Glutamine, transamination reactions

Problem 21.5 (a) What is the relationship between reactants and products in the transamination reaction (b) Which ketoacid is needed to give (i) alanine (ii) leucine (iii) serine (iv) glutamine (c) Which amino acids cannot be made by transamination 4... 

The amino acid and nucleotide biosynthetic pathways make repeated use of the biological cofactors pyridoxal phosphate, tetrahydrofolate, and A-adenosylmethionine. Pyridoxal phosphate is required for transamination reactions involving glutamate and for other amino acid transformations. One-carbon transfers require S-adenosyhnethionine and tetrahydrofolate. Glutamine amidotransferases catalyze reactions that incorporate nitrogen derived from glutamine. 

At pH 7.5 less than 1% of the compound exists in the open-chain a-keto-acid form while at pH 9 approximately 3% is in a form that reacts as a typical a-keto acid. Glutamine transaminase also catalyzes the transamination of glutamic acid y-A -methylamide the expected transamination product, a-keto-A-methylglutaramic acid has not yet been isolated from a transamination reaction mixture. However, this compound was... 

The following enzymic reactions were observed. Dialyzed extracts of S. glebosus catalyzed the conversion of mt/o-[U-,4C]inositol (31, Scheme 12) to aminodeoxy-scyHo-[U-14C]inositol (33) when both NAD+ and an amino donor were provided. NAD+ was presumably required in order to form keto-sct/Wo-[U-14C]inositol (32), and the amino donor (for example, L-glutamine) was required in order to convert 32 into 33. The transamination reaction was confirmed in a separate assay. Assays for reactions H and I (see Scheme 12) with S. glebosus were negative. 

Ammonia is produced by almost all cells in the body however, only the liver has the enzymatic machinery to convert it to urea. Therefore, extra-hepatic ammonia must be transported to the liver. However, anunonia in the blood is toxic to cells, and therefore the nitrogen from amino acid catabolism is transported in blood either as glutamine or alanine. Glutamine is synthesized from Glu and ammonia in an ATP-requiring reaction that is catalyzed by glutamine synthetase. Alanine is formed from pyruvate in a transamination reaction catalyzed by alanine transaminase (ALT). 

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. 

Two types of reactions play prominent roles in amino acid metabolism. In transamination reactions, new amino acids are produced when a-amino groups are transferred from donor a-amino acids to acceptor a-keto acids. Because transamination reactions are reversible, they play an important role in both amino acid synthesis and degradation. Ammonium ions or the amide nitrogen of glutamine can also be directly incorporated into amino acids and eventually other metabolites. 

Alanine is an allosteric inhibitor of glutamine synthetase, an enzyme with a central role in nitrogen metabolism in the cell. Alanine participates in transamination reactions and in the glucose-alanine cycle. 

Glutamine is an oi amino acid found in proteins. In mammals, glutamine is a non-essential amino acid, meaning it does not need to be present in the diet. Glutamine is classified as an amide because it is an amide derivative of glutamic acid (Reaction 1 below). Glutamine is a very important compound in transamination reactions. 

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). 

In muscle and brain, but not in liver, the purine nucleotide cycle allows Nlij to be released from amino acids (see Fig. 38.5). Nitrogen is collected by glutamate from other amino acids by means of transamination reactions. Glutamate then transfers its amino group to oxaloacetate to form aspartate, which supplies nitrogen to the purine nucleotide cycle (see Chapter 41). The reactions of the cycle release fumarate and NH4. The ammonium ion formed can leave the muscle in the form of glutamine. 

In nonlegumes, Mo deficiency hampers NOj" reduction and decreases the amounts of most amino acids. Addition of Mo to deficient plants has been found to increase the contents of glutamic acid, glutamine, a-alanine, serine, and aspartic acid in spinach Spinacea oleracea L.), cauliflower, tomato Lycopersicon esculentum Mill.) (Mulder et al., 1959), and maize (Berducou and Mache, 1963). However, decreases in the contents of some amino acids and amides during later stages of growth of Mo-fertilized crops can result from their incorporation into proteins or from subsequent metabolic reactions such as transamination reactions or conversion to amides (Possingham, 1957). 

The amino acids glutamate and glutamine are the principal donors of amino groups in transamination reactions. 

What are some common features in amino acid biosynthesis In the anabolism of amino acids, transamination reactions play an important role. Glutamate and glutamine are frequently the amino-group donors. The enzymes that catalyze transamination reactions frequently require pyridoxal phosphate as a coenzyme. One-carbon transfers also operate in the anabolism of amino acids. Carriers are required for the one-carbon groups transferred. Tetrahydrofolate is a carrier of methylene and formyl groups, and S-adenosylmethionine is a carrier of methyl groups. 

Alanine appears to be formed primarily by alanine dehydrogenase in A. cylindrica, C. licheniformey P. boryanumy and A. nidulanSy because the inhibition of formation of glutamate and glutamine by methionine sulfoximine is not accompanied by a comparable inhibition in the formation of alanine (Table I). Moreover, alanine formation by these species was not reduced in the presence of aminooxy acetate, an inhibitor of amino transfer reactions (I3,J4). However, in A. variabilis alanine appears to be formed by a transamination reaction, because synthesis of [ N]alanine from was strongly inhibited in the presence of... 

Two observations indicate that aspartate is formed primarily by a transamination reaction, probably from glutamate, rather than by a direct amination with NH4 in A. cylindrica. First, N-label in aspartate was reduced more than 90% by the presence of aminooxy acetate, while labeling of glutamate and glutamine was not reduced (IS). Second, [ N]aspartate was not detected when glutamate formation was inhibited either directly, by azaserine, or indirectly, when glutamine s)mthesis was inhibited by methionine sulfoximine (13). Moreover, activity of a glutamate-aspartate aminotransferase reaction has been detected in vitro in two strains of A. cylindrica (1,26). 

Asparagine and glutamine are important amide derivatives of the amino acids aspartic acid and glutamic acid. These two amides are also classed as amino acids (Table 4.1) and occur as components of proteins. They also occur as free amides and play an important role in transamination reactions. 

The glutamate may undergo further amination to give glutamine but, more importantly, may undergo transamination reactions with various keto acids to give amino acids as shown in Fig. 9.15. 

S) Amino Acid Amide Transaminases. Mention was made previously (see Amidases) of the observations by Greenstein and co-workers that the addition of pyruvic and other a-keto acids to liver extracts accelerated the formation of ammonia from glutamine and asparagine. This reaction has been shown by Meister and co-workers to involve a transamination reaction different in several respects from those previously discussed. The reaction for glutamine may be formulated as follows ... 

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