Metabolism — citric acid cycle (step 8)
Malate dehydrogenase: NAD⁺-linked oxidation
How the Malate dehydrogenase works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: L-Malate, NAD⁺, His base. Conditions: Mitochondrial matrix; equilibrium lies toward malate. Bonds formed: C=O (oxaloacetate); NADH; bonds broken: C–H (carbinol); O–H. Stereochemistry: Hydride to the Si/Re face of NAD⁺ (A-side). Rate and selectivity: Pulled forward by citrate synthase consuming oxaloacetate.
At a glance
Step 1: Hydride to NAD⁺
Malate's secondary alcohol carbon transfers a hydride directly to NAD⁺'s nicotinamide ring — the identical hydride-transfer oxidation mechanism as isocitrate dehydrogenase, just without a subsequent decarboxylation this time.
Step 2: NADH + oxocarbenium-like carbon
This generates the reduced cofactor NADH and an oxocarbenium-like intermediate at the substrate carbon.
Step 3: Loss of the O–H proton
Loss of the remaining proton gives oxaloacetate, regenerating the exact molecule that started the cycle — this final step closes the loop, and the NADH produced here (along with the NADH and FADH₂ generated at the three other oxidative steps of the cycle) carries the cycle's captured electrons onward to the electron transport chain.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- NAD+
- Nicotinamide adenine dinucleotide (oxidised). Hydride-accepting coenzyme in oxidations.
- NADH
- Nicotinamide adenine dinucleotide (reduced). Hydride-donating coenzyme.
- FAD
- Flavin adenine dinucleotide. Two-electron/one-electron redox coenzyme.
- CoA
- Coenzyme A. Thiol carrier that forms high-energy thioesters (acetyl-CoA).
- His
- Histidine. Amino acid whose imidazole acts as acid/base catalyst.
How it compares with related reactions
| Malate dehydrogenase | Aconitase | Citrate synthase | |
|---|---|---|---|
| Reagents | L-Malate, NAD⁺, His base | Citrate, [4Fe-4S] cluster (Fe₄ binds OH), enzyme base (Ser) and acid | Acetyl-CoA, oxaloacetate, citrate synthase (His274, His320, Asp375) |
| Conditions | Mitochondrial matrix; equilibrium lies toward malate | Mitochondrial matrix, pH ≈ 7.4, 37 °C | Mitochondrial matrix, pH ≈ 7.8, 37 °C |
| Stereochemistry | Hydride to the Si/Re face of NAD⁺ (A-side) | Anti elimination; the intermediate flips so water adds to the opposite carbon | Enolate attacks the si face of oxaloacetate; pro-S citrate |
| Rate / selectivity | Pulled forward by citrate synthase consuming oxaloacetate | Near-equilibrium; citrate:isocitrate ≈ 90:10 at equilibrium | Hydrolysis of citryl-CoA (ΔG°′ ≈ −32 kJ/mol overall) makes the step irreversible |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Malate dehydrogenase?
L-Malate, NAD⁺, His base. Conditions: Mitochondrial matrix; equilibrium lies toward malate.
Which bonds are formed and which are broken?
Formed: C=O (oxaloacetate); NADH. Broken: C–H (carbinol); O–H.
What is the stereochemical outcome?
Hydride to the Si/Re face of NAD⁺ (A-side)
What controls the rate or the selectivity?
Pulled forward by citrate synthase consuming oxaloacetate
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- Curved-arrow notation: rules and the mistakes students make