Doming and spin cascade in Ferric Haems: Femtosecond X-ray Absorption and X-ray Emission Studies

TL;DR

This study uses femtosecond X-ray techniques to reveal that ferric cytochrome c undergoes a spin state cascade leading to doming, a process previously unobserved in ferric hemes, suggesting a broader biological significance.

Contribution

It demonstrates for the first time that ferric cytochrome c's photocycle involves a cascade of excited spin states causing doming, revealing a new dynamic in ferric heme proteins.

Findings

Femtosecond X-ray methods show ferric Cyt c undergoes a spin state cascade.

The spin cascade causes the ferric heme to undergo doming.

This behavior may be common to all ferric hemes, implying biological relevance.

Abstract

The structure-function relationship is at the heart of biology and major protein deformations are correlated to specific functions. In the case of heme proteins, doming is associated with the respiratory function in hemoglobin and myoglobin, while ruffling has been correlated with electron transfer processes, such as in the case of Cytochrome c (Cyt c). The latter has indeed evolved to become an important electron transfer protein in humans. In its ferrous form, it undergoes ligand release and doming upon photoexcitation, but its ferric form does not release the distal ligand, while the return to the ground state has been attributed to thermal relaxation. Here, by combining femtosecond Fe K-edge X-ray absorption near-edge structure (XANES) studies and femtosecond Fe Kalpha and Kbeta X-ray emission spectroscopy (XES), we demonstrate that the photocycle of ferric Cyt c is entirely due to a cascade among excited spin states of the Iron ion, causing the ferric heme to undergo doming, which we identify for the first time. We also argue that this pattern is common to all ferric haems, raising the question of the biological relevance of doming in such proteins.