Structural insights into IVD enzyme function and isovaleric acidemia

Background

IVD is a key enzyme in leucine catabolism, catalyzing the conversion of isovaleryl-CoA to 3-methylcrotonyl-CoA. Defects in IVD function lead to toxic accumulation of metabolites such as isovaleric acid, resulting in isovaleric acidemia (IVA)-a life-threatening autosomal recessive disorder characterized by vomiting, metabolic acidosis, and neurological damage. Although IVD gene mutations are known to cause IVA, the enzyme's structural dynamics and complex substrate-binding mechanisms have long hindered precise mechanistic studies.

Research progress

By optimizing protein purification protocols, the team obtained cryo-EM structures of IVD in apo state and in complex with substrates (isovaleryl-CoA and butyryl-CoA) at high resolutions. The structures revealed that human IVD exists as a tetramer, with each monomer forming a unique "U-shaped" substrate channel composed of α-helical and β-sheet domains. This architecture enables selective recognition of short-branched chain substrates while excluding longer chains due to steric hindrance.

Further analysis highlighted IVD's distinct substrate selectivity within the ACAD family of acyl-CoA dehydrogenases. Unlike ACADM (which processes straight-chain C6-C12 substrates), IVD exhibits two defining features: preference for shorter branched-chain substrates (C4-C6). Structural comparisons between IVD and ACADM revealed critical differences in their active pockets. In IVD, residues L127 and L290 form a narrowed side-chain distance (compared to T121 and V284 in ACADM), creating spatial constraints that exclude substrates longer than C7 via a "bottleneck effect". Conversely, ACADM's Y400 and E401 residues restrict lateral flexibility with bulky side chains, while IVD's G406 and A407 reduce steric hindrance, enabling accommodation of branched chains.

The study identified glutamate residue E286 as the catalytic core, facilitating α-hydrogen abstraction, while the FAD cofactor stabilizes the tetramer through hydrogen bonds with T200, R312, and E411. Disease-associated mutations (e.g., A314V, E411K) disrupt FAD binding or distort the substrate pocket, reducing enzymatic activity by over 80%. For example, the E411K mutation replaces negatively charged glutamate with lysine, destabilizing FAD binding and tetramer integrity. These atomic-level insights clarify genotype-phenotype correlations, improving diagnostic accuracy and prognostic predictions.

Future prospects

The resolved structures provide a foundation for developing small-molecule therapeutics targeting the FAD-binding region or substrate pocket to stabilize mutant IVD and restore partial function. Additionally, the established "mutation-structure-function" framework enhances precision in IVA diagnosis and personalized treatment planning. This study not only advances our understanding of IVD's structural biology but also opens avenues for tackling this rare metabolic disorder.