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EPR and Electron Nuclear Double Resonance spectroscopies here characterize CO binding to the active-site A cluster of wild-type (WT) Acetyl-CoA Synthase (ACS) and two variants, F229W and F229A. The A-cluster binds CO to a proximal Ni (Ni p ) that bridges a [4Fe-4S] cluster and a distal Ni d . An alcove seen in the ACS crystal structure near the A-cluster, defined by hydrophobic residues including F229, forms a cage surrounding a Xe mimic of CO. Previously, we only knew WT ACS bound a single CO to form the A red -CO intermediate, containing Ni p (I)-CO with CO located on the axis of the d  z   2 odd-electron orbital (g ⊥ &gt; g || ∼ 2). Here, the two-dimensional field-frequency pattern of 2K-35 GHz 13 C-ENDOR spectra collected across the A red -CO EPR envelope reveals a second CO bound in the d  z   2 orbital's equatorial plane. This WT A-cluster conformer dominates the nearly conservative F229W variant, but 13 C-ENDOR reveals a minority "A" conformation with (g || &gt; g ⊥ ∼ 2) characteristic of a "cloverleaf" (e.g., d  x   2 -  y   2 ) odd-electron orbital, with Ni p binding two, apparently "in-plane" CO. Disruption of the alcove through introduction of the smaller alanine residue in the F229A variant diminishes conversion to Ni(I) ∼ 10-fold and introduces extensive cluster flexibility. 13 C-ENDOR shows the F229A cluster is mostly (60%) in the "A" conformation but with ∼20% each of the WT conformer and an "O" state in which d  z   2 Ni p (I) (g ⊥ &gt; g || ∼ 2) surprisingly lacks CO. This paper thus demonstrates the importance of an intact alcove in forming and stabilizing the Ni(I)-CO intermediate in the Wood-Ljungdahl pathway of anaerobic CO and CO 2 fixation.

journal of the american chemical society

<sup>13</sup>C Electron Nuclear Double Resonance Spectroscopy Shows Acetyl-CoA Synthase Binds Two Substrate CO in Multiple Binding Modes and Reveals the Importance of a CO-Binding “Alcove”

13C Electron Nuclear Double Resonance Spectroscopy Shows Acetyl-CoA Synthase Binds Two Substrate CO in Multiple Binding Modes and Reveals the Importance of a CO-Binding “Alcove”

¹³C Electron Nuclear Double Resonance Spectroscopy Shows Acetyl-CoA Synthase Binds Two Substrate CO in Multiple Binding Modes and Reveals the Importance of a CO-Binding “Alcove”