Citrate shuttle

Citrate may leave the mitochondria (citrate shuttle) to deliver acetyl CoA into the cytoplasm for fatty acid synthesis. 

The citrate shuttle transports acetyl CoA groups from the mitochondria to the cytoplasm for fatty acid synthesis. Acetyl CoA combines with oxaloacetate in the mitochondria to form citrate, but rather than continuing in the citric add cycle, citrate is transported into the cytoplasm. Factors that indirectly promote this process indude insuKn and high-energy status. 

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 ). 

Cytosolic generation of acetyl-CoA ( citrate shuttle ) This pathway is shown in Figure 18-13. Citrate synthesized from oxaloacetate and acetyl-CoA is transported to the cytosol via the tricarboxylate anion carrier system and cleaved to yield acetyl-CoA and oxaloacetate. 

See also Oxaloacetate, Fumarate, Citrate Shuttle, Shuttling Electron Carriers into the Mitochondrion, Figure 15.11... 

See also Citric Acid Cycle, Glyoxylate Cycle, Citric Acid Cycle Enzymes, Citric Acid Cycle Intermediates, Figure 14.3, Table 14.1, Citrate Shuttle... 

When acetyl-CoA accumulates in the mitochondrial matrix (for example, after a big meal), it must be moved to the cytoplasm where it can be used in fatty acid biosynthesis. Acetyl-CoA cannot pass directly through the inner membrane of the mitochondrion, however, and must be shuttled out of the mitochondrion on the back of oxaloacetate (to form citrate). The citrate shuttle system operates as follows (see Figure 18.31) ... 

Citrate lyase is a cytoplasmic enzyme that is important in the citrate shuttle, which moves excess acetyl-CoA from the mitochondrion to the cytoplasm. Citrate lyase catalyzes the reaction below ... 

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). 

See also Citric Acid Cycle, Anaplerotic Reaction, Citrate Shuttle... 

Nonetheless, mitochondria do not contain an acetyl CoA transporter, therefore a shuttle system, called the citrate shuttle, is required to move the C2 units across the membrane citrate transport out of the mitochondria provides a mechanism to stimulate fatty acid synthesis in the cytosol where acetil Co-A is cleaved back to oxaloacetate and acetyl-CoA by the ATP-citrate lyase (Figure 4) [98, 99]. 

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. 

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... 

Is part of mechanism for shuttling citrate from matrix to cytosol Imports fatty acids for fuel... 

FIGURE 21-10 Shuttle for transfer of acetyl groups from mitochondria to the cytosol. The mitochondrial outer membrane is freely permeable to all these compounds. Pyruvate derived from amino acid catabolism in the mitochondrial matrix, or from glucose by glycolysis in the cytosol, is converted to acetyl-CoA in the matrix. Acetyl groups pass out of the mitochondrion as citrate in the cytosol they are de-... 

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. 

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. 

VDAC plays a role in the regulated flux of metabolites—usually anionic species such as phosphate, chloride, organic anions, and the adenine nucleotides—across the outer membrane. VDAC appears to form an open p -barrel structure similar to that of the bacterial porins (Section 12.5.2). although mitochondrial porins and bacterial porins may have evolved independently. Some cytoplasmic kinases bind to VDAC, thereby obtaining preferential access to the exported ATP. In contrast, the inner membrane is intrinsically impermeable to nearly all ions and polar molecules. A large family of transporters shuttles metabolites such as ATP, pyruvate, and citrate across the inner mitochondrial membrane. The two faces of this membrane will be referred to as the matrix side and the cytosolic side (the latter because it is freely accessible to most small molecules in the cytosol). They are also called the N and P sides, respectively, because the membrane potential is negative on the matrix side and positive on the cytosolic side. 

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). 

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