Transamination oxaloacetate

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... 

Keto acids are acceptors and oxaloacetate Transaminates... giving rise to aspartate. 

PEP is carboxylated to oxaloacetate, transaminated to aspartate ar d shunted into the bundle sheath. Aspartate then is transaminated back to oxaloacetate in the mesophyll, reduced to malate by NADH and decarboxylated to pyruvate by NAD-linked malic enzyme in the bundle-sheath mitochondria. There is no net production of NAD(P)H so the chloroplasts have PSII and may be less effective at inhibiting photorespiration. Pyruvate is phosphorylated in the mesophyll back to PEP, using 2 ATP (Fig. 13.18). 

PEP carboxylase occurs in yeast, bacteria, and higher plants, but not in animals. The enzyme is specifically inhibited by aspartate, which is produced by transamination of oxaloacetate. Thus, organisms utilizing this enzyme control aspartate production by regulation of PEP carboxylase. Malic enzyme is found in the cytosol or mitochondria of many animal and plant ceils and is an NADPIT-dependent enzyme. 

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. 

Aspartate and Asparagine. Transamination of oxaloacetate forms aspartate. The conversion of aspartate... 

Figure 28-3. Formation of alanine by transamination of pyruvate. The amino donor may be glutamate or aspartate. The other product thus is a-ketoglutarate or oxaloacetate.

Transamination is the most common initial reaction of amino acid catabohsm. Subsequent reactions remove any additional nitrogen and restmcmre the hydrocarbon skeleton for conversion to oxaloacetate, a-ketoglutarate, pyruvate, and acetyl-CoA. 

The nitrogen contained in the amino acids is usually disposed of through the urea cycle. One of the early, if not the first, steps in amino acid catabolism involves a transamination using oxaloacetate or a-ketoglutarate as the amino-group acceptor. This converts the amino acid into a 2-keto acid, which can then be metabolized further. 

The alanine cycle accomplishes the same thing as the Cori cycle, except with an add-on feature (Fig. 17-11). Under conditions under which muscle is degrading protein (fasting, starvation, exhaustion), muscle must get rid of excess carbon waste (lactate and pyruvate) but also nitrogen waste from the metabolism of amino acids. Muscle (and other tissues) removes amino groups from amino acids by transamination with a 2-keto acid such as pyruvate (oxaloacetate is the other common 2-keto acid). 

It was then possible to confirm the existence of two transaminating systems, the original one utilizing pyruvate as amino acceptor, and a second which used oxaloacetate. Both enzymes were purified and found to be very specific for their substrates. The reactions catalyzed were freely reversible. 

The association between vitamin B6 deficiency and transamination emerged from 1945 when Schlenk and Fisher noted that pyridoxine-deficient rats had a diminished capacity for transamination. In the same year Gunsalus and his colleagues found transamination in Streptococcus faecalis depended on pydridoxal phosphate. The properties of the heat-stable component in purified glutamic-oxaloacetate transaminase were similar to those of pydridoxal phosphate. Later pyri-doxal phosphate was established as an essential coenzyme in many amino acid transformations. 

Transamination of alanine yields pyruvate catalysed by alanine transaminase (ALT) whilst aspartate produces oxaloacetate catalysed by aspartate transaminase (AST). All transaminase enzymes operate close to a true equilibrium (K eq 1, see Chapter 2) and... 

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. 

Non-essential amino acids are those that arise by transamination from 2-oxoacids in the intermediary metabolism. These belong to the glutamate family (Glu, Gin, Pro, Arg, derived from 2-oxoglutarate), the aspartate family (only Asp and Asn in this group, derived from oxaloacetate), and alanine, which can be formed by transamination from pyruvate. The amino acids in the serine family (Ser, Gly, Cys) and histidine, which arise from intermediates of glycolysis, can also be synthesized by the human body. 

