzyme was larger than that in mitochond rial AspAT, suggesting that the structure of the domain interface is r esponsible for the degree of movement of the small domain.