Amino acid dehydrogenases catalyze oxidative deamination or reductive amination between amino acids and their corresponding α-keto acids. Through NAD(H), NADP(H), or flavin cofactors, these enzymes connect nitrogen metabolism, central carbon metabolism, and cellular redox status. They are also important tools for chiral amino acid synthesis, amino acid quantification, and enzyme-coupled assays.

Keywords: amino acid dehydrogenases; glutamate dehydrogenase; leucine dehydrogenase; proline dehydrogenase; oxidative deamination; reductive amination; cofactor regeneration

1 Basic Concepts of Amino Acid Dehydrogenases

1.1 Major Catalytic Reactions

NAD(P)-dependent amino acid dehydrogenases generally catalyze the following reversible reaction:

L-Amino acid + NAD(P)⁺ + H₂O ⇌ α-Keto acid + NH₄⁺ + NAD(P)H + H⁺

In the oxidative deamination direction, an amino acid is converted into an α-keto acid, accompanied by the release of NH₄⁺ and production of NADH or NADPH. In the reductive amination direction, an L-amino acid is synthesized from an α-keto acid, NH₄⁺, and a reduced cofactor. The actual reaction direction is jointly determined by substrate concentration, NH₄⁺ level, cofactor redox ratio, pH, and the rate of product removal.

1.2 Classification of Amino Acid Dehydrogenases

Amino acid dehydrogenases can be classified according to substrate preference and cofactor type. Glutamate dehydrogenase catalyzes the interconversion of L-glutamate and α-ketoglutarate. Leucine dehydrogenase preferentially acts on branched-chain and hydrophobic substrates. Threonine dehydrogenase participates in threonine degradation. Proline dehydrogenase uses FAD to oxidize proline and operates through a mechanism distinct from that of reversible NAD(P)-dependent dehydrogenases.

Table 1. Major types of amino acid dehydrogenases

1.3 Differences from Aminotransferases and Amino Acid Oxidases

Amino acid dehydrogenases directly generate or consume reduced cofactors through redox reactions. Aminotransferases depend on pyridoxal phosphate and transfer amino groups between amino acids and α-keto acids, generally without directly releasing free NH₄⁺. Amino acid oxidases use FAD or FMN and can transfer electrons to oxygen to produce H₂O₂.Although all three enzyme classes can modify amino acid structures, their reaction principles and detection readouts are different.

2 Structure and Catalytic Mechanisms of Amino Acid Dehydrogenases

2.1 Substrate and Cofactor Recognition

Most NAD(P)-dependent amino acid dehydrogenases contain a cofactor-binding region and a substrate-binding region. The cofactor-binding region commonly forms a Rossmann-like fold that binds the NAD(P) cofactor and positions its nicotinamide ring appropriately for hydride transfer. The substrate-binding region recognizes the α-carboxyl group, α-amino group, and side chain of the amino acid.Following substrate entry, the enzyme transitions from an open to a closed conformation, positioning the substrate α-carbon and nicotinamide ring appropriately for hydride transfer.

2.2 Oxidative Deamination

Oxidative deamination begins with deprotonation of the substrate α-amino group. A hydride is subsequently transferred from the α-carbon to the nicotinamide ring of NAD(P)⁺, producing NAD(P)H and an imino acid intermediate.The imino acid is hydrolyzed in the presence of water to generate the corresponding α-keto acid and NH₄⁺. The enzyme-imposed spatial orientation of the substrate determines the reaction rate and stereoselectivity.

2.3 Reductive Amination

During reductive amination, an imine-like intermediate formed from an α-keto acid and NH₄⁺ acts as the acceptor. NADH or NADPH transfers a hydride to the α-carbon of this intermediate, producing an L-amino acid.High NH₄⁺ concentrations and continuous availability of reduced cofactors favor product synthesis. Excessively high salt concentrations may alter enzyme conformation, inhibit catalytic activity, and complicate product separation.

