Asparagine transamination

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

Transamination with Glutamine and Asparagine. Meister and collaborators have described two types of transaminase in which the amino donor is glutamine or asparagine. Transamination from glutamine to any of more than 30 a-keto acids leads to the formation of a-ketoglutaramic acid, which is hydrolyzed by a specific amidase (HI). The reaction... 

CINCEROVA A. 1969. Effect of trophic conditions on asparagine transamination in wheat plants. Biologia Plantarum, 11, 139-148. 

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). 

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. 

Aspartate and asparagine are converted into oxaloacetate, a citric acid cycle intermediate. Aspartate, a four-carbon amino acid, is directly transaminated to oxaloacetate. 

Asparagine is hydrolyzed by asparaginase to NH4 + and aspartate, which is then transaminated. 

Glucose may be converted to pyruvate, which forms oxaloacetate, which by transamination forms aspartate and, subsequently, asparagine. 

In transamination reactions, amino groups are transferred from one carbon skeleton to another. In reductive animation, amino acids are synthesized by the incorporating of free NH) or the amide nitrogen of glutamine or asparagine into a-keto acids. Ammonium ions are also incorporated into cellular metabolites by the animation of glutamate to form glutamine. 

AMINO ACIDS FORMING OXALOACETATE Both aspartate and asparagine are degraded to form oxaloacetate. Aspartate is converted to oxaloacetate with a single transamination reaction. Asparagine is initially hydrolyzed to yield aspartate and NHJ by asparaginase. 

Aspartate is involved in the control point of pyrimidine biosynthesis (Reaction 1 below), in transamination reactions (Reaction 2 below), interconversions with asparagine (reactions 3 and 4), in the metabolic pathway leading to AMP (reaction 5 below), in the urea cycle (reactions 2 and 8 below), IMP de novo biosynthesis, and is a precursor to homoserine, threonine, isoleucine, and methionine (reaction 7 below). It is also involved in the malate aspartate shuttle. 

See also Table 5.1, Genetic Code, Asparagine, Citrulline, Urea Cycle, Transamination in Amino Acid Metabolism, Citric Acid Cycle Intermediates in Amino Acid Metabolism, Essential Amino Acids... 

The illustrations here and here show the transamination reactions interconverting ot-ketoglutarate, glutamate, and glutamine (see here) and oxaloacetate, aspartate, and asparagine (see here). Notice in each case that one enzyme is primarily involved in the anabolic reactions (making an amino acid) whereas a different enzyme is involved in the catabolic pathway (breaking down an amino acid). 

Two of the enzymes involved in the methionine cycle, namely, MTA nucleosidase [46] and MTR kinase [47] were purified from plants and characterised. However, the plant enzyme catalysing the formation of KMB from MTR-P has not been characterised. In rat liver extracts, three enzymes are involved in the conversion [42]. The first enzyme isomerizes MTR-P to l-phospho-5-methylthioribulose (MTRu-P), the second enzyme produces two unidentified metabolites from MTRu-P, and the third enzyme catalyses the conversion of these two metabolites to KMB with the uptake of oxygen. Plants may produce KMB from MTR-P by similar enzymes. The last step of the methionine cycle that converts KMB to methionine is most likely a transamination, and this activity was also detected in avocado extracts in the presence of asparagine [45]. 

A number of reports have described the transamination of asparagine by crude extracts or by more purified preparations (cf. Lea and Miflin, this volume. Chapter 16 and Mazelis, this volume. Chapter 15 Wightman and Forest, 1978 Lloyd and Joy, 1978). The transamination of asparagine to a-ketosuccinamic acid has been found to take place using either glyoxylate or pyruvate as the amino acceptor, and to a lesser extent with oxaloacetate (Streeter, 1977). 

The transamination of asparagine in soybean leaf extracts was detected by Streeter (1977). Glyoxylic acid was the most active 2-oxo acid acceptor, followed by pyruvate, oxaloacetate, and 2-oxoglutarate. The breakdown of the product 2-oxosuccinamic acid to oxaloacetate and ammonia was also detected. 

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