Crystal structure of archaeal highly thermostable L-aspartate dehydrogenase/NAD/citrate ternary complex

Abstract

The crystal structure of the highly thermostable l-aspartate dehydrogenase (l-aspDH; EC 1.4.1.21) from the hyperthermophilic archaeon Archaeoglobus fulgidus was determined in the presence of NAD and a substrate analog, citrate. The dimeric structure of A. fulgidus l-aspDH was refined at a resolution of 1.9 Å with a crystallographic R-factor of 21.7% (Rfree = 22.6%). The structure indicates that each subunit consists of two domains separated by a deep cleft containing an active site. Structural comparison of the A. fulgidus l-aspDH/NAD/citrate ternary complex and the Thermotoga maritima l-aspDH/NAD binary complex showed that A. fulgidus l-aspDH assumes a closed conformation and that a large movement of the two loops takes place during substrate binding. Like T. maritima l-aspDH, the A. fulgidus enzyme is highly thermostable. But whereas a large number of inter- and intrasubunit ion pairs are responsible for the stability of A. fulgidus l-aspDH, a large number of inter- and intrasubunit aromatic pairs stabilize the T. maritima enzyme. Thus stabilization of these two l-aspDHs appears to be achieved in different ways. This is the first detailed description of substrate and coenzyme binding to l-aspDH and of the molecular basis of the high thermostability of a hyperthermophilic l-aspDH. © 2007 The Authors.