Malate dehydrogenase types

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

Malic enzyme (malate dehydrogenase (decarboxylating), EC 1.1.1.39) catalyzes reversible oxidative decarboxylation of malate to pyruvate. The enzyme uses NAD+ as an electron acceptor, but it is also able to utilize NADP+ with lower affinity (Drmota et al. 1996). With a subunit size of approximately 63 kDa, the Trichomonas hydrogenosomal malic enzyme belongs to the family of large, eukaryotic type of malic enzymes. In contrast, the approximately 40-kDa-subunit malic enzyme, located in the cytosol, belongs... 

Fig. 5.4. Two types of energy metabolism in cestodes. (a) Type 1 homolactate fermentation, (b) Type 2 Malate dismutation. Reaction 3 involves a carboxylation step decarboxylation occurs at 6, 7 and 10. Reducing equivalents are generated at reactions 6 and 7 one reducing equivalent is used at reaction 9. Thus, when the mitochondrial compartment is in redox balance and malate is the sole substrate, twice as much propionate as acetate is produced. Key 1, pyruvate kinase 2, lactate dehydrogenase 3, phosphoenolpyruvate carboxykinase 4, malate dehydrogenase 5, mitochondrial membrane 6 malic enzyme 7, pyruvate dehydrogenase complex 8, fumarase 9, fumarate reductase 10, succinate decarboxylase complex. indicates reactions at which ATP is synthesised from ADP cyt, cytosol mit, mitochondrion. (After Bryant Flockhart, 1986.)...

Brugerolle G, Metenier G (1973) Localisation intracellulaire et caracterisation de deux types de malate dehydrogenase chez Trichomonas vaginalis DonnA J Protozool 20 320-327... 

One illustration of this type of uncertainty comes from studies of paralogs of malate dehydrogenase. The mitochondrial paralog of malate dehydrogenase (mMDH) has long been... 

Subsequent metabolism of oxaloacetate (OAA) varies according to species. Three main types of C4 pathway are recognized, of which the most extensively studied is that shown by plants such as Zea mays (corn) (Fig. 2). In these plants (here called type-1 C4 plants) OAA is reduced to malate via NADP-malate dehydrogenase in mesophyll chloroplasts. Malate is then transported to bundle sheath chloroplasts and oxidatively decarboxylated by NADP-malic enzyme to produce pyruvate, CO2 and NADPH. Pyruvate is recycled to the mesophyll cells while the CO2 and NADPH are used in the RPP cycle in the bundle sheath chloroplast. The original C3 carbon acceptor (PEP) is regenerated from pyruvate in the mesophyll chloroplast by the activity of pyruvate, Pj dikinase [8] (Eq. 5). 

Fig. 2 Heat inactivation of Thermus PEPCs. Heat treatments were carried out with 0.1 mg/ml of wild-type ( , A) and mutant (O, A) Thermus PEPCs at 90 ( , O) and 95T (A, A). The residual activities were assayed at 60°C in the reaction mixture containing 10 mM potassium PEP, 10 mM KHCO3,10 mM MgS04,0.3 mM NADPH, 1.0 mM CoASAc, 0.1 M Ches-KOH, pH 8.6,2.0 U malate dehydrogenase from Thermus sp., and the enzyme. Relative activities were plotted against incubation time.

A second type of EMIT has been developed using the enzyme malate dehydrogenase as the enzymatic label. Research has shown that thyroxine competitively inhibits malate dehydrogenase. A conjugate prepared with thyroxine covalently bound close to the enzyme s active site shows very low specific activity that can be restored by binding of the thyroxine to arcP -thyroxine antibody. In this very specific assay for thyroxine, enzyme activity increases upon antibody binding, so that in a competitive assay for free thyroxine, activity decreases with increasing free thyroxine concentration. 

Figure 4. Schematic representation of the metabolic fate of alanine in hepatocytes. Note that striking differences may exist between mammalian cell types on the one hand and individual amino acids on the other (see text). Solid and broken arrow lines refer to metabolic conversions and transport routes, respectively, and circles in membranes refer to specific transporters. Numbers refer to enzymes involved in alanine metabolism 1, alanine transaminase 2, pyruvate carboxylase 3, malate dehydrogenase 4, glutamate dehydrogenase 5, glutamine synthetase.

Tripathi, A. K., Desai, P. V., Pradhan, A., Khan, S. I., Avery, M. A., Walker, L. A., and Tekwani, B. L. (2004). An alpha-proteobacterial type malate dehydrogenase may complement LDH function in Plasmodium falciparum. Cloning and biochemical characterization of the enzyme. Eur.. Biochem. 271,3488-3502. 

