Malate citrate shuttle

FIGURE 25.1 The citrate-malate-pyruvate shuttle provides cytosolic acetate units and reducing equivalents (electrons) for fatty acid synthesis. The shuttle collects carbon substrates, primarily from glycolysis but also from fatty acid oxidation and amino acid catabolism. Most of the reducing equivalents are glycolytic in origin. Pathways that provide carbon for fatty acid synthesis are shown in blue pathways that supply electrons for fatty acid synthesis are shown in red. 

Malate is not the only form in which C4 compounds are exported from mitochondria. Much oxaloacetate is combined with acetyl-CoA to form citrate the latter leaves the mitochondria and is cleaved by the ATP-dependent citrate-cleaving enzymes (Eq. 13-39). This, in effect, exports both acetyl-CoA (needed for lipid synthesis) and oxaloacetate which is reduced to malate within the cytoplasm. Alternatively, oxaloacetate may be transaminated to aspartate. The aspartate, after leaving the mitochondria, may be converted in another transamination reaction back to oxaloacetate. All of these are part of the nonequilibrium process by which C4 compounds diffuse out of the mitochondria before completing the reaction sequence of Eq. 17-46 and entering into other metabolic processes. Note that the reaction of Eq. 17-46 leads to the export of reducing equivalents from mitochondria, the opposite of the process catalyzed by the malate-aspartate shuttle which is discussed in Chapter 18 (Fig. 18-18). The two processes are presumably active under different conditions. 

In animals and fungi there is a similar dichotomy. NADPH can be generated by cytosolic malic enzyme which catalyses the reaction malate + NADP+ — pyruvate + COg + NADPH. Cytosolic malate derives from the following successive reactions the pyruvate/ citrate shuttle on the mitochondrial inner membrane takes pyruvate to the mitochondrion in exchange for citrate cytosolic ATP citrate lyase catalyses ATP + citrate + CoA-SH —> acetylCoA (CH3CO-S-C0A) + oxaloacetate and cytosolic malate dehydrogenase, which catalyses NADH + oxaloacetate NAD+ + malate. This scheme provides both acetylCoA and NADPH for subsequent long chain fatty acid synthesis (see section on Fatty acid synthesis ). 

This important anaplerotic reaction provides a means of replenishing L-malate in the citric acid cycle (Figure 14.18) and it also plays an important role in the citrate shuttle (Figure 18.31). 

Figure 17.4 Schematic view of citrate availability in mitochondria and in the cytosol. Citrate, which is produced in the TCA cycle inside mitochondria without ATP consumption, is exported into the cytosol using the citrate/malate antiport shuttle. Next, citrate is further catabolized via ATP-dependent citrate...

During the formation of carboxylic acid like lA, there will be shuttling of metabolites within the intracellular compartments, having the capability to utilize the enzymes of the respective compartments. Jaklitsch et al. (1991) reported that the CadA, which is e key enzyme for the biosynthesis of lA, is located in cytosol. Otiier important enzymes, such as citrate synthase and aconitase, are found in the mitochondria, but some residual level of these enzymes are also found in the cytosolic fraction. The depicted mechanism is that the ds-aconitate is transported to cytosol assisted by the malate-citrate antiporter. The biosynthetic pathway of LA in the citric acid cycle is illustrated in Fig. 10.5. 

Oxaloacetate is an intermediate of many metabolic pathways. It also plays a role in the malate-aspartate shuttle, which transfers high energy electrons into mitochondria. Citrate is formed by the condensation of oxaloacetate with acetyl CoA. A transamination reaction transfers an amino group from an amino acid to an a-keto acid. Transfer of the amino group from aspartate to a-ketoglutarate forms oxaloacetate and glutamate. In gluconeogenesis, pyruvate is carboxylated in mitochondria to form oxaloacetate. After transfer to the cytosol, the enzyme phosphoenolpyruvate carboxykinase catalyses the conversion of oxaloacetate to phosphoenolpyruvate. 

Supplies P, for oxidative phosphorylation Shuttles reducing equivalents (as malate) from matrix to cytosol Completes shuttling begun by malate-a-ketoglutarate shuttle Provides cytosolic citrate as source of acetyl-CoA for lipid synthesis... 

Fatty acids are generated cytoplasmically while acetyl-CoA is made in the mitochondrion by pyruvate dehydrogenase.This implies that a shuttle system must exist to get the acetyl-CoA or its equivalent out of the mitochondrion. The shuttle system operates in the following way Acetyl-CoA is first converted to citrate by citrate synthase in the TCA-cycle reaction. Then citrate is transferred out of the mitochondrion by either of two carriers, driven by the electroos-motic gradient either a citrate/phosphate antiport or a citrate/malate antiport as shown in Figure 2-2. 

Fatty acid synthesis and degradation. Fatty acids are synthesized in the cytosol by the addition of two-carbon units to a growing chain on an acyl carrier protein. Malonyl CoA, the activated intermediate, is formed by the carboxylation of acetyl CoA. Acetyl groups are carried from mitochondria to the cytosol as citrate by the citrate-malate shuttle. In the cytosol, citrate is cleaved to yield acetyl CoA. In addition to transporting acetyl CoA, citrate in the cytosol stimulates acetyl CoA carboxylase, the enzyme catalyzing the committed step. When ATP and acetyl CoA are abundant, the level of citrate increases, which accelerates the rate of fatty acid synthesis (Figure 30.8). 

Both malate enzyme and citrate lyase are part of the shuttle system that transports two-carbon units from the mitochondrion to the cytosol. Malate enzyme also generates reducing power in the form of NADPH, which is used for fatty acid synthesis however, the pentose phosphate pathway (see the text. Section 20.3) also serves as a source of NADPH, so that fatty acid synthesis can continue even if malate enzyme is deficient. Recall from page 515 of the text that malate can cross the mitochondrial membrane. Citrate lyase is more critical to fatty acid synthesis because it is required to generate acetyl CoA from citrate in the cytosol. Without cytosolic acetyl CoA, fatty acid synthesis cannot take place, and the cells cannot grow and divide. 

The noncompartmented model consisted of 73 transformers (reaction rates and transport step), balancing 53 metaboHtes. The compartmented approach was buUt on 95 transformers and 75 metabolites. While most of the simulated fluxes were similar in both approaches, a fraction of approximately 15% of totally available ATP was missing in the compartmented model compared to the noncompartmented approach. This was due to the assumed activity of the citrate-pyruvate shuttle that imports pyruvate (via pyruvate/H+ symporter) into mitochondria by exporting citrate (via citrate/malate antiporter). In cytoplasm, citrate is further... 

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