Aspartate Transcarbamoylase (ATCase) from E. coli.
The enzyme has two catalytic trimers and three regulatory dimers--12 subunits in all. The inititial display shows the Backbone of the unliganded enzyme (T-state) in the "top" view. The catalytic subunits are colored green; the regulatory dimers are colored red (cf. Fig. 6.3 in Campbell).
The numbered "Highlight" buttons display "side" views of the T- and R-states of ATCase. The T-state catalytic trimers are green; the regulatory dimers are red.
The R-state catalytic trimers are blue; the regulatory dimers are yellow. The following paragraphs briefly describe the displays.
1. Separate Structures: "Side" views are shown. The R-state catalytic trimers move apart about 12 Å. The regulatory dimers rotate. The R-state structure has the substrate analogs, phosphonoacetamide (PAM) and malonic acid (MAL), bound in the active sites of the catalytic subunits
2. Superimposed Structures: The T-state structure (no bound ligands) is compared to three R-state structures. All of the R-state structures have PAM and MAL bound in the active sites. The CTP or ATP, when present, are bound to the regulatory subunits. The ligands are Spacefill, colored CPK.
The relatively small conformational changes caused by ATP or CTP binding to the R-state hide the changes that occur in the active sites. These changes cause activation (ATP) or inhibition (CTP) of ATCase.
Note: The coordinates used in these displays have only the alpha carbons of the proteins (CA). Thus, Backbone and Spacefill are the only Chime display options available. The original PDB coordinates (1at1.pdb - 8at1.pdb) have atomic coordinates for all subunits.
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