The oxidation of L-malate in most living organisms is catalyzed by two distinct types of pyridine nucleotide-dependent enzymes. In one case the principal product is oxaloacetate, while in the other it is pyruvate and CO2. The enzymes of the malate-oxaloacetate class, which utilize NAD+, have been referred to as simple dehydrogenases, while enzymes of the malate-pyruvate type, which, in contrast, use NADP+, have been designated decarboxylating dehydrogenases and are commonly known as malic enzymes (1). 

In dehydrogenase-type enzyme-catalyzed reactions, the NAD" " as a cofactor is required for the enzymatic reaction. With substrate oxidation, NAD" " is simultaneously reduced to NADH. NADH contains a tertiary amine and can be acted as the co-reactant of Ru(bpy)3 (Fig. 16A). The ECL intensity is increased in proportion to the concentration of the substrates. However, for the NADH-depleting enzyme, such as the determination of pyruvate using malate dehydrogenase, the determinations... 

However, when Syrian (golden) hamster melanoma cells were fused with non-melanin-producing C3H mouse cells [104], and the hybrid line was isolated by the appropriate selection, no melanin production was observed in the hybrid cells, nor were tyrosinase and dopa oxidase activities detected in their extracts [104]. The parental melanoma cells had the enzymic activities and produced pigment. Nevertheless, in the hybrid cells, which possessed the expected hybrid karyotype, the lactate and malate dehydrogenase isozyme patterns were combinations of both parental types [104]. Thus, one might conclude that in the non-melanin-producing C3H parental cells there existed a specific diffusible enzyme... 

Fig. 6. Distribution after centrifugation in a sucrose density gradient of activities of the five aromatic synthetic enzymes encoded in the arom cluster in N. crassa (and of the reference activity—Neurospora malate dehydrogenase) from an arom9 qal mutant [47], This mutant has very low constitutive DHQase activity (about 8% of wild type).

Thus as the malate passes into the cytoplasm it takes reducing potential with it. Once in the cytoplasm a different type of malate dehydrogenase releases the oxaloacetate and causes the reduction of cytoplasmic NAD . In this way not only oxaloacetate but surplus energy in the form of reducing potential is transferred from the mitochondria to the cytoplasm. 

Some plants, such as corn and sugar cane, have evolved an auxiliary C4-dicarboxylic acid cycle< > that cooperates with the reductive pentose cycle in the photosynthetic assimilation of CO2. In plants with this cycle (sometimes referred to as the Hatch and Slack cycle), chloroplasts in the mesophyll cells near the surface on the leaf contain three C4-pathway specific enzymes pyruvate, phosphate-dikinase that directly converts pyruvate into phosphoenolpyruvate (PEP) with ATP, PEP carboxylase that catalyzes the carboxyla-tion of PEP to oxaloacetate, and malate dehydrogenase that finally reduces oxaloacetate to malate with NADPH. The purpose of these steps is apparently to incorporate CO2 and NADPH into malate in order to translocate them to the vascular bundle sheath cells, where they are again released by the action of a NADP-dependent malic enzyme. The malic enzyme is located in the bundle sheath chloroplasts together with the en mes of the Calvin cycle. CO2 is then reduced to carbohydrates while pyruvate is presumably transported back to the mesophyll cells. Besides the malate-type C4-plants, there is a second and larger group of species (aspartate type) that contains little malic enzyme and utilizes aspartate as the COj carrier. 

Nonspecific disruption of membrane permeability was proposed for difenopenten-ethyl ethyl 4-[(4-trifluoromethyl)phenoxyl-2-pentanoate, which uncoupled and inhibited respiration in corn and soybean mitochondria. This was considered to be a secondary effect of this aryloxy-phenoxypropionate analogue (see Chapter 3) due to the relatively high in vitro concentration. A similar conclusion was made for another aryl-oxyphenoxypropionate, diclofop, which uncoupled state 4 respiration, inhibited state 3 respiration, and caused swelling of isolated wheat and wild oat mitochondria. The inhibition of respiration was proposed to be due to an effect prior to the electron transport chain, that is, an effect on succinate or malate dehydrogenase. A potent inhibition of state 3 respiration was also reported for the herbicide UK J72J (2-ethylamino-4-amino-5-thiomethyl-6-chloropyrimidine), with succinate as substrate.Difenzoquat also inhibits state 3 respiration in isolated wheat and wild oat mitochondria however, this was not thought to be a significant contribution to the chlorotic contact symptoms observed with this translocated herbicide. Difenzoquat also possesses a weak bipyridinium-type PSI electron acceptor activity which causes chlorosis. ... 

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