What Is 3-methylglutaconyl-CoA hydratase

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Last updated: April 15, 2026

Quick Answer: 3-methylglutaconyl-CoA hydratase is an enzyme involved in the leucine catabolism pathway, encoded by the AUH gene in humans. Mutations in this gene can cause 3-methylglutaconic aciduria type I, a rare autosomal recessive metabolic disorder first described in 1981.

Key Facts

3-methylglutaconyl-CoA hydratase is encoded by the AUH gene located on chromosome 1p32.3
The enzyme catalyzes the conversion of 3-methylglutaconyl-CoA to 3-hydroxy-3-methylglutaryl-CoA in leucine degradation
Deficiency leads to 3-methylglutaconic aciduria type I, affecting fewer than 1 in 100,000 newborns
The AUH gene spans 14 exons and was first cloned in 1998
Over 30 pathogenic mutations in AUH have been linked to the disorder as of 2023

Overview

3-methylglutaconyl-CoA hydratase is a mitochondrial enzyme essential for the breakdown of the amino acid leucine. It functions specifically in the fifth step of leucine catabolism, ensuring metabolic intermediates are processed efficiently and safely.

Deficiencies in this enzyme lead to a rare inherited disorder known as 3-methylglutaconic aciduria type I. This condition disrupts normal energy metabolism and results in the accumulation of organic acids, which can cause neurological symptoms and developmental delays.

Enzyme function: Catalyzes the hydration of 3-methylglutaconyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), a critical intermediate in energy production pathways.
Gene location: The AUH gene resides on the short arm of chromosome 1 at position 1p32.3, spanning approximately 10 kilobases of genomic DNA.
Protein structure: The enzyme is a homodimer, with each subunit consisting of 497 amino acids and a molecular weight of about 55 kDa.
Metabolic pathway: Operates exclusively in the mitochondrial matrix as part of the leucine degradation cascade, downstream of isovaleryl-CoA dehydrogenase.
Discovery timeline: First identified in human liver mitochondria in 1981, with the gene cloned and sequenced in 1998 by Japanese researchers.

How It Works

The enzyme functions through a stereospecific hydration mechanism, adding a water molecule across the double bond of 3-methylglutaconyl-CoA. This reaction is essential to prevent toxic buildup and redirect carbon flux toward the citric acid cycle.

Substrate specificity:3-methylglutaconyl-CoA is the primary substrate, with no significant activity toward other CoA esters, ensuring pathway fidelity.
Catalytic mechanism: Employs a glutamate residue as a catalytic base to activate water, enabling nucleophilic attack on the α,β-unsaturated thioester.
Reaction product:HMG-CoA is formed, which is further cleaved into acetyl-CoA and acetoacetate by HMG-CoA lyase for energy production.
Enzyme kinetics: Exhibits a Km of 18 μM for 3-methylglutaconyl-CoA and a turnover number (kcat) of approximately 12 sec⁻¹ under physiological conditions.
Regulation: Activity is modulated by mitochondrial pH and CoA availability, though no allosteric regulators have been definitively identified.
Deficiency impact: Mutations in AUH reduce enzyme activity to less than 10% of normal, leading to urinary excretion of 3-methylglutaconic and 3-methylglutaric acids.

Comparison at a Glance

Below is a comparison of 3-methylglutaconyl-CoA hydratase with other enzymes involved in branched-chain amino acid metabolism:

Enzyme	Substrate	Gene	Disease Association	Incidence
3-methylglutaconyl-CoA hydratase	3-methylglutaconyl-CoA	AUH	3-methylglutaconic aciduria type I	<1 in 100,000
Isobutyryl-CoA dehydrogenase	Isobutyryl-CoA	ACAD8	Isobutyryl-CoA dehydrogenase deficiency	~1 in 200,000
Short/branched-chain acyl-CoA dehydrogenase	2-methylbutyryl-CoA	ACADSB	2-methylbutyryl-CoA dehydrogenase deficiency	Rare
Isobutyryl-CoA mutase	Isobutyryl-CoA	IBU	Propionic acidemia variant	Extremely rare
Enoyl-CoA hydratase	trans-2-enoyl-CoA	ECHS1	Multiple acyl-CoA dehydrogenase deficiency	~1 in 100,000

This table highlights the specificity of 3-methylglutaconyl-CoA hydratase within leucine metabolism and contrasts its genetic and clinical profile with related enzymes. While all are involved in fatty acid or amino acid oxidation, only AUH mutations cause isolated 3-methylglutaconic aciduria without systemic acidemia.

Why It Matters

Understanding 3-methylglutaconyl-CoA hydratase is vital for diagnosing and managing rare metabolic disorders, particularly in pediatric neurology and genetic counseling. Its role underscores the importance of mitochondrial enzymes in maintaining metabolic homeostasis.

Diagnostic marker: Elevated urinary 3-methylglutaconic acid is a hallmark, used in tandem with genetic testing for AUH mutations.
Neurological impact: Untreated deficiency can cause hypotonia, developmental delay, and spastic paraplegia, typically appearing in early childhood.
Genetic testing: Over 30 pathogenic variants in AUH have been documented, including missense, nonsense, and splice-site mutations.
Treatment approach: No cure exists, but low-leucine diets and supportive care may reduce symptom severity in some patients.
Research significance: The enzyme's role in mitochondrial RNA degradation adds complexity, as AUH also functions in RNA metabolism.
Public health: Newborn screening panels do not routinely include this disorder, leading to potential underdiagnosis in global populations.

Continued research into 3-methylglutaconyl-CoA hydratase enhances our understanding of inborn errors of metabolism and may lead to targeted therapies in the future.