Mechanism of disproportionation of asparate transcarbamoylase molecules lacking one regulatory subunit

A cooperative Escherichia coli aspartate transcarbamoylase without regulatory subunits

Here we report the isolation, kinetic characterization, and X-ray structure determination of a cooperative Escherichia coli aspartate transcarbamoylase (ATCase) without regulatory subunits. The native ATCase holoenzyme consists of six catalytic chains organized as two trimers bridged noncovalently by six regulatory chains organized as three dimers, c(6)r(6). Dissociation of the native holoenzyme produces catalytically active trimers, c(3), and nucleotide-binding regulatory dimers, r(2). By introducing specific disulfide bonds linking the catalytic chains from the upper trimer site specifically to their corresponding chains in the lower trimer prior to dissociation, a new catalytic unit, c(6), was isolated consisting of two catalytic trimers linked by disulfide bonds. Not only does the c(6) species display enhanced enzymatic activity compared to the wild-type enzyme, but the disulfide bonds also impart homotropic cooperativity, never observed in the wild-type c(3). The c(6) ATCase was crystallized in the presence of phosphate and its X-ray structure determined to 2.10 A resolution. The structure of c(6) ATCase liganded with phosphate exists in a nearly identical conformation as other R-state structures with similar values calculated for the vertical separation and planar angles. The disulfide bonds linking upper and lower catalytic trimers predispose the active site into a more active conformation by locking the 240s loop into the position characteristic of the high-affinity R state. Furthermore, the elimination of the structural constraints imposed by the regulatory subunits within the holoenzyme provides increased flexibility to the c(6) enzyme, enhancing its activity over the wild-type holoenzyme (c(6)r(6)) and c(3). The covalent linkage between upper and lower catalytic trimers restores homotropic cooperativity so that a binding event at one or so active sites stimulates binding at the other sites. Reduction of the disulfide bonds in the c(6) ATCase results in c(3) catalytic subunits that display kinetic parameters similar to those of wild-type c(3). This is the first report of an active c(6) catalytic unit that displays enhanced activity and homotropic cooperativity.

Association of the catalytic subunit of aspartate transcarbamoylase with a zinc-containing polypeptide fragment of the regulatory chain leads to increases in thermal stability

The regulatory enzyme aspartate transcarbamoylase (ATCase), comprising 2 catalytic (C) trimers and 3 regulatory (R) dimers, owes its stability to the manifold interchain interactions among the 12 polypeptide chains. With the availability of a recombinant 70-amino acid zinc-containing polypeptide fragment of the regulatory chain of ATCase, it has become possible to analyze directly the interaction between catalytic and regulatory chains in a complex of simpler structure independent of other interactions such as those between the 2 C trimers, which also contribute to the stability of the holoenzyme. Also, the effect of the interaction between the polypeptide, termed the zinc domain, and the C trimer on the thermal stability and other properties can be measured directly. Differential scanning microcalorimetry experiments demonstrated that the binding of the zinc domain to the C trimer leads to a complex of markedly increased thermal stability. This was shown with a series of mutant forms of the C trimer, which themselves varied greatly in their temperature of denaturation due to single amino acid replacements. With some C trimers, for which tm varied over a range of 30 degrees C due to diverse amino acid substitutions, the elevation of tm resulting from the interaction with the zinc domain was as large as 18 degrees C. The values of tm for a variety of complexes of mutant C trimers and the wild-type zinc domain were similar to those observed when the holoenzymes containing the mutant C trimers were subjected to heat denaturation.(ABSTRACT TRUNCATED AT 250 WORDS)