Oxaloacetate. aspartate from

Fig. 1.6 Reactions of the malate-aspartate shuttle showing the transport of reducing equivalents from cytoplasm to mitochondria. a-KG, a-Ketoglutarate Asp, aspartate Glu, glutamate OAA, oxaloacetate aspartate aminotransferase (1) and malate dehydrogenase (2)...

The enzymes are, respectively (1) mitochondrial malate dehydrogenase (2) cytoplasmic malate dehydrogenase (3) phosphoenolpyruvate carboxykinase (4) pyruvate kinase and pyruvate dehydrogenase. The acetyl-CoA could then condense with oxaloacetate (produced from a second molecule of aspartate) to yield citrate. Aspartate could, therefore, continue to supply acctyl-CoA, which would continue to fuel the citric acid cycle. 

To give a real example, have a closer look on main functions and cycle of magnesium in green plants. Control on autocatalysis depends on the principal functions of Mg, that is, on photosynthesis when substantial parts of Mg taken up by roots are allocated to chlorophyll and rubisco synthesis, less will be available for other metabolic pathways, reducing the turnovers there unless there are lots of Mg around like in marine plants. In addition, the tricarboxylate cycle (citrate cycle) requires Mg (besides Fe and Mn) to produce the enzymes hence some Mg (as well as Fe, Mn) must be invested to produce the citrate (malate, oxaloacetate (aspartate)) ions delivered by the roots to render Mg (and other metals) in turn bioavailable by means of complexation and resorption of almost neutral complex entities. Furthermore, the tricarboxylate cycle is coupled to biosynthesis of amino acids by redox transamination hence there will be both competition at the metal center(s) and possible extraction of metal ions from enzymes once NHj and electrons are... 

Alanine from pyruvate aspartate from oxaloacetate glutamate from a-ketoglutarate. 

Fig. 21. Synthesis of alanine from glucose, with recovery of glutamate through a loop involving oxaloacetate, aspartate, fumarate, and malate.

Oxaloacetate, generated from pyruvate by pyruvate carboxylase or from amino acids that form intermediates of the TCA cycle, does not readily cross the mitochondrial membrane. It is either decarboxylated to form PEP by the mitochondrial PEPCK or it is converted to malate or aspartate (see Figs. 31.7B and 31.7C). The conversion of oxaloacetate to malate requires NADH. PEP, malate, and aspartate can be transported into the cytosol. 

Oxaloacetate, produced from malate or aspartate in the cytosol, is converted to PEP by the cytosolic PEPCK (see Eig. 31.7A). 

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... 

Rosenberg, L.L., Capindale,J.B., Whatley, F.R. Formation of oxaloacetate and aspartate from phosphoenol-pyruvate in spinach leaf chloroplast extract. Nature (London) 181, 632-633 (1958)... 

Kato C, Kurihara T, Kobashi N, Yamane H, Nishiyama M (2004) Conversion of feedback regulation in aspartate kinase by domain exchange. Biochem Bioph Res Co 316 802-808 Kim YH, Park JS, Cho JY, Cho KM, Park YH, Lee J (2004) Proteomic response analysis of a threonine-overproducing mutant of Escherichia coli. Biochem J 381 823-829 Klaffl S, Eikmanns BJ (2010) Genetic and functional analysis of the soluble oxaloacetate decarboxylase from Corynebacterium glutamicum. J Bacterid 192 2604-2612 Komatsubara S, Kisumi M, Murata K, Chibata 1 (1978) Threonine production by regulatory mutants of Serratia marcescens. Appl Environ Microbiol 35 834-840 Kotaka M, Ren J, Lockyer M, Hawkins AR, Stammers DK (2006) Structures of R- and T-state Escherichia coli aspartokinase 111 mechanisms of the allosteric transition and inhibition by lysine. J Biol Chem 281 31544-31552... 

Seven of the eleven nonessential amino acids are synthesized either from pyruvate or from the citric acid cycle intermediates oxaloacetate and a-keto-glutarate. Alanine is biosynthesized hy transamination of pyruvate, aspartate from oxaloacetate, and glutamate from a-ketoglutarate. The mechanisms of... 

FIGURE 14.22 Glutamate aspartate aminotransferase, an enzyme conforming to a double-displacement bisnbstrate mechanism. Glutamate aspartate aminotransferase is a pyridoxal phosphate-dependent enzyme. The pyridoxal serves as the —NH, acceptor from glntamate to form pyridoxamine. Pyridoxamine is then the amino donor to oxaloacetate to form asparate and regenerate the pyridoxal coenzyme form. (The pyridoxamine enzyme is the E form.)... 

Glyoxysomes do not contain all the enzymes needed to run the glyoxylate cycle succinate dehydrogenase, fumarase, and malate dehydrogenase are absent. Consequently, glyoxysomes must cooperate with mitochondria to run their cycle (Figure 20.31). Succinate travels from the glyoxysomes to the mitochondria, where it is converted to oxaloacetate. Transamination to aspartate follows... 

A possible explanation for the superiority of the amino donor, L-aspartic add, has come from studies carried out on mutants of E. coli, in which only one of the three transaminases that are found in E. coli are present. It is believed that a branched chain transaminase, an aromatic amino add transaminase and an aspartate phenylalanine aspartase can be present in E. coli. The reaction of each of these mutants with different amino donors gave results which indicated that branched chain transminase and aromatic amino add transminase containing mutants were not able to proceed to high levels of conversion of phenylpyruvic add to L-phenylalanine. However, aspartate phenylalanine transaminase containing mutants were able to yield 98% conversion on 100 mmol l 1 phenylpyruvic acid. The explanation for this is probably that both branched chain transaminase and aromatic amino acid transminase are feedback inhibited by L-phenylalanine, whereas aspartate phenylalanine transaminase is not inhibited by L-phenylalanine. In addition, since oxaloacetate, which is produced when aspartic add is used as the amino donor, is readily converted to pyruvic add, no feedback inhibition involving oxaloacetate occurs. The reason for low conversion yield of some E. coli strains might be that these E. cdi strains are defident in the aspartate phenylalanine transaminase. 

Aminotransferase (transaminase) reactions form pymvate from alanine, oxaloacetate from aspartate, and a-ketoglutarate from glutamate. Because these reactions are reversible, the cycle also serves as a source of carbon skeletons for the synthesis of these amino acids. Other amino acids contribute to gluconeogenesis because their carbon skeletons give rise to citric acid cycle... 

It was observed that glutamate and aspartate are diverted predominantly to the synthesis of cell substance rather than to the formation of oxalate. It is not inconsistent to see oc-ketoglutarate being formed from glutamate, while no oxaloacetic acid can be detected in the medium containing aspartate, as the oxaloacetic acid is known to be extremely unstable (2), (62), (Hi). The relatively low yields of oxalic acid, derived... 

The aspartate and glutamate produced by these reactions, plus those taken up from the lumen, are metabolised to oxaloacetate and oxoglutarate, respectively, as discussed above. The a-NH2 group in these amino acids is transferred to pyruvate to form alanine, which is released and then taken up by the liver, where the NH2 group is converted to ammonia and then to urea. 

The oxaloacetate is then transported from mitochondrion into the cytosol but not directly, since there is no transporter for oxaloacetate in the mitochondrial membrane. This problem is solved by conversion of oxaloacetate to aspartate, by transamination, and it is the aspartate that is transported across the inner mitochondrial membrane to the cytosol, where oxaloacetate is regenerated from aspartate by a cytosolic aminotransferase enzyme. 

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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