2.4 Cofactor Selectivity

NADP(H) contains an additional 2′-phosphate group compared with NAD(H). The cofactor-binding sites of NADP-dependent enzymes generally contain basic residues that stabilize this phosphate group, whereas the corresponding positions in NAD-dependent enzymes more commonly contain small or acidic residues.Cofactor selectivity should be determined by comparing Kₘ, kcat, and kcat/Kₘ under NAD(H)- and NADP(H)-dependent conditions. The occurrence of a reaction at a single cofactor concentration is insufficient to establish cofactor preference.

Table 2. Major differences between NAD- and NADP-dependent amino acid dehydrogenases

Note: Reaction direction is also affected by substrate concentration, enzyme source, and cellular metabolic state and cannot be determined solely from cofactor type.

3 Representative Amino Acid Dehydrogenases

3.1 Glutamate Dehydrogenase

Glutamate dehydrogenase (GDH or GLDH) catalyzes the reversible interconversion of L-glutamate and α-ketoglutarate. Glutamate is an important intracellular amino-group donor, whereas α-ketoglutarate is an intermediate of the tricarboxylic acid cycle. GDH can directly regulate the allocation of nitrogen and carbon fluxes.

Mammalian GDH is located mainly in mitochondria, where it forms oligomeric structures and is subject to allosteric regulation by molecules such as ADP, GTP, and leucine. Microbial NAD-GDH generally favors glutamate oxidation, whereas NADP-GDH is commonly involved in NH₄⁺ assimilation and glutamate synthesis. Cofactor preference and reaction equilibrium differ among species.

3.2 Leucine Dehydrogenase

Leucine dehydrogenase (LeuDH) is found mainly in microorganisms such as Bacillus species and generally uses NAD(H) to catalyze the reversible interconversion of L-leucine and α-ketoisocaproate. Its substrate-binding pocket preferentially accommodates branched-chain and hydrophobic side chains and may also accept certain valine, isoleucine, and non-natural α-keto acid substrates.

LeuDH exhibits high selectivity for L-configured products and is suitable for branched-chain amino acid detection and chiral amino acid synthesis. Engineering of the substrate-binding pocket can improve its acceptance of bulky, aromatic, or non-natural substrates.

3.3 Threonine Dehydrogenase

Threonine dehydrogenase uses NAD⁺ to oxidize L-threonine to 2-amino-3-ketobutyrate, which can subsequently be converted into glycine and acetyl-CoA. Unlike typical α-amino acid oxidative deamination, this reaction primarily oxidizes the side-chain hydroxyl group of threonine.Threonine dehydrogenase inhibitors can be used to determine whether changes in threonine degradation and downstream one-carbon metabolism depend on this enzyme.

3.4 Proline Dehydrogenase

Proline dehydrogenase (ProDH) is an FAD-dependent enzyme that catalyzes the oxidation of L-proline to Δ¹-pyrroline-5-carboxylate and transfers electrons to respiratory chain-associated acceptors.

ProDH participates in proline catabolism, mitochondrial redox regulation, and stress responses. It does not directly generate NADH and does not perform reversible reductive amination through the typical NAD(P)-dependent mechanism.

4 Metabolic Functions of Amino Acid Dehydrogenases

4.1 Nitrogen Assimilation and Release

When nitrogen is abundant and amino acid synthesis is required, NADPH-dependent GDH can use NH₄⁺ and α-ketoglutarate to synthesize glutamate. Glutamate subsequently donates amino groups to other α-keto acids through aminotransferases, generating different amino acids.

Under amino acid catabolic conditions, NAD-dependent dehydrogenases release NH₄⁺ and generate α-keto acids. NH₄⁺ enters excretory or reassimilation pathways, whereas α-keto acids enter the tricarboxylic acid cycle, gluconeogenesis, or other catabolic pathways.

4.2 Carbon Skeletons and Energy Metabolism

α-Ketoglutarate generated by GDH can enter the tricarboxylic acid cycle, whereas α-ketoisocaproate generated by LeuDH can enter branched-chain amino acid catabolism.Amino acid dehydrogenases also alter the NAD(P)⁺/NAD(P)H ratio, linking amino acid availability to mitochondrial respiration, reducing-equivalent supply, and biosynthesis.

