Malate urea cycle

In 1937 Krebs found that citrate could be formed in muscle suspensions if oxaloacetate and either pyruvate or acetate were added. He saw that he now had a cycle, not a simple pathway, and that addition of any of the intermediates could generate all of the others. The existence of a cycle, together with the entry of pyruvate into the cycle in the synthesis of citrate, provided a clear explanation for the accelerating properties of succinate, fumarate, and malate. If all these intermediates led to oxaloacetate, which combined with pyruvate from glycolysis, they could stimulate the oxidation of many substances besides themselves. (Kreb s conceptual leap to a cycle was not his first. Together with medical student Kurt Henseleit, he had already elucidated the details of the urea cycle in 1932.) The complete tricarboxylic acid (Krebs) cycle, as it is now understood, is shown in Figure 20.4. 

The first suggestion that substrates in carbohydrate oxidation might exert catalytic effects on the oxidation of other intermediates (cf.earlier demonstration of such action in the urea cycle by Krebs and Henseleit, 1932 see Chapter 6) arose from the work of Szent-Gyorgi (1936). He demonstrated that succinate and its 4C oxidation products catalytically stimulated the rate of respiration by muscle tissues. He also observed that reactions between the 4C intermediates were reversible and that if muscle was incubated with oxaloacetate, fumarate and malate made up 50-75% of the products, 2-oxoglutarate 10-25% and, significantly, 1-2% of the C was converted to citrate. These observations were... 

However, the urea cycle also causes a net conversion of oxaloacetate to fumarate (via aspartate), and the regeneration of oxaloacetate (Fig. 18-12) produces NADH in the malate dehydrogenase reaction. Each NADH molecule can generate up to 2.5 ATP during mitochondrial... 

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... 

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. 

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). 

Citrin is an aspartate-glutamate antiporter that has a role both in the urea cycle and in the malate aspartate shuttle. It is necessary for the transport of aspartate produced in the mitochondria into the cytosol, where it is used by AS. Its role in the malate-aspartate shuttle is to transport cytosolic NADH reducing equivalents into the mitochondria, where they are used in oxidative phosphorylation. Defects in citrin cause citrullinemia type II. Patients manifest later-onset intermittent hyperammonemic encephalopathy as in HHH syndrome. 

Pyrophosphate is rapidly hydrolyzed, and so the equivalent of four molecules of ATP are consumed in these reactions to synthesize one molecule of urea. The synthesis of fumarate by the urea cycle is important because it links the urea cycle and the citric acid cycle (Figure 23.17). Fumarate is hydrated to malate, which is in turn oxidized to oxaloacetate. Oxaloacetate has several possible fates (1) transamination to aspartate, (2) conversion into glucose by the gluconeogenic pathway, (3) condensation with acetyl CoA to form citrate, or (4) conversion into pyruvate. 

The uiea cycle may be considered to be a mitochondrial pathway, as carbamyl phosphate synthase and ornithine transcarbamylase are mitochondrial enzymes however, the enzymes catalyzing subsequent steps of the pathway arc cytosolic-The steps leading to conversion of citrulline to ornithine occur in the cytosol. Hence, the pathway is shared by the mitochondrial and cytosolic compartments. The fumarate produced by the urea cycle is converted to malate by a cytoplasmic form of fumarase. Mittxihondrial fumarase is part of the Krebs cycle. Cytoplasmic malate can enter the mitochondrion by means of a transport system, such as the malate/phosphate exchanger or the ma ate/a-ketoglutaratc exchanger. These transport systems are membrane-bound proteins. 

The answer is e. (Murray, pp 505-626. Scriver, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) All the compounds listed are intermediates of the citric acid cycle. However, only fumarate is an intermediate of both the citric acid and urea cycles. It and arginine are produced from argininosuccinate. Once produced by the urea cycle, fumarate enters the citric acid cycle and is converted to malate and then oxidized to oxaloacetate. Depending upon the organism s needs, oxaloacetate can either enter gluconeogenesis or react with acetyl CoA to form citrate. 

The urea cycle converts NH4 to urea, a less toxic molecule. The sources of the atoms in urea are shown in color. Cit-rulline is transported across the inner membrane by a carrier for neutral amino acids. Ornithine is transported in exchange for H+ or citrulline. Fumarate is transported back into the mitochondrial matrix (for reconversion to malate) by carriers for a-ketoglutarate or tricarboxylic acids. 

After its transport back into the mitochondrial matrix, fumarate is hydrated to form malate, a component of the citric acid cycle. The oxaloacetate product of the citric acid cycle can be used in energy generation, or it can be converted to glucose or aspartate. The relationship between the urea cycle and the citric acid cycle, often referred to as the Krebs bicycle, is outlined in Figure 15.2 ... 

Oxaloacetate is formed in the glyoxylate, citric acid, and urea cycles as a result of catalysis by malate dehydrogenase ... 

Aspartate is involved in the control point of pyrimidine biosynthesis (Reaction 1 below), in transamination reactions (Reaction 2 below), interconversions with asparagine (reactions 3 and 4), in the metabolic pathway leading to AMP (reaction 5 below), in the urea cycle (reactions 2 and 8 below), IMP de novo biosynthesis, and is a precursor to homoserine, threonine, isoleucine, and methionine (reaction 7 below). It is also involved in the malate aspartate shuttle. 

Fumarase is an enzyme of the citric acid cycle, glyoxylate cycle, and urea cycle that catalyzes addition of water to the double bond of fumarate to form L-malate. 

L-malate is an intermediate in the citric acid cycle, urea cycle, amino acid metabolism, the glyoxylate cycle, and shuttles across membranes of the cell (Figure 18.31). 

In the citric acid cycle (and urea cycle), L-malate is produced by addition of HO water to the molecule fiimarate catalyzed by the enzyme fumarate hydratase. D-Malate cannot be produced by the enzyme. 

L-malate is converted to oxaloacetate by action of the enzyme malate dehydrogenase. NADH is another product of this reaction. The same reactions occur in the urea cycle as well. 

Although the major route for aspartate degradation involves its conversion to oxaloacetate, carbons from aspartate can form fumarate in the urea cycle (see Chapter 38). This reaction generates cytosolic fumarate, which must be converted to malate (using cytoplasmic fumarase) for transport into the mitochondria for oxidative or anaplerotic purposes. An analogous sequence of reactions occurs in the purine nucleotide cycle. Aspartate reacts with inosine monophosphate (IMP) to... 

The fumarate formed in the urea cycle is converted to malate and then to oxaloacetate by enzymes of the citrate cycle and is available for the production of more aspartate by transamination. 

Aspartic Acid, as we have seen, not only can transfer its amino group to keto acids but also can supply one nitrogen directly to the urea cycle. In the second process, there arise succinoarginine and fumarate, which becomes malate by the addition of one water molecule. Malate, in turn, is dehydrogenated to oxaloacetate. The latter is also the product of the transamination of aspartate. 

The fumarate produced in step [4] is converted via malate to oxaloacetate [6, 7], from which aspartate is formed again by transamination [9]. The glutamate required for reaction [9] is derived from the glutamate dehydrogenase reaction [8], which fixes the second NH4 " in an organic bond. Reactions [6] and [7] also occur in the tricarboxylic acid cycle. However, in urea formation they take place in the cytoplasm, where the appropriate isoenzymes are available. 

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