Fumarase Fumarate

Succinyl CoA is cleaved by succinate thiokinase (also called succinyl CoA synthetase), producing succinate and ATP (or GTP). This is an example of substrate-level phosphory lation. Succinate is oxidized to fumarate by succinate dehydrogenase, producing FADH2. The enzyme is inhibited by oxaloacetate. Fumarate is hydrated to malate by fumarase (fumarate hydratase), and malate is oxidized to oxaloacetate by malate dehy drogenase, producing NADH. 

When a V/K profile is represented by Eq. (75) we cannot a priori tell whether the group with pK has to be ionized and the group with pK2 protonated, or vice versa, since the shape of the pH profile will be exactly the same. For example, fumarase (fumarate hydratase) has an imidazole (p/f 7.1) that must be protonated and a carboxyl group (p/f 5.85) that must be ionized for reaction of malate, whereas the imidazole must be neutral and the carboxyl protonated for reaction of fumarate (7S). It is common for such reverse protonation to be required in one direction of an enzymic reaction. 

This method works very well for deuterium isotope effects and has been used to measure 0 isotope effects on fumarase (fumarate hydratase) (109), and C ones on malic enzyme (110). 

Dehydratases remove water, as in fumarase (fumarate hydratase) ... 

Fumarase, fumarate hydrolase (EC 4.2.1.2) the tricarboxyhc acid cycle enzyme which reversibly converts fumarate to malate by adding water to the double bond. In contrast to other hydrolyases, which require either pyridoxal phosphate or metal ions as cofactors, F. has no cofactor requirement. It is a tetra-mer (M, 194,000, 1784 amino acids, no disulfide bridges) of identical subunits M, 48,500). It exists as a number of isoenzymes. 

In the presence of fumarase (fumarate hydratase, EC 4.2.12) malate undergoes reversible dehydration to generate fumarate (citric acid cycle). In this process (a franx-elimination), the hydroxyl at C-2 and the pro-R hydrogen at C-3 are lost (Scheme 13.23). 

Optically inactive starting materials can give optically active products only if they are treated with an optically active reagent or if the reaction is catalyzed by an optically active substance The best examples are found m biochemical processes Most bio chemical reactions are catalyzed by enzymes Enzymes are chiral and enantiomerically homogeneous they provide an asymmetric environment m which chemical reaction can take place Ordinarily enzyme catalyzed reactions occur with such a high level of stereo selectivity that one enantiomer of a substance is formed exclusively even when the sub strate is achiral The enzyme fumarase for example catalyzes hydration of the double bond of fumaric acid to malic acid m apples and other fruits Only the S enantiomer of malic acid is formed m this reaction... 

The reaction is reversible and its stereochemical requirements are so pronounced that neither the cis isomer of fumaric acid (maleic acid) nor the R enantiomer of malic acid can serve as a substrate for the fumarase catalyzed hydration-dehydration equilibrium... 

The enzyme fumarase catalyzes the stereospecific addition of water to fumarate to form L-malate. A standard solution of fumarase, with a concentration of 0.150 tM, gave a rate of reaction of 2.00 tM mim under conditions in which the concentration of the substrate was significantly greater than K. The rate of reaction for a sample, under identical conditions, was found to be 1.15 tM mimh What is the concentration of fumarase in the sample ... 

Biosynthesis ofS(— )-M llc Acid. Aqueous fumaric acid is converted to levorotatory malic acid by the intracellular enzyme, fumarase, which is produced by various microorganisms. A Japanese process for continuous commercial production of S(—)-mahc acid from fumaric acid is based on the use of immobilized Brevibacteriumflavum cells in carrageenan (32). The yield of pyrogen-free S(—)-mahc acid that is suitable for pharmaceutical use is ca 70% of the theoretical. 

Fumarate is hydrated in a stereospecific reaction by fumarase to give L-malate (Figure 20.17). The reaction involves fraw5-addition of the elements of water across the double bond. Recall that aconitase carries out a similar reaction. 

Steps 7-8 of Figure 29.12 Hydration and Oxidation The final two steps in the citric acid cycle are the conjugate nucleophilic addition of water to fumarate to yield (S)-malate (L-malate) and the oxidation of (S)-malate by NAD+ to give oxaloacetate. The addition is cataiyzed by fumarase and is mechanistically similar to the addition of water to ris-aconitate in step 2. The reaction occurs through an enolate-ion intermediate, which is protonated on the side opposite the OH, leading to a net anti addition. 

Defects of the Krebs cycle. Fumarase deficiency was reported in children with mitochondrial encephalomyop-athy. Usually, there is developmental delay since early infancy, microcephaly, hypotonia and cerebral atrophy, with death in infancy or early childhood. The laboratory hallmark of the disease is the excretion of large amounts of fumaric acid and, to a lesser extent, succinic acid in the urine. The enzyme defect has been found in muscle, liver and cultured skin fibroblasts [16]. 

S. Except for oxido-reductases, transferases, and hydrolases, most ligases (enzymes that catalyze bond formation) are entirely substrate specific. Thus, fumarate hydratase (or fumarase) reversibly and stereospecifically adds water to fumaric acid to produce (S)-( — )-malic acid only (8) (Figure 1), and another enzyme, mesaconase, adds water to mesaconic acid to form (+ )-citramalic acid (9) (Figure 2). Although no extensive studies are available, it appears that neither fumarase nor mesaconase will add water stereospecifically to any other a,(3-unsaturated acid. 

The enzyme fumarase catalyses the stereospecific iram -addition of water to fumaric acid giving (5)-malic acid, and the reverse reaction, the rrans-elimination of water from (S )-malic acid ... 

Carbonate dehydratase [Zn " ]— carbonic anhydrase Fumarate hydratase— fumarase ... 

A general type of chemical reaction between two compounds, A and B, such that there is a net reduction in bond multiplicity (e.g., addition of a compound across a carbon-carbon double bond such that the product has lost this 77-bond). An example is the hydration of a double bond, such as that observed in the conversion of fumarate to malate by fumarase. Addition reactions can also occur with strained ring structures that, in some respects, resemble double bonds (e.g., cyclopropyl derivatives or certain epoxides). A special case of a hydro-alkenyl addition is the conversion of 2,3-oxidosqualene to dammara-dienol or in the conversion of squalene to lanosterol. Reactions in which new moieties are linked to adjacent atoms (as is the case in the hydration of fumarate) are often referred to as 1,2-addition reactions. If the atoms that contain newly linked moieties are not adjacent (as is often the case with conjugated reactants), then the reaction is often referred to as a l,n-addition reaction in which n is the numbered atom distant from 1 (e.g., 1,4-addition reaction). In general, addition reactions can take place via electrophilic addition, nucleophilic addition, free-radical addition, or via simultaneous or pericycUc addition. 

Fumarate NADH Formation Fumarase, Malate Dehydrogenase Argininosuccinate Lyase ... 

Fumarase, inhibition by cisplatin, 37 195 Fumarate reductase cysteine distribution, 38 240-241 EPR, 47 452-456 ground-state properties, 47 23... 

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