Figure 6
Comparison of Form-I and -II St AckA. (A) Plot of residual distance between sequentially equivalent Cα atoms against residue number obtained after pairwise superposition of A-subunit of Form-I and A- and B-subunits of Form-II St AckA. Labeling scheme – e.g. IIA-IIB corresponds to distances between equivalent Cα atoms after superposition of Form-II B-subunit on Form-II A-subunit. The insets show the variable segment (residues 230–300) Cα atom distances highlighting large conformational differences (corresponding to ~18% of the total length of the enzyme). Region corresponding to residues 251–260 (refer Additional file 1: Figure S1 for fit of the electron density) are marked by vertical dotted lines. (B) Structural superposition of the A-subunit of Form-I (salmon-red) with A- (yellow) and B- (cyan) subunits of the Form-II St AckA highlighting the structural differences of the variable segment. Secondary structures corresponding to the core helices and variable segments (labels corresponding to I-A, II-A and II-B subunits are enclosed using ○, □ and Δ shapes, respectively) are labeled. Met230 and Lys300 occur at the ends of the variable segment. (C) Form-II St AckA dimer highlighting regions of variable segment (A-subunit, yellow except for the variable segment shown in bright-orange; B-subunit, cyan with variable segment highlighted in blue). Citrate (pink, CIT-501) bound at the dimeric interface is shown in ball and stick representation. Secondary structures corresponding to the variable segments of each subunit are labeled.