Malate formation

Both of these reactions are followed by exergonic reactions. The equilibrium of the reaction malate oxaloacetate (step 8) lies in favor of malate formation, so at equilibrium the concentration of oxaloacetate will be low. The next reaction in the cycle (oxaloacetate + acetyl-CoA — citrate) (step 1) is, however, exergonic, and the oxaloacetate is removed to condense with acetyl-CoA. Similarly, the conversion of citrate to isocitrate is endergonic, and at equilibrium the reaction favors the formation of citrate. The next reaction in the cycle (isocitrate—>2-oxoglutarate) is exergonic, and so the isocitrate is removed thus allowing this reaction to proceed. 

Various enzymes, lysozyme, catalase, phosphatase, malate/formate dehydrogenase, adenylase kinase, pymvate decarboxylase, alkaline phospatase, serum albumin, lipoprotein, interferon, growth factor, polypeptide hormone DNA, RNA, y-globulin, proteinase, carboxypeptidase, endotoxins, pyrogen, IgG, trypsin inhibitor, transferrin, casein, L-benzoyl arginine ethyl ester, cytocrome C IgG, heparin... 

Fumarase catalyzes the conversion of fumarate to malate. It has a Km of 5 pM for fumarate and a Vm of 50 pmol/min/ mg of protein when measured in the direction of malate formation. The concentration of fumarate required to give a velocity of 25 pmol/min/mg protein is... 

Although the equilibrium of this reaction favors malate formation, in vivo the reaction proceeds toward the formation of oxaloacetate, since the latter is rapidly removed by the citrate synthase reaction to initiate the next round of the cycle. 

The increase in glutamate favors transamination of oxaloacetate and limits oxaloacetate availability for phosphoenolpyruvate synthesis. When the [NADH]/[NAD+] ratio is low, malate formation occurs more readily. The cytosolic PEPCK is relatively unaffected by the mitochondrial [NADH]/[NAD+] ratio. Once malate and aspartate are transported to the cytosol and they are reconverted to oxaloacetate, cytosolic PEPCK can convert it to phosphoenolpyruvate. 

Available evidence indicates that malic enzyme operates in two steps oxidation of malate to form enzyme-bound oxaloacetate, and decarboxylation of this material (27, 28). Malic enzyme will also decarboxylate oxaloacetate (28). Isotope effects indicate that oxidation and decarboxylation are separate steps and that both steps are partially rate determining (29). When the enzyme is supplied with oxaloacetate in the presence of NADPH, the substrate partitions in part toward malate formation and in part toward pyruvate formation (30). 

Group 1 sulfate reducers utilize lactate, pyruvate, malate, formate, and alcohols as their energy source but can only convert these compounds to acetate. They do not have TCA cycle and excrete acetate as waste products. 

Various enzymes, lysozyme, catalase, phosphatase, malate/formate dehydrogenase, adenylate kinase, pyruvate decarboxylase, alkaline phosphatase, serum albumin, lipoprotein, interferon, growth factor, polypeptide hormone... 

By measuring the nitrate content of leaf extracts through anion chromatography, we accidently observed that nitrate disappearance was always accompanied by malate accumulation (Fig.3). Since reduction of nitrate to NH, produces OH", one might consider malate accumulation as a compensatory acid production. However, the ratio of malate formed to nitrate reduced appeared to be rather variable, and at present our data allow as yet no clearcut conclusions on a causal relation between nitrate reduction and malate formation in leaves. 

This is one of three enzymes originally considered to be a single malic enzyme responsible for the conversion of malate to pyruvate. This enzyme also decarboxylates added oxalacetate. The other two enzymes utilise NAD instead of NADP. One (E.C. 1.1.1.38) also decarboxylates oxalacetate the other (E.C. 1.1.1.39) does not. Radiochemical assay may be accomplished by measuring malate formation from pyruvate and C labelled bicarbonate [496]. 

The equilibrium constant (0.05 mol/1) favors malate formation. It is usually assumed that in vivo the reaction is toward malate utilization rather than synthesis (Walker, 1962). The activity is usually less than PEP carboxylase (Table 5.1). 

The regulation of malate enzyme is poorly understood. Although the equilibrium constant favors malate formation, the CO2 release tends to drive the reaction toward decarboxylation (Harary et al., 1953). Hence the suggestion that the in vivo reaction is toward malate utilization rather than malate synthesis (Walker, 1962 Ting and Dugger, 1965). 

