# Structures of two LarA‐like nickel‐pincer nucleotide cofactor‐utilizing enzymes with a single catalytic histidine residue

## TL;DR

This paper reveals structural and functional insights into two LarA-like enzymes that use a unique nickel cofactor and a single histidine residue for catalysis.

## Contribution

The study identifies a new octameric structure and active site residues in LarA homologs lacking a typical catalytic histidine.

## Key findings

- Two LarA homologs lacking one catalytic histidine form a novel octameric structure.
- Structures with and without the NPN cofactor reveal new active site residues and substrate recognition mechanisms.
- Genomic analysis suggests these enzymes may be involved in carbohydrate metabolism.

## Abstract

The nickel‐pincer nucleotide (NPN) cofactor catalyzes the racemization/epimerization of α‐hydroxy acids in enzymes of the LarA family. The established proton‐coupled hydride transfer mechanism requires two catalytic histidine residues that alternately act as general acids and general bases. Notably, however, a fraction of LarA homologs (LarAHs) lack one of the active site histidine residues, replacing it with an asparaginyl side chain that cannot participate in acid/base catalysis. Here, we investigated two such LarAHs and solved their cryo‐electron microscopic structures with and without loaded NPN cofactor, respectively. The structures revealed a consistent octameric assembly that is unprecedented in the LarA family and unveiled a new set of active site residues that likely recognize and process substrates differently from those of the well‐studied LarAHs. Genomic context analysis suggested their potential involvement in carbohydrate metabolism. Together, these findings lay the groundwork for expanding the breadth of reactions and the range of mechanisms of LarA enzymes.