Enzyme malate synthase

The enzyme malate synthase also synthesises (S )-malic acid from glyoxylic acid (OHC-CO2H) and the thioester acetyl-coenzyme A (CH3CO.SC0A this may be regarded as a source of the nucleophile CH2CO.SC0A). 

While the syntheses described here should produce chiral acetate samples, methods are needed to prove the existence of chirality and to determine its extent. The first method was devised independently by the Comforth and Arigoni groups and is still widely used [122,128,129]. It depends on the use of two enzymes, malate synthase and fumarase. Thus, overall, the sample of acetic acid is first converted to its CoA derivative (either with acetate kinase and phosphotransacetylase or chemically with ClCOOC2H5 and Co ASH) and then to malate and fumarate ... 

Scheme 10 General reaction mechanism of the CoA-dependent Claisen-type condensing enzymes, malate synthase, a-isopropylmalate synthase, citrate synthase, and homocitrate synthase.

In the glyoxylate cycle, L-malate is created by combining glyoxylate with acetyl-CoA in a reaction catalyzed by the enzyme malate synthase. 

Glyoxylate is a two carbon compound generated in the glyoxylate cycle by action of the enzyme isocitrate lyase on isocitrate (the other product of the reaction is succinate). Glyoxylate is combined with acetyl-CoA to form malate by the enzyme malate synthase. 

V. Tugarinov, L.E. Kay, Quantitative C and NMR relaxation studies of the 723-residue enzyme malate synthase G reveal a dynamic binding interface. Biochemistry 44 (2005) 15970-15977. 

Refined solution structure of the 82-kDa enzyme malate synthase... 

The method of analysis developed is for acetic acid and involves the use of two enzymes. Chiral acetic acid [as 1.26)] is irreversibly condensed as its CoA-derivative with glyoxylic acid (/.27), using the enzyme malate synthase, to give malic acid 1.28). The condensation occurs with loss of hydrogen isotope by a primary kinetic isotope effect (A h kj). This means that loss of H is favoured over loss of D which is in turn favoured over loss of T. The result is a high retention of tritium. 

This procedure involves the enzymatic conversion of the chiral acetate (C(HDT)COO ), obtained from experiments involving chiral methyl groups, to labeled malate. The enzymes used in this procedure include acetate kinase, phosphotransacetylase, malate synthase, and fumarase. 

ABORTIVE COMPLEXES MALATE DEHYDROGENASE MALATE SYNTHASE (S)-2-METHYLMALATE DEHYDRATASE Malate condensing enzyme,... 

The glyoxylate cycle is active in the germinating seeds of some plants and in certain microorganisms that can live on acetate as the sole carbon source. In plants, the pathway takes place in glyoxysomes in seedlings. It involves several citric acid cycle enzymes and two additional enzymes isocitrate lyase and malate synthase. 

Balance Sheet for the Citric Acid Cycle The citric acid cycle has eight enzymes citrate synthase, aconitase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase, and malate dehydrogenase. 

A key enzyme in the glyoxylate cycle is isocitrate lyase, which cleaves isocitrate (Eq. 13-40) to succinate and glyoxylate. The latter is condensed with a second acetyl group by the action of malate synthase (Eq. 13-38). The L-malate formed in this reaction is dehydrogenated to the regenerating substrate oxalo-... 

Topic LI). Thus, animals cannot convert fatty acids into glucose. In contrast, plants have two additional enzymes, isocitrate lyase and malate synthase, that enable them to convert the carbon atoms of acetyl CoA into oxaloacetate. This is accomplished via the glyoxylate pathway, a route involving enzymes of both the mitochondrion and the glyoxysome, a specialized membranous plant organelle. 

Malate synthase does not incorporate hydrogen from water into malate so there is no enolization of acetyl-CoA furthermore, for this enzyme, a normal isotope effect has been determined kH>/c2H = 3.8 0.1 [129]. 

The enzyme s malate synthase, and that s just what it can do, Glyoxylate plus acetyl is simply two plus two. 

MLS1 Malate synthase, enzyme of the glyoxylate cycle, involved in utilization of non-fermentable carbon sources expression is subject to carbon catabolite repression localizes in peroxisomes during growth in oleic acid medium... 

The formation of the glyoxalate hydrate is obviously unproductive and is probably an artifact resulting from the assay being done on the isolated system. Bern-hard found that many enzymes can bind to other enzymes that occupy adjacent positions in metabolic pathways (28), transferring substrates and products without the intermediate involvement of solvent. In the case of malate synthase, the oxidation of glycolate to glyoxalate might produce material in the unhydrated form, which would be used directly. In that case, the equilibrium isotope effect on hydration would not be observed. 

Enol Mechanism for Claisen Enzymes. An alternative mechanism for Claisen enzymes involves initial conversion of the nucleophilic substrate into the corresponding enol. In this case, addition of the carbon of the enol to the carbonyl of the electrophilic reactant can be assisted by acid-base catalysis. Application of the mechanism to the malate synthase reaction is shown in the Scheme... 

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