Structural models of WT CA and the W23F/V26I mutant illustrating contacts between the side chains of Trp23 and Val26 or the substituted Phe23 and Ile26 residues. (A) WT. Model number 10 in the 1GWP.pdb structure file of the N-terminal CA domain (Tang, Ndassa, and Summers, 2002) demonstrates the closest approach of the Trp23 (blue) and Val26 (magenta) side chains. Note that for clarity, the terminal methyl group of Met39 (yellow), which “hangs” over Trp23 is not shown. (B) W23F/V26I mutant. A possible model for the double mutant was constructed by mutating the Trp23 and Val26 residues and then slightly moving the Ile26 and Met39 side chains. Phe23 is shown in blue and Ile26 in magenta. This figure was kindly provided by Stewart R. Durell (NCI, NIH).