Modulation of ligand–heme reactivity by binding pocket residues demonstrated in cytochrome c' over the femtosecond–second temporal range

<jats:p>The ability of hemoproteins to discriminate between diatomic molecules, and the subsequent affinity for their chosen ligand, is fundamental to the existence of life. These processes are often controlled by precise structural arrangements in proteins, with heme pocket residues driving reactivity and specificity. One such protein is cytochrome c', which has the ability to bind nitric oxide (<jats:styled-content style="fixed-case">NO</jats:styled-content>) and carbon monoxide (<jats:styled-content style="fixed-case">CO</jats:styled-content>) on opposite faces of the heme, a property that is shared with soluble guanylate cycle. Like soluble guanylate cyclase, cytochrome c' also excludes <jats:styled-content style="fixed-case">O</jats:styled-content><jats:sub>2</jats:sub> completely from the binding pocket. Previous studies have shown that the <jats:styled-content style="fixed-case">NO</jats:styled-content> binding mechanism is regulated by a proximal arginine residue (<jats:styled-content style="fixed-case">R</jats:styled-content>124) and a distal leucine residue (<jats:styled-content style="fixed-case">L</jats:styled-content>16). Here, we have investigated the roles of these residues in maintaining the affinity for <jats:styled-content style="fixed-case">NO</jats:styled-content> in the heme binding environment by using various time‐resolved spectroscopy techniques that span the entire femtosecond–second temporal range in the <jats:styled-content style="fixed-case">UV</jats:styled-content>‐vis spectrum, and the femtosecond–nanosecond range by <jats:styled-content style="fixed-case">IR</jats:styled-content> spectroscopy. Our findings indicate that the tightly regulated <jats:styled-content style="fixed-case">NO</jats:styled-content> rebinding events following excitation in wild‐type cytochrome c' are affected in the <jats:styled-content style="fixed-case">R</jats:styled-content>124<jats:styled-content style="fixed-case">A</jats:styled-content> variant. In the <jats:styled-content style="fixed-case">R</jats:styled-content>124<jats:styled-content style="fixed-case">A</jats:styled-content> variant, vibrational and electronic changes extend continuously across all time scales (from fs–s), in contrast to wild‐type cytochrome c' and the <jats:styled-content style="fixed-case">L</jats:styled-content>16<jats:styled-content style="fixed-case">A</jats:styled-content> variant. Based on these findings, we propose a <jats:styled-content style="fixed-case">NO</jats:styled-content> (re)binding mechanism for the <jats:styled-content style="fixed-case">R</jats:styled-content>124<jats:styled-content style="fixed-case">A</jats:styled-content> variant of cytochrome c' that is distinct from that in wild‐type cytochrome c'. In the wider context, these findings emphasize the importance of heme pocket architecture in maintaining the reactivity of hemoproteins towards their chosen ligand, and demonstrate the power of spectroscopic probes spanning a wide temporal range.</jats:p>