What Is 3-hydroxyacyl-CoA dehydrogenase

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Last updated: April 15, 2026

Quick Answer: 3-hydroxyacyl-CoA dehydrogenase (HADH) is an enzyme involved in fatty acid oxidation, catalyzing the third step of the mitochondrial beta-oxidation pathway. It converts 3-hydroxyacyl-CoA to 3-ketoacyl-CoA, using NAD+ as a cofactor to produce NADH for ATP generation.

Key Facts

HADH catalyzes the third step in mitochondrial beta-oxidation of fatty acids
The enzyme uses NAD+ as a cofactor to produce NADH
Deficiency in HADH can cause metabolic disorders like HADH deficiency
HADH deficiency is linked to hyperinsulinemic hypoglycemia in infants
The gene encoding HADH is located on chromosome 4q22.3 in humans

Overview

3-hydroxyacyl-CoA dehydrogenase (HADH) is a crucial enzyme in the mitochondrial beta-oxidation pathway, responsible for breaking down fatty acids to generate energy. It specifically catalyzes the third step in this metabolic cycle, converting 3-hydroxyacyl-CoA to 3-ketoacyl-CoA, a reaction essential for producing acetyl-CoA and reducing equivalents.

Found primarily in liver and muscle mitochondria, HADH plays a key role in maintaining energy homeostasis during fasting or prolonged exercise. Its activity supports ATP production by enabling the efficient utilization of fatty acids as fuel, particularly when glucose availability is low.

Enzyme classification: HADH is classified under EC 1.1.1.35, part of the short-chain dehydrogenase/reductase family, and acts specifically on 3-hydroxyacyl-CoA intermediates.
Reaction specificity: It oxidizes the hydroxyl group at the 3-carbon position of fatty acyl-CoA chains, typically those with 4 to 10 carbon atoms.
Cofactor dependence: The enzyme requires NAD+ as a cofactor, reducing it to NADH, which enters the electron transport chain to produce ATP.
Subcellular location: HADH is localized in the mitochondrial matrix, where fatty acid beta-oxidation occurs in eukaryotic cells.
Gene location: The HADH gene is located on chromosome 4q22.3 and spans approximately 12 kilobases with 5 exons.

How It Works

The enzymatic function of 3-hydroxyacyl-CoA dehydrogenase is tightly integrated into the four-step fatty acid beta-oxidation cycle. Each cycle shortens the fatty acid chain by two carbons, generating acetyl-CoA, NADH, and FADH2 for energy production.

Substrate binding: 3-hydroxyacyl-CoA binds to the active site of HADH, positioning the 3-hydroxyl group for oxidation in a stereospecific manner.
NAD+ interaction:NAD+ binds adjacent to the hydroxyl group, accepting a hydride ion during the oxidation reaction, forming NADH and a keto group.
Catalytic mechanism: A conserved tyrosine residue in the active site acts as a catalytic base, deprotonating the hydroxyl group to facilitate hydride transfer.
Stereochemistry: HADH specifically acts on L-3-hydroxyacyl-CoA isomers, distinguishing it from enzymes that process D-isomers.
Product release: The resulting 3-ketoacyl-CoA is released and enters the next step, catalyzed by thiolase, which cleaves it into acetyl-CoA and a shortened acyl-CoA.
Regulation: HADH activity is modulated by NADH/NAD+ ratios, ensuring enzyme function aligns with cellular energy demands.

Comparison at a Glance

The following table compares HADH with other key enzymes in fatty acid metabolism to highlight functional distinctions.

Enzyme	Reaction Catalyzed	Coenzyme	Carbon Chain Preference	Associated Disorders
3-hydroxyacyl-CoA dehydrogenase (HADH)	Oxidation of L-3-hydroxyacyl-CoA to 3-ketoacyl-CoA	NAD+	C4–C10	HADH deficiency, hyperinsulinism
Acyl-CoA dehydrogenase	Dehydrogenation of acyl-CoA to trans-2-enoyl-CoA	FAD	Varies by type (SCD, MCD, etc.)	MCAD deficiency (1 in 10,000 births)
Enoyl-CoA hydratase	Hydration of trans-2-enoyl-CoA to L-3-hydroxyacyl-CoA	None	All chain lengths	Rare hydratase deficiencies
3-ketoacyl-CoA thiolase	Cleavage of 3-ketoacyl-CoA into acetyl-CoA and acyl-CoA	None	C4 and longer	Thiolase deficiency (very rare)
Carnitine palmitoyltransferase I	Transfers long-chain acyl groups to carnitine	Carnitine	C12–C18	CPT I deficiency (neonatal onset)

This comparison underscores HADH’s unique role in the beta-oxidation pathway, particularly its NAD+-dependent oxidation and association with metabolic disorders involving insulin dysregulation. Unlike other dehydrogenases that use FAD, HADH directly contributes to NADH pools, influencing mitochondrial respiration efficiency.

Why It Matters

Understanding HADH is vital for diagnosing and managing rare metabolic diseases and improving knowledge of cellular energy metabolism. Its dysfunction can disrupt energy production and lead to life-threatening conditions, especially in infants.

Diagnostic marker: Elevated levels of 3-hydroxybutyryl-carnitine in blood can indicate HADH deficiency, aiding newborn screening.
Hyperinsulinism link: Mutations in HADH are associated with hyperinsulinemic hypoglycemia, where insulin remains high despite low blood sugar.
Therapeutic implications: Patients may require frequent feeding or diazoxide to suppress insulin secretion and prevent hypoglycemia.
Genetic counseling: Autosomal recessive inheritance means siblings of affected individuals have a 25% recurrence risk.
Research applications: HADH is studied in models of insulin regulation and fatty acid metabolism, offering insights into diabetes and obesity.
Evolutionary conservation: The enzyme is highly conserved across mammals, indicating its fundamental role in energy metabolism.

In summary, 3-hydroxyacyl-CoA dehydrogenase is a pivotal enzyme in fatty acid catabolism, linking lipid metabolism to energy production and insulin regulation. Its study continues to inform both clinical medicine and biochemical research.