4.3 Influence of Enzyme Source on Function

Mammalian GDH is subject to complex allosteric regulation and is suitable for studying nutrient sensing and mitochondrial metabolism. Microbial GDH and LeuDH generally exhibit high expression levels, stability, and in vitro catalytic activity, making them more suitable for enzymatic assays and biomanufacturing.

Enzymes with the same name may differ substantially in cofactor preference, optimal pH, oligomeric state, and substrate spectrum. The specific enzyme source and activity definition should therefore be documented in each experiment.

5 Biocatalytic and Analytical Applications of Amino Acid Dehydrogenases

5.1 Chiral Amino Acid Synthesis

Reductive amination can directly convert α-keto acids into L-amino acids with relatively few by-products. Enzyme stereoselectivity can also reduce the need for chemical resolution.LeuDH may be prioritized for branched-chain or hydrophobic α-keto acids, whereas α-ketoglutarate is suitable for GDH-based systems. Other substrates may require screening of homologous enzymes or protein engineering to expand the substrate range. After the reaction, product identity and conversion should be confirmed by HPLC or LC–MS. The enantiomeric excess should also be determined for chiral products.

5.2 Cofactor Regeneration

NADH and NADPH are relatively expensive, making cofactor-regeneration systems necessary for many biocatalytic processes. NAD-dependent amino acid dehydrogenases can be coupled with formate dehydrogenase or NAD-dependent glucose dehydrogenase. NADP-dependent GDH can be combined with NADP-dependent glucose-6-phosphate dehydrogenase.The hydride-supply rate of the regeneration enzyme should exceed the cofactor-consumption rate of the target reaction to prevent cofactor availability from becoming the rate-limiting step.

5.3 Amino Acid and NH₄⁺ Detection

NADH and NADPH exhibit characteristic absorbance at 340 nm. In oxidative deamination assays, amino acids and NAD(P)⁺ are added, and an increase in A₃₄₀ is recorded. In reductive amination assays, α-keto acids, NH₄⁺, and NAD(P)H are added, and a decrease in A₃₄₀ is recorded.GDH can be used to determine glutamate or NH₄⁺, whereas LeuDH can be used to measure leucine and certain branched-chain amino acids. Endogenous NAD(P)H, other dehydrogenases, and reducing substances in complex samples may generate background signals. No-enzyme, no-substrate, and heat-inactivated controls should be included.

6 Products Related to Amino Acid Dehydrogenases

6.1 Glutamate Dehydrogenases

6.2 Leucine Dehydrogenases

6.3 Activity Assays and Pathway Inhibitors

6.4 NADH Cofactor-Regeneration Products

6.5 NADPH- and NADH-Regeneration Products

Amino acid dehydrogenases regulate amino acids, NH₄⁺, α-keto acids, and cellular reducing equivalents through oxidative deamination and reductive amination. GDH primarily connects glutamate metabolism with the tricarboxylic acid cycle, LeuDH is suitable for branched-chain amino acid conversion and chiral synthesis, and threonine dehydrogenase and ProDH represent distinct substrate- and cofactor-dependent catalytic mechanisms.

For more related articles, please see below:

[1] Functional Roles of Auxiliary Enzymes in Enzyme-Coupled Colorimetric Reactions: Mutarotase, Oxidases, Peroxidases, and Dehydrogenases

[2] Comparison of Enzymatic Glucose Detection Methods: Glucose Oxidase, Glucose Dehydrogenase, and Hexokinase Methods

[3] Structure, Function, and Applications of Glutamate Dehydrogenase

[4] Technical Analysis and Clinical Applications of Lactate Dehydrogenase

[5] Metabolism and Oxidative Stress Detoxification, and Its Detection Applications

[6] Review of Glucose-6-phosphate Dehydrogenase (G6PD): Structural Features, Metabolic Functions, and Research Applications