The particular arrangement of catalytic and regulatory subunits in this enzyme affords the complex with strongly allosteric behaviour with respect to its substrates.

The discussion of structure, catalytic center, and allosteric site that follows is based on the prokaryotic version of ATCase, specifically E. coli's.

The ATCase holoenzyme is made of two catalytic trimers that are in contact and held together by three regulatory dimers, so the native form of the enzyme contains six chains of each type, with a total molecular weight of 310 kDa.

[8] The three-dimensional arrangement of the catalytic and regulatory subunits involves several ionic and hydrophobic stabilizing contacts between amino acid residues.

[9] This compound is a strong inhibitor of ATCase and has a structure that is thought to be very close to that of the transition state of the substrates.

[11] These studies, in addition to investigations using site-directed mutagenesis of specific amino acids, have identified several residues that are crucial for catalysis, such as Ser52, Thr53, Arg54, Thr55, Arg105, His134, Gln137, Arg167, Arg229, Glu231, and Ser80 and Lys84 from an adjacent catalytic chain.

[7] In general, the rate enhancement of ATCase is achieved by orientation and stabilization of substrates, intermediates, and products rather than by direct involvement of amino acid residues in the catalytic mechanism.

[3] Comparison of the crystal structures of the T and R forms of ATCase show that it swells in size during the allosteric transition, and that the catalytic subunits condense during this process.

The overall outcome of these structural changes is that the two domains of each catalytic chain come closer together, ensuring a better contact with the substrates or their analogues.

Studies have confirmed that the position of the 240s loop directly affects substrate binding in the corresponding active site.

[16] The regulatory and catalytic subunits exist as fused protein homologs, providing strong evidence that they would interact together.