"Role of the malate-aspartate shuttle on the metabolic ..

Web Figure 12.5.C Metabolite shuttles across the inner membrane of mitochondria. The two shuttles shown have both been demonstrated to work in isolated mitochondria. (Top) The malate/aspartate shuttle uses isoforms of two enzymes, malate dehydrogenase (1), and aspartate aminotransferase (2), to interconvert malate and aspartate, which are then exchanged using two transporters, the glutamate/aspartate (A) and the 2-oxoglutarate (α-ketoglutarate) (B). The electrogenic glutamate/aspartate transporter provides directionality so that reducing equivalents are transferred into the mitochondrial matrix (see Web Figure 12.5.B). (Bottom) The malate/oxaloacetate (OAA) shuttle uses malate dehydrogenase isoforms (1) in the matrix and in the cytosol to interconvert malate and OAA, which are then exchanged by the OAA transporter (C). This shuttle can theoretically transfer reducing equivalents in either direction depending on the redox conditions on the two sides of the membrane. However, in plant cells it may only serve to export reducing equivalents. (Modified from Siedow and Day 2000.)

The malate-aspartate shuttle and the glycerol-3 ..

Why do brain cells use shuttles that pass electrons from NADH ..

Schematic of the malate/aspartate shuttle

[47] Cheeseman AJ, Clark JB. Influence of the malate-aspartate shuttle on oxidative metabolism is a synaptosomes. J Neurochem 1988; 50: 1559-65.

The normal role of the malate-aspartate shuttle in mitochondria

As a component of the malate/aspartate shuttle, AGC1 transfers the reducing equivalents of NADH + H + from the cytosol into mitochondria ( Indiveri et al., 1987; Palmieri, 2004 ).

Atp Synthesis | Adenosine Triphosphate | Citric Acid Cycle

Although isolated plant mitochondria can oxidize added NADH and NADPH directly by the two external NAD(P)H dehydrogenases (see Web Topic 12.3), they can also use metabolic shuttles (Web Figure 12.5.C). The malate/oxaloacetate shuttle uses malate dehydrogenase in the cytosol and in the matrix to catalyze the interconversion of malate (reduced) and oxaloacetate (oxidized). These two compounds are exchanged by the oxaloacetate transporter (Oliver and McIntosh 1995, Palmieri, L. et al. 2008b). This exchange, however, is not driven by the electrochemical proton gradient across the inner membrane, so it can only move reducing equivalents from a relatively reduced compartment to a relatively oxidized compartment. Since the mitochondrial matrix is much more reduced than the cytosol with respect to the NADH/NAD ratio, the oxaloacetate transporter is likely to work primarily in the export of reducing equivalents (Krömer and Heldt 1991; Wigge et al. 1993).

An Essential Role of the Mitochondrial Electron ..

The transporter for malate2– also transports succinate2– and is therefore called the dicarboxylate transporter. Malate2– or succinate2– can, in turn, exchange for citrate2– on the tricarboxylate transporter. Malate2– can also exchange for 2-oxoglutarate2– via a special transporter. An interesting transporter, the oxaloacetate transporter, exchanges oxaloacetate2– for malate2– (or citrate2–), which permits the shuttling of reducing equivalents (see below). The joint activity of these transporters allows plant mitochondria to affect the net import or export of most citric acid cycle intermediates (Laloi 1999, Palmieri, F. et al. 2011). Finally, the glutamate/aspartate transporter facilitates the uptake of glutamate. In yeast mitochondria, glutamate is taken up together with a proton in exchange for an aspartate carrying two negative charges corresponding to a net transport of one H+. It is therefore driven by both the electrical and the chemical proton gradient. A similar transporter is present in plant mitochondria, but a proton co-transport has not been shown, so the transport may be passive in plants. Recently, several additional transporters for metabolites have been identified (Picault et al. 2004, Palmieri, F. et al. 2011), and it is likely that certain setups of transporters are only expressed under special developmental conditions, like the mobilization of storage nutrients in developing seedlings.

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Web Figure 12.5.B Transporters in the inner membrane of plant mitochondria. All of these transporters are directly or indirectly driven by the electrical and/or chemical part of the proton gradient. Inorganic phosphate (Pi) uptake is driven by the ∆pH (hydroxyl anion moving in the opposite direction) and the resulting Pi gradient is used to drive the uptake of dicarboxylate anions (malate and succinate) by the dicarboxylate transporter. The dicarboxylates can, in turn, exchange for 2-oxoglutarate (also called α-ketoglutarate) or citrate. In the latter case, citrate is protonated on one of its acid groups to maintain transport electroneutrality. The exchange of glutamate (in) for aspartate (out) is electrogenic in baker’s yeast; this means that it must be driven by the electrochemical proton gradient since electroneutral glutamate (glutamate2– + 2 H+) is transported against negatively charged aspartate (Cavero et al. 2003). The latter transporter is important for the operation of the malate/aspartate shuttle (see Web Figure 12.5.C). Similarly, the electroneutral malate/oxaloacetate exchange is involved in the malate/oxaloacetate shuttle. (Modified from Siedow and Day 2000.)