Only a few studies in the literature reported malate production from fumarate using immobilized baker s yeast, Saccharomyces cerevisiae. Nevertheless, only one-third of the specific activity of yeast conversion is achieved as compared with the bacterial system (Oliveira et al., 1994). Neufeld et al. (1991) studied L-malate formation by immobilized S. cerevisiae that was amplified for fumarase in the presence of a surfactant. The highest specific activity... 

We detenuined the influence of oxy- and ketocarboxylic acids (succinate, fumarate, adipinate, a-ketoglutarate, isocitrate, tartrate, E-malate) on the luminescence intensity of the Eu-OxTc complex. These substances interact as polydentate ligands similarly to citrate with the formation of ternary complexes with Eu-OxTc. As to succinate, fumarate, adipinate and a-ketoglutarate this they cannot effectively coordinate with EiT+ and significant fluorescence enhancement was not observed. 

Alkaline (and also acidic) ester hydrolysis of /3-poly(L-malate) is accompanied by side reactions leading to the formation of fumarate, maleate and/or racemiza-tion, especially at elevated temperatures. The above assays thus underestimate the polymer contents due to the formation of small amounts of 2-4% fumarate (unpublished results). This fraction of fumarate increases for the hydrolysis of more concentrated polymer solutions. 

Molecules of interest that contain free amino groups can be coupled in aqueous solution to /S-poIy(L-malate) as amides using carbodiimides such as the water-soluble l-ethyl-3(3-dimethyIaminopropyl)carbodiimide hydrochloride (EDC) [2,12,20,21]. By this method, the molecules are attached randomly. A selective amide bond formation at the carboxylate terminus can be achieved... 

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

There are also voices critical of the rTCA cycle Davis S. Ross has studied kinetic and thermodynamic data and concludes that the reductive, enzyme-free Krebs cycle (in this case the sequence acetate-pyruvate-oxalacetate-malate) was not suitable as an important, basic reaction in the life evolution process. Data on the Pt-catalysed reduction of carbonyl groups by phosphinate show that the rate of the reaction from pyruvate to malate is much too low to be of importance for the rTCA cycle. In addition, the energy barrier for the formation of pyruvate from acetate is much too high (Ross, 2007). 

MnP is the most commonly widespread of the class II peroxidases [72, 73], It catalyzes a PLC -dependent oxidation of Mn2+ to Mn3+. The catalytic cycle is initiated by binding of H2O2 or an organic peroxide to the native ferric enzyme and formation of an iron-peroxide complex the Mn3+ ions finally produced after subsequent electron transfers are stabilized via chelation with organic acids like oxalate, malonate, malate, tartrate or lactate [74], The chelates of Mn3+ with carboxylic acids cause one-electron oxidation of various substrates thus, chelates and carboxylic acids can react with each other to form alkyl radicals, which after several reactions result in the production of other radicals. These final radicals are the source of autocataly tic ally produced peroxides and are used by MnP in the absence of H2O2. The versatile oxidative capacity of MnP is apparently due to the chelated Mn3+ ions, which act as diffusible redox-mediator and attacking, non-specifically, phenolic compounds such as biopolymers, milled wood, humic substances and several xenobiotics [72, 75, 76]. 

The change with the concentration cannot be due to the reversible formation and decomposition of a lactone of the ordinary type because we get the effect with ethyl malate as well as with malic add. The change cannot be due to a reversible conversion of laevo-malic acid into dextro-malic add, because then a solution of equivalent amounts of dextro- and laevo-malic acids would become optically active on addition of salts, adds and bases. Hydrochloric add or sodium hydroxide imparts no activity to a solution of d/-malic add. The changes on adding electrolytes to a solution of dextro-malic add are equal and opposite in sign to the changes in laevo-malic acid under the same conditions. 

The reaction mixture for a coupled assay includes the substrates for the initial or test enzyme and also the additional enzymes and reagents necessary to convert the product of the first reaction into a detectable product of the final reaction. The enzyme aspartate aminotransferase (EC 2.6.1.1), for instance, results in the formation of oxaloacetate, which can be converted to malic acid by the enzyme malate dehydrogenase (EC 1.1.1.37) with the simultaneous conversion of NADH to NAD+, a reaction which can be followed spectropho-tometrically at 340 nm ... 

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