What Is (3S)-3-hydroxyacyl-CoA hydro-lyase

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Last updated: April 10, 2026

Quick Answer: (3S)-3-hydroxyacyl-CoA hydro-lyase, classified as EC 1.1.1.35, is an oxidoreductase enzyme that catalyzes the third step of mitochondrial beta-oxidation, converting 3-hydroxyacyl-CoA to 3-ketoacyl-CoA using NAD+ as a cofactor. This essential enzyme breaks down both medium-chain and short-chain fatty acids, providing critical energy metabolism for cellular function.

Key Facts

Enzyme classification: EC 1.1.1.35 oxidoreductase, acting on CH-OH groups with NAD+/NADP+ as acceptor
Catalyzes the third step of beta-oxidation pathway in mitochondrial fatty acid metabolism
Converts 3-hydroxyacyl-CoA substrate to 3-ketoacyl-CoA product using NAD+ cofactor
Two main isoforms exist: HAD (prefers medium-chain substrates) and SCHAD (prefers short-chain, branched, and steroid substrates)
Deficiency causes hyperinsulinemic hypoketotic hypoglycemia and altered organic acid concentrations in affected individuals

Overview

(3S)-3-hydroxyacyl-CoA hydro-lyase, also known as 3-hydroxyacyl-CoA dehydrogenase (HAD), is a critical mitochondrial enzyme involved in the complete degradation of fatty acids through the beta-oxidation pathway. This enzyme catalyzes the third and crucial oxidation step of this metabolic process, converting 3-hydroxyacyl-CoA substrates into 3-ketoacyl-CoA products. Classified under EC number 1.1.1.35, it belongs to the oxidoreductase family of enzymes that facilitate electron transfer reactions essential for energy production.

The enzyme plays a fundamental role in cellular energy metabolism by enabling the breakdown of both medium-chain fatty acids (6–12 carbons) and short-chain fatty acids (fewer than 6 carbons), which are subsequently converted into acetyl-CoA molecules. These acetyl-CoA units enter the citric acid cycle to generate ATP, the primary energy currency of cells. Additionally, multiple isoforms of this enzyme exist, including the long-chain specific variant and the short-chain specific variant (SCHAD), each optimized for different substrate chain lengths and compositions, ensuring comprehensive fatty acid metabolism across diverse physiological conditions.

How It Works

The enzyme operates through a highly coordinated oxidation-reduction mechanism within the mitochondrial matrix, where fatty acids undergo sequential processing. Here are the key mechanistic features:

NAD+ Cofactor Requirement: The enzyme requires nicotinamide adenine dinucleotide (NAD+) as an essential electron acceptor, which gets reduced to NADH during the catalytic cycle, subsequently feeding reducing equivalents into the electron transport chain for ATP synthesis.
Hydroxyl Group Oxidation: The enzyme catalyzes oxidation of the hydroxyl (-OH) group on carbon 3 of the acyl-CoA substrate, converting this secondary alcohol into a ketone (C=O) functional group, fundamentally altering substrate reactivity.
Three-Step Beta-Oxidation Cycle: Operating as the third step following acyl-CoA dehydrogenase and enoyl-CoA hydratase, this enzyme bridges two sequential oxidation reactions that progressively shorten fatty acid chains by two-carbon units per cycle.
Multiple Isoform Specificity: HAD prefers medium-chain substrates, while short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) exhibits broader substrate specificity, metabolizing short-chain acyl-CoAs, branched-chain acyl-CoAs, and even steroid molecules, ensuring metabolic flexibility across diverse physiological states.
Multifunctional Enzyme Complexes: In some organisms and tissues, this enzyme exists as part of larger multifunctional protein complexes containing additional activities including enoyl-CoA hydratase, 3-oxoacyl-CoA thiolase, and enoyl-CoA isomerase, enabling coordinated sequential reactions.

Key Comparisons

Enzyme Variant	Preferred Substrate	Primary Location	Metabolic Role
Long-Chain HAD	Medium-chain fatty acids (C6-C12)	Mitochondrial matrix	Initial fatty acid degradation stages
Short-Chain SCHAD	Short-chain, branched, steroids	Mitochondrial matrix	Final degradation stages, steroid metabolism
Enoyl-CoA Hydratase	Double bonds in fatty acids	Mitochondrial matrix	Precedes HAD in beta-oxidation cycle
3-Ketoacyl-CoA Thiolase	3-ketoacyl-CoA products	Mitochondrial matrix	Final cleavage step, produces acetyl-CoA

Why It Matters

This enzyme is biochemically significant because it represents a major control point in fatty acid catabolism, directly impacting cellular energy status and metabolic homeostasis. Efficient beta-oxidation enables cells to extract maximum energy from dietary fats and mobilized storage triglycerides, particularly important during fasting, sustained physical activity, and metabolic stress conditions.

Energy Production Impact: Each cycle of beta-oxidation generates one NADH and one FADH2, which collectively produce approximately 5 ATP molecules through oxidative phosphorylation, making this enzyme central to fat-dependent energy generation.
Clinical Significance: Genetic mutations or deficiencies in 3-hydroxyacyl-CoA dehydrogenase cause serious metabolic disorders characterized by hypoketotic hypoglycemia, accumulation of toxic acylcarnitine species, cardiac myopathy, and skeletal muscle weakness, particularly during fasting or illness.
Metabolic Disease Markers: Elevated urinary organic acids and plasma acylcarnitine profiles in deficient patients serve as diagnostic biomarkers for newborn screening programs, enabling early intervention and dietary management.
Drug Development Target: Understanding HAD mechanisms informs development of therapeutics for metabolic disorders, obesity management, and mitochondrial diseases affecting fatty acid oxidation capacity.

Understanding (3S)-3-hydroxyacyl-CoA hydro-lyase function remains essential for comprehending human metabolism, treating genetic deficiency disorders, and developing targeted interventions for metabolic and mitochondrial diseases. This enzyme exemplifies how individual catalytic steps within major metabolic pathways are carefully regulated and coordinated to maintain cellular energy homeostasis and prevent accumulation of potentially toxic lipid metabolites.