Hydration of fumarate to malate

Hydration of Fumarate to Malate The reversible hydra- The equilibrium of this reaction lies far to the left under... 

Carry out the matrix multiplications in equation 5.4-4 for the three chemical reactions involved in the hydration of fumarate to malate in the pH range 5 to 9. 

The enzyme fumarase catalyzes the hydration of fumarate to malate. This enzyme is known to be reversibly inhibited by succinate. Reaction velocities were determined in triplicate at different substrate concentrations, in... 

TABLE 4.2 Rate of Hydration of Fumarate to Malate by Fumarase at various Substrate Concentrations"... 

TABLE 4.3 Estimates of the Catalytic Parameters for the Fumarase-Catalyzed hydration of Fumarate to Malate ... 

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. 

This enzyme [EC 4.2.1.34], also known as mesaconase and mesaconate hydratase, catalyzes the conversion of (5 )-2-methylmalate to 2-methylfumarate and water. The enzyme will also catalyze the hydration of fumarate to (5 )-malate. 

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

The next step is the hydration of fumarate to form 1-malate. Fumarase catalyzes a stereospecific trans addition of a hydrogen atom and a hydroxyl group. The hydroxyl group adds to only one side of the double bond of fumarate hence, only the 1 isomer of malate is formed. 

The conversion of fumarate to malate is a hydration reaction, not a redox reaction. 

Reactions analogous to those of the TCA-cycle are found in the biosynthesis of leucine and lysine (Fig. 4). Similarly, the sequence of reactions represented by the dehydrogenation of succinate by a fla-voenzyme, followed by hydration of fumarate to ma-late, then dehydrogenation of malate by a NAD-linked dehydrogenase, finds a counterpart in the initial stages of fatty acid degradation (see). 

The electrophiJic addition of water to a molecule containing a double bond, known as hydration, is a reaction that is important biologically. One example is the hydration of fumarate to form malate, one of the steps in the citric acid cycle (also known as the Krebs cycle). 

In the next step, fumarase catalyses the hydration of fumarate to L-malate ... 

Generally speaking, these distinctions have not been observed by biochemists. Stereoselective has been little used, and stereospecific has been used to cover almost all aspects of the impact of stereochemical influences on reactions in living tissues or enzyme systems. Consider, for instance, the enzymatic hydration of fumarate by the enzyme, fumarase. Since there is a relationship between the structure of the substrate and product, the process could be described as stereospecific. Yet the definition of stereospecific requires that it be shown that the isomer of fumaric acid gives rise to a product which is stereochemically different from L-malate. Since the enzyme, however, does not catalyze any reaction with the (Z)-isomer (maleic acid) it is not clear whether stereospecific actually applies. 

Addition of water to a double bond is a reaction that we find in several biochemical pathways. For instance, the citric acid cycle is a key metabolic pathway for the complete oxidation of the sugar glucose and the release of the majority of the energy used by the body. It is also the source of starting materials for the s)m-thesis of the biological molecules needed for life. The next-to-last reaction in the citric acid cycle is the hydration of a molecule of fumarate to produce a molecule called malate. 

In reaction 7, a hydration adds water to the double bond of fumarate to yield malate, which is a secondary alcohol. 

Acid-catalyzed hydration of isolated double bonds is also uncommon in biological pathways. More frequently, biological hydrations require that the double bond be adjacent to a carbonyl group for reaction to proceed. Fumarate, for instance, is hydrated to give malate as one step in the citric acid cycle of food metabolism. Note that the requirement for an adjacent carbonyl group in the addition of water is the same as that we saw in Section 7.1 for the elimination of water. We ll see the reason for the requirement in Section 19.13, but might note for now that the reaction is not an electrophilic addition but instead occurs... 

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. 

This enzyme is highly stereospecific it catalyzes hydration of the trans double bond of fumarate but not the cis double bond of maleate (the cis isomer of fumarate). In the reverse direction (from L-malate to fumarate), fumarase is equally stereospecific D-malate is not a substrate. 

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).]... 

Cleavage of argininosuccinate Argininosuccinate is cleaved to yield arginine and fumarate. The arginine formed by this reaction serves as the immediate precursor of urea. Fumarate produced in the urea cycle is hydrated to malate, providing a link with sev eral metabolic pathways. For example, the malate can be trans ported into the mitochondria via the malate shuttle and reenter... 

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. 

Another example is provided by malic acid, a chiral molecule which also contains a prochiral center (see Eq. 9-74). In this case replacement of the pro-R or pro-S hydrogen atom by another atom or group would yield a pair of diastereoisomers rather than enantiomers. Therefore, these hydrogen atoms are diastereotopic. When L-malic acid is dehydrated by fumarate hydratase (Chapter 13) the hydrogen in the pro-R position is removed but that in the pro-S position is not touched. This can be demonstrated by allowing the dehydration product, fumarate, to be hydrated to malate in 2HzO (Eq. 9-74). The malate formed contains deuterium in the pro-R position. If this malate is now isolated and placed with another portion of enzyme in H20, the deuterium is removed cleanly. The fumarate produced contains no deuterium. 

The fumarate released in the urea cycle links the urea cycle with the TCA cycle. This fumarate is hydrated to malate, which is oxidized to oxaloacetate. The carbons of oxaloacetate can stay in the TCA cycle by condensation with acetyl-CoA to form citrate, or they can leave the TCA cycle either by gluconeogenesis to form glucose or by transamination to form aspartate as shown in figure 22.9. Because Krebs was involved in the discoveries of both the urea cycle and the TCA cycle, the interaction between the two cycles shown in figure 22.9 is sometimes referred to as the Krebs bicycle. 

Fumarate is converted to malate (4C) by fumarase this is a hydration reaction requiring the addition of a water molecule. 

The synthesis of fumarate by argininosuccinase links the urea cycle to the citric acid cycle (Fig. 2). Fumarate is an intermediate of this latter cycle which is then hydrated to malate, which in turn is oxidized to oxaloacetate (see Topic LI). 

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