Fig 2.

Crystal structure of the Roseiflexus castenholzii malonyl-CoA reductase (RfxMCR) N-terminal region bound with NADP⁺ and malonate semialdehyde (MSA). (A) Overall structure of NADP⁺–MSA-bound RfxMCR-N is shown as ribbon (left) and surface models (right). The cofactor NADP⁺ (shown in yellow) and the reaction intermediate MSA (shown in salmon) are resolved in the substrate-binding pocket of the SDR1 domain (blue), but not in the SRD2 domain (violet). The regions in SDR2 without a clear electron density map (Glu359-Arg371, Ser415-Ala421, and Pro505-Ser514) are labeled and indicated with dashed lines. The α10 and α11 helices, which cover the NADP⁺–MSA binding site, are shown as ribbons. (B) NADP⁺–MSA binding in the RfxMCR-N substrate-binding pocket. The amino acid residues coordinating NADP⁺ (yellow) and MSA (salmon) are labeled and shown in stick form. Hydrogen-bonding interactions are labeled with the corresponding distances and indicated with dashed lines. (C) Overall enzymatic activity of mutant forms of full-length RfxMCR. Members of the catalytic triad (Thr165–Tyr178–Lys182) and Tyr172 and Arg175, which are necessary for coordinating MSA, were mutated to Ala residues. All the enzymatic data were obtained from triplicate experiments.