Oxaloacetate-malate system

There are two shuttle systems the glycerol-3-phosphate shuttle found in skeletal muscle and nerve cells and the oxaloacetate-malate shuttle foimd in heart and liver cells. Because skeletal muscle produces the majority of the ATP for the body, it is the glycerol-3-phosphate shuttle that is used most commonly when discussing metabolic energy )delds. In Example 22.1, calculation of the ATP harvest of glycolysis is based on this shuttle. 

The oxaloacetate-malate shuttle system is more efficient. In this system, cytoplasmic NADH reduces oxaloacetate to malate. 

Other bacteria are able to use as electron donors compounds that constitute the reduced components of redox systems with E g values more negative than that of the quinone/quinol redox system (E g - OV) which binds at the Qg site of BPS [Fig. 3(c)]. Examples of such electron donors are H2S (E g of S/ HgS = -0.23V), which is used by some purple sulfur bacteria (including the versatile Chromatium), and malic acid (E of oxaloacetate/malate = -0.17V), which is used by some purple- and green non-sufur bacteria. Organisms using such electron donors have an enzyme that catalyses the generalized reaction shown in eq. 2 this enzyme is the functional equivalent of the succinate UQ oxidoreductase of complex II of the mitochondrial electron transport chain. 

The second electron shuttle system, called the malate-aspartate shuttle, is shown in Figure 21.34. Oxaloacetate is reduced in the cytosol, acquiring the electrons of NADH (which is oxidized to NAD ). Malate is transported across the inner membrane, where it is reoxidized by malate dehydrogenase, converting NAD to NADH in the matrix. This mitochondrial NADH readily enters the electron transport chain. The oxaloacetate produced in this reaction cannot cross the inner membrane and must be transaminated to form aspartate, which can be transported across the membrane to the cytosolic side. Transamination in the cytosol recycles aspartate back to oxaloacetate. In contrast to the glycerol phosphate shuttle, the malate-aspartate cycle is reversible, and it operates as shown in Figure 21.34 only if the NADH/NAD ratio in the cytosol is higher than the ratio in the matrix. Because this shuttle produces NADH in the matrix, the full 2.5 ATPs per NADH are recovered. 

The bioluminescent determinations of ethanol, sorbitol, L-lactate and oxaloacetate have been performed with coupled enzymatic systems involving the specific suitable enzymes (Figure 5). The ethanol, sorbitol and lactate assays involved the enzymatic oxidation of these substrates with the concomitant reduction of NAD+ in NADH, which is in turn reoxidized by the bioluminescence bacterial system. Thus, the assay of these compounds could be performed in a one-step procedure, in the presence of NAD+ in excess. Conversely, the oxaloacetate measurement involved the simultaneous consumption of NADH by malate dehydrogenase and bacterial oxidoreductase and was therefore conducted in two steps. 

The reaction will proceed toward oxaloacetate formation in the cell if low product concentration is maintained. Oxidation of NADH by the mitochondrial electron transport system and utilization of oxaloacetate in the formation of citrate shifts the malate-oxaloacetate reaction toward oxaloacetate production. 

Figure 2.3(A). The Krebs cycle is initiated with the condensation of oxaloacetate and acetylCoA and ends with the formation of oxaloacetate from malate. Two carbons enter the cycle and two carbons are released as C02 in one turn of the cycle in steady state, the cycle is therefore a catalytic system (intermediates are neither accumulated nor depleted) analogous to a super-enzyme. The cycle is considered the hub of cell metabolism, the final and common pathway for the complete catabolism of most carbon fuels.

It is often necessary to move electrons into mitochondria for disposal via oxidative phosphorylation. However, NADH and FADH2 do not penetrate the inner mitochondrial membrane. Instead, such electrons may first be passed to dihydroxyacetone phosphate or to oxaloacetate to make glycerol-3-phosphate and malate, respectively. These compounds can penetrate the inner mitochondrial membrane via the porters described earlier and oxidized there by mitochondrial NAD+ or FAD. These systems are termed the glycerol-3-phosphate and malate shuttles, respectively, and they are described in greater detail in Chapter 18. 

Figure 16.28. Compartmental Cooperation. Oxaloacetate utilized in the cytosol for gluconeogenesis is formed in the mitochondrial matrix by carboxylation of pyruvate. Oxaloacetate leaves the mitochondrion by a specific transport system (not shovm) in the form of malate, -which is reoxidized to oxaloacetate in the cytosol.

Figure 15.11b shows the malate/aspartate shuttle system, which is particularly active in liver and heart. It uses malate, aspartate, and oxaloacetate to shuttle cytoplasmic electrons from NADH into the mitochondrial matrix. In this shuttle, NADH reduces oxaloacetate to malate, which travels through an inner membrane transport system that ultimately exchanges the malate for an ot-ketoglutarate. To do... 

In the presence of oxygen and the cytochrome system, succinate is finally transformed to fumarate by a dehydrogenation process, and in turn, fumarate is converted into malate, which itself is dehydrogenated to form oxaloacetate. 

This particular form of the system of dicarboxylic acids had to be abandoned when Krebs showed that a-ketoglutarate and citrate, in addition to succinate, fumarate, malate and oxaloacetate, also re-establish the respiration of a muscle pulp. The case of a-ketoglutaric acid, in the scheme of Szent-Gyorgyi, did not present an insurmountable difficulty since the oxidative decarboxylation of a-ketoglutarate pelds succinic acid. 

The enzyme can be measured in a kinetic manner by a coupled enzyme reaction system. The oxaloacetate formed by aspartate aminotransferase is converted to malate by including malate dehydrogenase in the assay system. This is accompanied by the oxidation of NADH to NAD which can be followed spectrophotometrically at 340 nm. 

Figure 17.2 Anaerobic metabolic pathways involved in SA production in wild-type Anaerobiospirillum succiniciproducens. PEP, phosphoenolpyruvate OAA, oxaloacetate MAE, malate FUM, fumarate El, nonspecific protein of the phosphotransferase system (PTS) HPr, non-specific phosphoryl carrier protein of PTS pyk, pyruvate kinase pfo.

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