In the malate shuttle (left)—which operates in the heart, liver, and kidneys, for example-oxaloacetic acid is reduced to malate by malate dehydrogenase (MDH, [2a]) with the help of NADH+HT In antiport for 2-oxogluta-rate, malate is transferred to the matrix, where the mitochondrial isoenzyme for MDH [2b] regenerates oxaloacetic acid and NADH+HT The latter is reoxidized by complex I of the respiratory chain, while oxaloacetic acid, for which a transporter is not available in the inner membrane, is first transaminated to aspartate by aspartate aminotransferase (AST, [3a]). Aspartate leaves the matrix again, and in the cytoplasm once again supplies oxalo-acetate for step [2a] and glutamate for return transport into the matrix [3b]. On balance, only NADH+H"" is moved from the cytoplasm into the matrix ATP is not needed for this. 

Pyruvate is first transported from the cytosol into mitochondria or is generated from alanine within mitochondria by transamination, in which the a-amino group is removed from alanine (leaving pyruvate) and added to an a-keto carboxylic acid (transamination reactions are discussed in detail in Chapter 18). Then pyruvate carboxylase, a mitochondrial enzyme that requires the coenzyme biotin, converts the pyruvate to oxaloacetate (Fig. 14-17) ... 

The carbon skeletons of six amino acids are converted in whole or in part to pyruvate. The pyruvate can then be converted to either acetyl-CoA (a ketone body precursor) or oxaloacetate (a precursor for gluconeogenesis). Thus amino acids catabolized to pyruvate are both ke-togenic and glucogenic. The six are alanine, tryptophan, cysteine, serine, glycine, and threonine (Fig. 18-19). Alanine yields pyruvate directly on transamination with... 

The carbon skeletons of asparagine and aspartate ultimately enter the citric acid cycle as oxaloacetate. The enzyme asparaginase catalyzes the hydrolysis of asparagine to aspartate, which undergoes transamination with a-lcetoglutarate to yield glutamate and oxaloacetate (Fig. 18-29). 

In plants of tropical origin, the first intermediate into which 14C02 is fixed is oxaloacetate, a four-carbon compound. This reaction, which occurs in the cytosol of leaf mesophyll cells, is catalyzed by phosphoenolpyru-vate carboxylase, for which the substrate is HC03, not C02. The oxaloacetate thus formed is either reduced to malate at the expense of NADPH (as shown in Fig. 20-23b) or converted to aspartate by transamination ... 

Glucogenic amino acids (see Table 14-4) derived from the breakdown of stored seed proteins also yield precursors for gluconeogenesis, following transamination and oxidation to succinyl-CoA, pyruvate, oxaloacetate, fumarate, and a-ketoglutarate (Chapter 18)—all good starting materials for gluconeogenesis. 

Alanine and aspartate are synthesized from pyruvate and oxaloacetate, respectively, by transamination from glutamate. Asparagine is synthesized by amidation of aspartate, with glutamine donating the NH4. These are nonessential amino acids, and their simple biosynthetic pathways occur in all organisms. 

Malate is oxidized to oxaloacetate by malate dehydrogenase (Figure 9.7). This reaction produces the third and final NADH of the cycle. [Note Oxaloacetate is also produced by the transamination of the amino acid, aspartic acid.]... 

Alanine, aspartate, and glutamate are synthesized by transfer of an amino group to the a-keto acids pyruvate, oxaloacetate, and a-keto-glutarate, respectively. These transamination reactions (Figure 20.12, and see p. 248) are the most direct of the biosynthetic pathways. Glutamate is unusual in that it can also be synthesized by the reverse of oxidative deamination, catalyzed by glutamate dehydrogenase (see p. 249). 

Biotin acts as a carboxyl group carrier in a series of carboxylation reactions, a function originally suggested by the fact that aspartate partially replaces biotin in promoting the growth of the yeast Torula cremonis. Aspartate was known to arise by transamination from oxaloacetate, which in turn could be formed by carboxylation of pyruvate. Subsequent studies showed that biotin was needed for an enzymatic ATP-dependent reaction of pyruvate with bicarbonate ion to form oxaloacetate (Eq. 14-3). This is a (3 carboxylation coupled to the hydrolysis of ATP. 

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