Glutaminase

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Glutaminase is an enzyme that catalyzes the hydrolysis of L-β-glutamine into L-glutamate and ammonia. Glutaminase is a kind of amidase. This enzyme is also contained in some bacteria and plant roots, but it has strong activity in higher animals. The optimal pH for enzymes in animal kidney and liver is 8.0, and the optimal pH for enzymes in cerebral cortex and omentum is 8-9, which is different in nature. Enzyme from E. coli, pH 4.7-5.1. This enzyme can be inhibited by glutamate, and has two types: phosphate-activated and inactivated. In the organism, the ammonia produced by the glutamine transferred from the peripheral tissues has the effect of regulating the alkali storage (kidney) and the synthesis of urea (liver) in the body.

Figure 1. Enzyme structure of glutaminase.

Distribution

Glutaminase is expressed and active in portal vein liver cells, and glutaminase, like glutamate dehydrogenase, generates NH3 (ammonia) for urea synthesis. Glutaminase is also expressed in the epithelial cells of the renal tubules, where the ammonia produced is excreted in the form of ammonium ions. This excretion of ammonium ions is an important mechanism of kidney acid-base regulation. During chronic acidosis, glutaminase is induced in the kidneys, which leads to an increase in the amount of ammonium ions excreted. Glutaminase can also be found in the intestine, so the hepatic portal vein ammonia can be as high as 0.26 mM (arterial blood ammonia is 0.02 mM).

Functions

One of the most important functions of glutaminase is found in the axon ends of neurons in the central nervous system. Glutamate is the most used excitatory neurotransmitter in the central nervous system. After being released into the synapse for neurotransmission, glutamate is quickly taken up by nearby astrocytes and converted into glutamine. This glutamine is then provided to the presynaptic ends of neurons, where glutaminase converts it back to glutamate for loading into synaptic vesicles. Although both "kidney type" (GLS1) and "liver type" (GLS2) glutaminase are expressed in the brain, it is reported that GLS2 is only present in the nucleus of central nervous system neurons.

Related enzyme

• Transglutaminase

Transglutaminase, also known as transglutaminase (TGase), is a monomeric protein with an active center and a molecular weight of about 38,000 composed of 331 amino groups. It can catalyze the intramolecular and intermolecular covalent cross-linking of protein and polypeptide. So as to improve the structure and function of the protein, and have significant effects on the properties of the protein such as foaming, emulsification, emulsification stability, thermal stability, water retention, and gel capacity, thereby improving the flavor, taste, texture and Appearance etc. Traditional meat processing technology usually adds a large amount of salt and phosphoric acid to improve its water holding capacity, consistency and texture. Recently, foods with less salt and less phosphoric acid have been widely promoted, but their texture and physical properties are not satisfactory. TG enzyme can replace part of the quality improver-phosphate that is usually added in the processing of meat products to produce low-salt meat products.

Figure 2. Enzyme structure of transglutaminase.

• TG function

The main functional factor of TG is transglutaminase. This enzyme is widely present in humans, advanced animals, plants and microorganisms, and can catalyze the cross-linking between or within protein molecules, the connection between proteins and amino acids, and the hydrolysis of glutamine residues in protein molecules. Through these reactions, the functional properties of various proteins can be improved, such as nutritional value, texture structure, taste and shelf life.

Applications

• Improve food texture.

TG can improve many important properties of proteins by catalyzing the cross-linking between protein molecules. For example, when the enzyme is used to produce recombinant meat, it can not only bond the minced meat together, but also cross-link various non-meat proteins to the meat protein, which significantly improves the taste, flavor, tissue structure and nutrition of meat products.

• Improve the nutritional value of protein.

TG can covalently cross-link certain essential amino acids (such as lysine) to the protein to prevent the Maillard reaction from destroying the amino acids, thereby improving the nutritional value of the protein. Transglutaminase can also introduce lacking amino acids into proteins with unsatisfactory amino acid composition. People in developing countries are particularly interested in this.

• Form a heat-resistant and water-resistant film.

The casein cross-linked by the enzyme can be dehydrated to obtain a water-insoluble film. This film can be decomposed by chymotrypsin, so it is an edible film that can be used as a food packaging material.