Fig. 7.

Crystal structure of CaAC in complex with ATPαS. a CaAC (E497K/C566D) homodimers form an intertwined helical superstructure within the crystal lattice (blue, red, gray indicate individual superhelices, yellow indicates 8 homodimers within a single asymmetric unit). b Each homodimer consists of two antiparallel arranged monomers and harbors 2 active sites at the dimer interface, occupied by the ATP analog: ATP-Sp-αS. Arrows indicate β4/5 loops, N-termini are labeled, chain A: blue/green, chain B: green/red. c Comparison of the β4/5 loop orientation in CaAC. Overlay of the six monomers of CaAC found in the loop-proximal (green) position close to the catalytic core, or the loop-distal (orange) position. The protein backbone is visualized as a ribbon with selected residues and the base head group shown as sticks. d CaAC active site; the adenine base and phosphate tail of ATP-Sp-αS are anchored by different monomers. Residues of the phosphate binding monomer are marked (*). The previously mutated residues E497K (orange), C566D (red) form hydrogen bonds to the adenine base. The ribose is tilted perpendicular to the adenine plane and points towards β2*, β3*. The conserved arginine, R577 (α4) orients towards Pα. The ion B site is occupied by Ca²⁺, which is octahedrally coordinated among others by two conserved aspartates, D457* and D501*. The phosphate tail of ATP-Sp-αS is anchored to the α1/ β1-loop via polar interactions. mFo-DFc omit map for ATP-Sp-αS contoured at 5σ, contour level is shown as gray mesh. e Magnification of the adenine base binding residues. Neighboring residues of D566, situated on β5 and mutated in RhACs-6 × are colored in red. f Magnification of the residues, involved in nucleotide phosphate binding, coloring as in d. Gray dashed lines indicate hydrogen bonds or metal coordination between 2.3 Å and 3.6 Å in length