# Human Oxygenase Variants Employing a Single Protein FeII Ligand Are Catalytically Active

## TL;DR

This study shows that a human oxygenase can function with only one iron ligand, challenging previous assumptions about its structure and function.

## Contribution

The study demonstrates that AspH variants with a single FeII ligand retain catalytic activity, offering new insights into oxygenase design.

## Key findings

- H725A and H679A AspH variants retain significant catalytic activity despite losing one FeII ligand.
- Crystal structures confirm that a single histidine ligand is sufficient for metal coordination.
- The findings suggest broader implications for understanding and designing 2OG oxygenases.

## Abstract

Aspartate/asparagine‐β‐hydroxylase (AspH) is a human 2‐oxoglutarate (2OG) and FeII oxygenase that catalyses C3 hydroxylations of aspartate/asparagine residues of epidermal growth factor‐like domains (EGFDs). Unusually, AspH employs two histidine residues to chelate FeII rather than the typical triad of two histidine and one glutamate/aspartate residue. We report kinetic, inhibition, and crystallographic studies concerning human AspH variants in which either of its FeII binding histidine residues are substituted for alanine. Both the H725A and, in particular, the H679A AspH variants retain substantial catalytic activity. Crystal structures clearly reveal metal‐ligation by only a single protein histidine ligand. The results have implications for the functional assignment of 2OG oxygenases and for the design of non‐protein biomimetic catalysts.