Fumarase fumarate hydratase

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

Fumarate hydratase (fumarase), which is discussed in Chapter 13, catalyzes the reversible hydration of fumaric acid to malic acid (Eq. 13-11). It was one of the first enzymes whose pH dependence was studied intensively. A bell-shaped pH dependence... 

Other enzymes in the aconitase family include isopropylmalate isomerase and homoaconitase enzymes functioning in the chain elongation pathways to leucine and lysine, both of which are pictured in Fig. 17-18.90 There are also iron-sulfur dehydratases, some of which may function by a mechanism similar to that of aconitase. Among these are the two fumarate hydratases, fumarases A and B, which are formed in place of fumarase C by cells of E. coli growing anaerobically.9192 Also related may be bacterial L-serine and L-threonine dehydratases. These function without the coenzyme pyridoxal phosphate (Chapter 14) but contain iron-sulfur centers.93-95 A lactyl-CoA... 

Fumarase. See Fumarate hydratase Fumarase-aspartase family 685 Fumarate 481s, 516s, 683s Fumarate hydratase (fumarase) 526, 683,688 acid-base catalysis 471 concerted reaction 685 Fumarase A 688 Fumarase B 688 Fumarase C 683 mechanism 683 - 685 pH dependence 684 rates of substrate exchange 684 turnover number of 683 Fumarate reductase 785 Fumarylpyruvate 690s Function of state R 476 Fungal infections 20 Fungi 20... 

By application of the CIP rules the order of priority of the atoms directly attached to the chirality centre is O > C(0,0,(0)) > C(C,H,H) > H. The atom or group of lowest priority, hydrogen in this case, is already oriented away from the observer. Therefore the sequence of the remaining three groups can be directly deduced from the formula, and these are easily seen to be arranged in a counter-clockwise sense to the observer. It therefore follows that the formula represents (S)-2-hydroxysuccinic acid (formerly known as L-malic acid). The compound is produced in the citric acid cycle from fumaric acid by fumarate-hydratase (fumarase). 

Stevens and Stevens (1979) measured the hydration dependence of glucose-6-phosphate dehydrogenase, hexokinase, fumarate hydratase (fumarase), and glucose-6-phosphate isomerase (phosphoglucose isom-erase) over the range 0.1-0.6 h. Serum albumin was used as a carrier protein to buffer the water content. The hydration isotherms of the enzymes and the serum albumin were assumed to be similar. For the first three enzymes activity was detected (0.05% of full solution activity) near 0.2 h. Activity was measurable for the isomerase at 0.15 h. In all cases, even at 0.3 h, the activity in the powder was less than 5% of the solution rate. Diffusion of substrates in the powder may be rate limiting. The amount of albumin in the powder affected the rate. 

Fumarate hydratase. The most studied enzyme of this group is probably the porcine mitochondrial fumarate hydratase (fumarase see also Chapter 9), a tetramer of 48.5-kDa subunits with a turnover number of 2 x 10 s T It accelerates the hydration reaction more than lO -fold. A similar enzyme, the 467-residue fumarase C whose three-dimensional structure is known, is foxmd in cells of E. coli when grown aerobically. The product of the fumarate hydratase reaction is L-malate (S-malate). The stereospecificity is extremely high. If the reaction is carried out in HjO an atom of H is incorporated into the pro-R position, i.e., the proton is added strictly from the re face of the trigonal carbon (Eq. 13-12). To obtain L-malate the hydroxyl must have been added from the opposite side of the double bond. Such anti (trans) addition is much more common in both nonenzymatic and enzymatic reactions than is addition of both H and OH (or -Y) from the same side (syn, cis, or adjacent addition). For concerted addition it is a natural result of stereoelectronic control. Almost all enzymatic addition and elimination reactions involving free carboxylic acids are anti with the proton entering from the re face. 

In this reaction, water is added across the double bond of fumarate by fumarate hydratase (fumarase) to yield the hydroxy compound L-malate ... 

Isocitrate Lyase Malate Synthase Succinate Dehydrogenase Fumarate Hydratase (Fumarase)... 

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. 

Fumarate is hydrated to malate in a freely reversible reaction cat alyzed by fumarase (also called fumarate hydratase, see Figure 9.6). [Note- Fumarate is also produced by the urea cycle (see p. 251), in purine synthesis (see p. 293), and during catabolism of the amino acids, phenylalanine and tyrosine (see p. 261).]... 

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. 

Fig. 25-15). In every case it is NH3 or an amine, rather than an OH group, that is eliminated. However, the mechanisms probably resemble that of fumarate hydratase. Sequence analysis indicated that all of these enzymes belong to a single fumarase-aspartase family.64 65 The three-dimensional structure of aspartate ammonia-lyase resembles that of fumarate hydratase, but the catalytic site lacks the essential HI 88 of fumarate hydratase. However, the pKa values deduced from the pH dependence of Vmax are similar to those for fumarase.64 3-Methylaspartate lyase catalyzes the same kind of reaction to produce ammonia plus czs-mesaconate.63 Its sequence is not related to that of fumarase and it may contain a dehydroalanine residue (Chapter 14).66... 

Step 7 is the reversible hydration of fumarate to form malate, catalyzed by fumarate hydratase (which is usually called fumarase). 

There is still a third possible mechanism for the fumarate hydratase reaction. The proton and hydroxyl groups may be added simultaneously in a concerted reaction. However, observed kinetic isotope effects are not consistent with this mechanism. In 1997 the structure of fumarase C of E. coli was reported. Each active site of the tetrameric enzyme is formed using side chains from three different subunits. The H188 imidazole is hydrogen bonded to an active site water molecule and is backed up by the E331 carboxy-late which forms a familiar catalytic pair. However, these results have not clarified the exact mode of substrate binding nor the details of the catalytic mechanism. Structural studies of fumarate hydratase from yeast and the pig are also in progress. 

Fumarate is hydrated. Fumarate is converted to L-malate in a reversible stereospecific hydration reaction catalyzed by fumarase (also referred to as fumarate hydratase) ... 

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) ... 

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). 

FIG. 4.2 Malate metabolism in mitochondria from body wall muscle of adult Ascaris smm. (1) Fumarase (2) malic enzyme (3) pyruvate dehydrogenase complex (4) complex I (5) succinate-coenzyme Q reductase (complex II, fumarate reductase) (6) acyl CoA transferase (7) methylmalonyl CoA mutase (8) methyl-malonyl CoA decarboxylase (9) propionyl CoA condensing enzyme (10) 2-methyl acetoacetyl CoA reductase (11) 2-methyl-3-oxo-acyl CoA hydratase (12) electron-transfer flavoprotein (13) 2-methyl branched-chain enoyl CoA reductase (14) acyl CoA transferase. 

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