# Neutrons & MnSOD: Past & Future

## TL;DR

This paper explores the structure and function of MnSOD, focusing on how neutrons and X-rays reveal proton and electron transfer mechanisms and challenges in studying its native substrate.

## Contribution

The paper highlights recent advancements in neutron crystallography and X-ray spectroscopy to uncover protonation states and catalytic mechanisms in MnSOD.

## Key findings

- Neutron crystallography and X-ray spectroscopy revealed novel protonation states of residues like Gln143 and Tyr34 in MnSOD.
- Strong short and low-barrier hydrogen bonds in MnSOD's active site contribute to its rapid catalytic turnover.
- Product inhibition by hydrogen peroxide leads to a MnSOD product-inhibited complex, with recent studies offering insights into its resolution.

## Abstract

Human manganese superoxide dismutase (MnSOD) is a tetrameric enzyme that plays a critical role in catalyzing the dismutation of superoxide radicals into molecular oxygen and hydrogen peroxide. This involves a redox cycle where the catalytic manganese alternates between oxidized (Mn3+) and reduced (Mn2+) states to facilitate coupled proton-electron transfer reactions (CPETs) that drive MnSOD function. The focus of the first half of this talk is to summarize the recent advancements in neutron protein crystallography and X-ray spectroscopy that have significantly enhanced our understanding of proton and electron transfer events within MnSOD, and revealed novel protonation states of key residues, such as Gln143 and Tyr34.

Additionally, the enzyme’s active site contains unique hydrogen bonds, including strong short hydrogen bonds (SSHBs) and low-barrier hydrogen bonds (LBHBs), which facilitate the enzyme’s exceptionally rapid catalytic turnover. Furthermore, product inhibition, driven by hydrogen peroxide accumulation, has been identified as a regulatory mechanism that shifts the enzyme’s catalytic pathway, leading to the formation of a “MnSOD product-inhibited complex”, which is resolved in the rate limiting step. Recent studies have characterized this complex, offering new insights into how MnSOD alleviates product inhibition. However, despite these advances, key questions remain regarding MnSOD's interaction with its native substrate, superoxide, and the precise structural mechanisms of substrate binding. The second half of the talk will focus on overcoming the current technological limitations and challenges to trapping the native substrate-bound MnSOD enzyme, which is essential to unraveling the full complexity of MnSOD catalysis.