Tertiary and quaternary conformational changes in aspartate transcarbamylase: A normal mode study

Aspartate transcarbamylase (ATCase) initiates the pyrimidine biosynthetic p athway in Escherichia coli. Binding of aspartate to this allosteric enzyme induces a cooperative transition between the tensed (T) and relaxed (R) sta tes of the enzyme which involves large quaternary and tertiary rearrangemen ts. The mechanisms of the transmission of the regulatory signal to the acti ve site (60 PL away) and that of the cooperative transition are not known i n detail, although a large number of single, double, and triple site-specif ic mutants and chimeric forms of ATCase have been obtained and kinetically characterized. A previous analysis of the very low-frequency normal modes o f both the T and R state structures of ATCase identified some of the large- amplitude motions mediating the intertrimer elongation and rotation that oc cur during the cooperative transition (Thomas et al., J.Mol. Biol. 257: 107 0-1087, 1996; Thomas et al., J. Mol. Biol. 261:490-506, 1996). As a complem ent to that study, the deformation of the quaternary and tertiary structure of ATCase by normal modes below 5 cm(-1) is investigated in this article. The ability of the modes to reproduce the domain motions occurring during t he transition is analyzed, with special attention to the interdomain closur e in the catalytic chain, which has been shown to be critical for homotropi c cooperativity. The calculations show a coupling between the quaternary mo tions and more localized motions involving specific residues. The particula r dynamic behavior of these residues is examined in the Light of biochemica l results to obtain insights into their role in the transmission of the all osteric signal. (C) 1999 Wiley-Liss, Inc.