Malate dehydrogenase
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Malate dehydrogenase (EC 1.1.1.37) (MDH) is an enzyme that reversibly catalyzes the oxidation of malate to oxaloacetate using the reduction of NAD+ to NADH. This reaction is part of many metabolic pathways, including the citric acid cycle. Other malate dehydrogenases, which have other EC numbers and catalyze other reactions oxidizing malate, have qualified names like malate dehydrogenase (NADP+).
Isozymes
Several
Humans and most other mammals express the following two malate dehydrogenases:
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Protein families
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Evolution and structure
In most organisms, malate dehydrogenase (MDH) exists as a
Each subunit of the malate dehydrogenase dimer has two distinct domains that vary in structure and functionality. A parallel
Malate dehydrogenase has also been shown to have a mobile loop region that plays a crucial role in the enzyme's catalytic activity. Studies have shown that conformational change of this loop region from the open conformation to the closed conformation after binding of substrate enhances MDH catalysis through shielding of substrate and catalytic amino acids from solvent. Studies have also indicated that this loop region is highly conserved in malate dehydrogenase.[6]
Mechanism
The active site of malate dehydrogenase is a hydrophobic cavity within the protein complex that has specific binding sites for the substrate and its
Mechanistically, malate dehydrogenase catalyzes the oxidation of the hydroxyl group of malate by utilizing NAD+ as an electron acceptor. This oxidation step results in the elimination of a proton and a hydride ion from the substrate. NAD+ receives the hydride ion (specifically, the hydride ion is transferred to the nicotinamide ring of the NAD+) and becomes reduced to NADH while concomitantly, the His-195 residue on the enzyme accepts the proton.
Studies have also identified a mobile loop in malate dehydrogenase that participates in the catalytic activity of the enzyme. The loop undergoes a conformational change to shield the substrate and catalytic amino acids from the solvent in response to the binding of the malate dehydrogenase:coenzyme complex to substrate. This flipping of the loop to the up position to cover the active site also promotes enhanced interaction of the catalytically important amino residues on the enzyme with the substrate. Additionally, the movement of the loop has been shown to correlate with the rate determining step of the enzyme.[13]
Function
Reaction
Malate dehydrogenases catalyzes the interconversion of malate to oxaloacetate. In the citric acid cycle, malate dehydrogenase is responsible for catalyzing the regeneration of oxaloacetate This reaction occurs through the oxidation of hydroxyl group on malate and reduction of NAD+. The mechanism of the transfer of the hydride ion to NAD+ is carried out in a similar mechanism seen in lactate dehydrogenase and alcohol dehydrogenase. The ΔG'° of malate dehydrogenase is +29.7 kJ/mol and the ΔG (in the cell) is 0 kJ/mol.[11]
Other pathways
Malate dehydrogenase is also involved in
Kinetics
Kinetic studies show that malate dehydrogenase enzymatic activity is ordered. The cofactor NAD+/NADH is bound to the enzyme before the substrate.[15] The Km value for malate, i.e., the concentration at which the enzyme activity is half-maximal, is 2 mM. The Kcat value is 259.2 s−1.[16]
Effect of pH on catalytic activity
Additionally, pH levels control specificity of substrate binding by malate dehydrogenase due to proton transfer in the catalytic mechanism.[17] A histidine moiety with a pK value of 7.5 has been suggested to play a role in the pH-dependency of the enzyme. Studies have indicated that the binding of the enol form oxaloacetate with the malate dehydrogenase:NADH complex forms much more rapidly at higher pH values.[12] Additionally, L-malate binding to malate dehydrogenase is promoted at alkaline conditions. Consequently, the non-protonated form malate dehydrogenase binds preferentially to L-malate and the enol form of oxaloacetate. In contrast, D-malate, hydroxymalonate, and the keto form of oxaloacetate have been found to bind exclusively to the protonated form of the enzyme. Specifically, when the histidine is protonated, the His residue can form a hydrogen bond with the substrate's carbonyl oxygen, which shifts electron density away from the oxygen and makes it more susceptible to nucleophilic attack by hydride. This promotes the binding of malate dehydrogenase to these substrates. As a result, at lower pH values malate dehydrogenase binds preferentially to D-malate, hydroxymalonate, and keto-oxaloacetate.[18]
Allosteric regulation
Because malate dehydrogenase is closely tied to the citric acid cycle, studies have proposed and experimentally demonstrated that citrate is an allosteric regulator of malate dehydrogenase depending on the concentrations of L-malate and NAD+. This may be due to deviations observed in the kinetic behavior of malate dehydrogenase at high oxaloacetate and L-malate concentrations. Experiments have shown that
Glutamate has also been shown to inhibit malate dehydrogenase activity. Furthermore, it has been shown that alpha ketoglutarate dehydrogenase can interact with mitochondrial aspartate aminotransferase to form a complex, which can then bind to malate dehydrogenase, forming a ternary complex that reverses inhibitory action on malate dehydrogenase enzymatic activity by glutamate. Additionally, the formation of this complex enables glutamate to react with aminotransferase without interfering activity of malate dehydrogenase. The formation of this ternary complex also facilitates the release of oxaloacetate from malate dehydrogenase to aminotransferase. Kinetically, the binding of malate dehydrogenase to the binary complex of alpha ketoglutarate dehydrogenase and aminotrannferase has been shown to increase reaction rate of malate dehydrogenase because the Km of malate dehydrogenase is decreased when it is bound as part of this complex.[20]
Interactive pathway map
Click on genes, proteins and metabolites below to link to respective articles.[§ 1]
- ^ The interactive pathway map can be edited at WikiPathways: "GlycolysisGluconeogenesis_WP534".
References
- ^ PMID 12537350.
- PMID 9834842.
- PMID 10075524.
- S2CID 469660.
- ^ Banaszak LJ, Bradshaw RA (1975). "Malate dehydrogenase". In Boyer PD (ed.). The Enzymes. Vol. 11 (3rd ed.). New York: Academic Press. pp. 369–396.
- ^ PMID 7849603.
- PMID 3076279.
- PMID 8476859.
- PMID 1507230.
- S2CID 26167967.
- ^ ISBN 978-0-470-54784-7.
- ^ PMID 31361.
- PMID 3422557.
- PMID 15358789.
- S2CID 35435579.
- PMID 7462244.
- PMID 25606688.
- PMID 217361.
- PMID 1567375.
- PMID 2899080.
Further reading
- Guha A, Englard S, Listowsky I (February 1968). "Beef heart malic dehydrogenases. VII. Reactivity of sulfhydryl groups and conformation of the supernatant enzyme". The Journal of Biological Chemistry. 243 (3): 609–15. PMID 5637713.
- McReynolds MS, Kitto GB (February 1970). "Purification and properties of Drosophila malate dehydrogenases". Biochimica et Biophysica Acta (BBA) - Enzymology. 198 (2): 165–75. PMID 4313528.
- Wolfe RG, Neilands JB (July 1956). "Some molecular and kinetic properties of heart malic dehydrogenase". The Journal of Biological Chemistry. 221 (1): 61–9. PMID 13345798.
External links
- Malate+dehydrogenase at the U.S. National Library of Medicine Medical Subject Headings (MeSH)