One Active Site, Two Reactions: Crystal Structure Reveals How Rare Enzyme ApnO Defies Classification

Enzymes are usually tidy things. For more than half a century, biochemists have sorted them into classes using the Enzyme Commission number system, a hierarchical scheme built on seven broad reaction types that tells researchers at a glance what a given protein does. Most enzymes slot neatly into a single category, reflecting the long-standing assumption that one active site catalyzes one kind of chemical transformation. But biology occasionally produces exceptions that refuse to fit the filing system, and a new structural study published in the Journal of Biological Chemistry examines one of the most intriguing of them in atomic detail.

The enzyme in question is D-apionate oxidoisomerase, abbreviated ApnO, a protein first discovered in 2018 that belongs to a very small group of enzymes that cannot be clearly assigned to a single Enzyme Commission class. Alongside ketol-acid reductoisomerase, known as KARI, ApnO stands out because its single active site supports two distinct reactions. Until now, however, ApnO had never undergone detailed functional and structural analysis, leaving open the question of how, at the molecular level, one catalytic pocket manages to accommodate chemistry that classification rules say should require separate enzymes.

To answer that question, researchers at Ehime University turned to X-ray crystallography, the technique that remains the gold standard for resolving the three-dimensional arrangement of atoms within a protein. By crystallizing ApnO and diffracting X-rays through the crystals, the team determined its structure at a level of detail sufficient to identify not only the fold of the protein but also the positions and coordination geometries of the metal ions sitting at the heart of its active site. Those metal ions turned out to hold the key to understanding how this unusual enzyme works.

The first surprise came from a comparison of overall architecture. Despite showing no significant sequence homology with KARI, meaning the two proteins share little detectable similarity in their linear chains of amino acids, ApnO turned out to closely resemble KARI in its overall three-dimensional structure. This is a classic example of structural convergence or distant evolutionary relationship hidden at the sequence level: two proteins can arrive at similar folds while their primary sequences drift beyond recognition. For enzymologists, the resemblance was immediately suggestive, because KARI is the other famous member of the small set of enzymes that defy single-class classification.

KARI’s catalytic mechanism depends on two magnesium ions, designated Mg(I) and Mg(II), which sit at two distinct binding positions, site I and site II, within the active site. In KARI, both metal ions adopt octahedral coordination geometry, a highly symmetric arrangement in which six ligands surround the metal at the vertices of an octahedron. This divalent metal pair helps stabilize the charged intermediates that arise during the reductoisomerase reaction. The expectation going into the study was that ApnO, with its structurally similar fold, might employ a comparable metal arrangement.

Instead, the crystal structure revealed something strikingly different. At the position equivalent to KARI’s Mg(II) site, ApnO binds zinc or manganese ions, and the coordination geometry there is tetrahedral rather than octahedral, meaning four ligands surround the metal in a pyramid-like arrangement. Meanwhile, at the position equivalent to Mg(I), a manganese ion adopts the expected octahedral geometry. The result is a hybrid active site in which one metal position mirrors the canonical arrangement seen in KARI while the other departs from it in both metal identity and geometry, a combination not previously observed in this structural family.

Distinguishing which specific metal ions occupy which sites in a crystal structure is not trivial, because metals of similar size can be difficult to tell apart by electron density alone. The research team overcame this challenge by exploiting the anomalous dispersion effect, a physical phenomenon that occurs when X-ray wavelengths are tuned near the absorption edges of particular elements. At those wavelengths, atoms of a given element scatter X-rays in a characteristic way, allowing zinc and manganese to be identified and distinguished unambiguously within the crystal. This careful metal identification gave the researchers confidence in their assignment of Zn(II) or Mn(II) to site II and Mn(II) to site I.

The detailed comparison between ApnO and KARI extended beyond the metals themselves to the protein residues that surround them. The zinc or manganese ion at site II in ApnO and the magnesium ion at the corresponding position in KARI differ not only in the type of metal ion and in coordination geometry but also show no conservation of the surrounding active-site residues. In other words, the amino acids that grip the metal in ApnO are not the same, nor even recognizably related, to those that grip the metal in KARI. This strongly suggested that the site II metal-binding capability was acquired independently in the two enzymes, an example of convergent evolution at the level of a single catalytic position.

Site I tells a different evolutionary story. Because the metal ion at that position is coordinated in a comparable octahedral fashion in both enzymes, the researchers concluded that this metal-binding site likely shared a common evolutionary origin, passed down from a shared ancestor even as the rest of the proteins diverged. The picture that emerges is therefore a mosaic: one metal position inherited from a common ancestor, the other invented separately in each enzyme lineage, together producing an active site capable of catalyzing two distinct reactions within a single structural framework. Such mosaic evolution may be a general feature of the rare enzymes that blur the boundaries of the Enzyme Commission classification.

The study also carries a broader message about structural biology in the era of accurate computational prediction. With modern machine-learning tools now able to predict protein structures with remarkable precision, one might ask why laborious experimental structure determination remains necessary. The ApnO work provides a clear answer: the direct identification of metal ions and the discovery of novel binding modes at active sites cannot be reliably revealed by predicted models alone. Metal identity, coordination geometry, and anomalous scattering behavior are experimental observables, and in metalloenzymes they can make the difference between a plausible guess and a verified mechanism. By combining crystallography with anomalous dispersion analysis, the Ehime University team has not only explained how ApnO manages its dual chemistry but also offered new insight into the molecular evolution of enzymes that defy the general paradigm in which a single enzyme’s activity is classified under a single reaction mode.

Subject of Research: Structural basis of dual-reaction catalysis by the rare enzyme D-apionate oxidoisomerase at a single active site

Article Title: Structural basis for enzyme catalyzing two distinct reactions at a single active site

Article References: Structural basis for enzyme catalyzing two distinct reactions at a single active site. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: ApnO, D-apionate oxidoisomerase, ketol-acid reductoisomerase, X-ray crystallography, enzyme classification, metal coordination, anomalous dispersion, molecular evolution, active site, zinc, manganese, Journal of Biological Chemistry