Enzyme glutamine synthetase

As mentioned in Section II., Meister and his co-workers (64) have studied extensively the substrate specificity of the enzyme glutamine synthetase from... 

Some tumour cells possess the enzyme glutamine synthetase and so are able to synthesise glutamine from glutamate as expected, proliferation of these tumour cells is not dependent upon the presence of glutamine in the culture medium. 

Glutamine Synthetase. The enzyme glutamine synthetase is very important in the control of nitrogen metabolism in plants. It catalyses... 

This process of transdifferentiation is preceded by a significant decline in the activity of those enzymes that are recognized as markers of neuroglial specificity. This is the case of the enzyme glutamine synthetase, the marker of astrocytes, which... 

Figure 3.1 Proteins come in different shapes and sizes (a) the enzyme glutamine synthetase, (b) the protein fibrin, and (c) the calcium pump protein.

AG° for this reaction is -16.3 kJ mol 1, and hence it is thermodynamically favorable. The enzyme glutamine synthetase catalyzes this reaction in animal cells (Chap. 15). 

The following reaction is catalyzed by the enzyme glutamine synthetase ... 

The enzyme glutamine synthetase is regulated covalently and allosterically. The covalent modification is an adenylylation catalyzed by the enzyme adenylyl transferase (AT). AT catalyzes the reaction in which a specific tyrosine residue in glutamine synthetase reacts with ATP to form an ester between the phenolic hydroxyl group and the phosphate of the resultant AMP. That tyrosine residue lies very close to a catalytic site. Adenylylation inactivates the adjacent catalytic site. A glutamine synthetase molecule with all 12 sites adenylylated is completely inactive, whereas partial adenylylation yields partial inactivation. 

Glutamine is synthesized in pigeon liver homogenates from glutamic acid and ammonia. The reaction requires ATP and a specific enzyme, glutamine synthetase. ADP is formed and inorganic phosphate is released in the reaction. The mechanism of the reaction is similar to that involved in glutathione biosynthesis. 

Regulatory Enzymes.—Studies of two multi-subunit regulatory enzymes, glutamine synthetase and aspartate transcarbamylase are reported. 

The enzyme glutamine synthetase mediates the synthesis of the amino acid glutamine (Gin, 8) from the amino acid glutamate (Glu, 7) and ammonium ion ... 

Glutamine accounts for more than half of the total intramuscular free amino acid pool, making it one of the most abundant and versatile amino acids in the plasma and skeletal muscle. Glutamine is predominantly synthesized and stored in the skeletal muscle by the action of the enzyme glutamine synthetase. Adipose tissue, lungs, hver, and brain are also sites of synthesis of glutamine. 

Thus operdn for histidine utilization has a unique property compared to the lac and ara operons of E. coli. In the lac and ara operons, specific proteins regulate transcription. These proteins are specialized for this regulatory role and do not perform any other function. In the case of the histidine utilization operon, however, the complex enzyme glutamine synthetase interacts with the regulatory elements of the operon. Hence, this enzyme serves as the regulatory protein (Tyler et al., 1974). 

Isotopic labeling has also been cleverly used to demonstrate the existence of enzyme-bound intermediates that do not readily dissociate into solution. The enzyme glutamine synthetase catalyzes the formation of glutamine from ATP and ammonia possibly through a tightly bound glutamyl phosphate intermediate. 

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