FIGURE 2.

Citrate and selected alternate substrates in binding site 1. Substrate is shown as a stick figure with carbon atoms in magenta and oxygen atoms red. Binding site residues are shown with carbon atoms in green, oxygen atoms red, nitrogen atoms blue, and sulfur atoms yellow. Ionic hydrogen bonds are shown as dotted orange lines. A, citrate in site 1 of the Cys-less control CTP binds to three positively charged side chains. B, citrate in the K83C mutant binds to two positively charged side chains. C, malate in the K83C mutant can bind to two positively charged side chains, as could citrate. D, citrate in the R189C mutant binds to two positively charged side chains. E, malate in the R189C mutant can bind to two positively charged side chains, as could citrate. F, pyruvate in the R189C mutant can bind to two positively charged side chains, as could citrate.