Transaminases reactions catalyzed

Next to reactions catalyzed by transaminases, hydrolase-catalyzed reactions also lead to limitations regarding the equilibrium. This problem occurs during ester synthesis, because this condensation reaction produces water. The equilibrium is shifted by high amounts of water towards the reactants therefore, an efficient removal is necessary to reach high conversions. Here, two process setups of Unichema Chemie B V will be discussed illustrating in situ product removal [41]. The first setup is based on azeotropic distillation of the water produced... 

This enzyme [EC 2.6.1.42], also referred to as transaminase B, catalyzes the reversible reaction of leucine with a-ketoglutarate (or, 2-oxoglutarate) to produce 4-methyl-2-oxopentanoate and glutamate. The pyridoxal-phosphate-dependent enzyme will also utilize isoleucine and valine as substrates. However, this enzyme is distinct from that of valine pyruvate aminotransferase [EC 2.6.1.66]. See also Leucine Aminotransferase... 

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... 

Fig. 3. Reactions catalyzed by (a) aspartate transaminase and (b) alanine transaminase.

The reactions catalyzed by transaminases are anergonic as they do not require an input of metabolic energy. They are also freely reversible, the direction of the reaction being determined by the relative concentrations of the amino acid-keto acid pairs. Pyridoxal phosphate is not just used as the coenzyme in transamination reactions, but is also the coenzyme for several other reactions involving amino acids including decarboxylations, deaminations, racemizations and aldol cleavages. 

The alanine racemization catalyzed by alanine racemase is considered to be initiated by the transaldimination (Fig. 8.5).26) In this step, PLP bound to the active-site lysine residue forms the external Schiff base with a substrate alanine (Fig. 8.5, 1). The following a-proton abstraction produces the resonance-stabilized carbanion intermediates (Fig. 8.5, 2). If the reprotonation occurs on the opposite face of the substrate-PLP complex on which the proton-abstraction proceeds, the antipodal aldimine is formed (Fig. 8.5,3). The subsequent hydrolysis of the aldimine complex gives the isomerized alanine and PLP-form racemase. The random return of hydrogen to the carbanion intermediate is the distinguishing feature that differentiates racemization from reactions catalyzed by other pyridoxal enzymes such as transaminases. Transaminases catalyze the transfer of amino group between amino acid and keto acid, and the reaction is initiated by the transaldimination, followed by the a-proton abstraction from the substrate-PLP aldimine to form a resonance-stabilized carbanion. This step is common to racemases and transaminases. However, in the transamination the abstracted proton is then tranferred to C4 carbon of PLP in a highly stereospecific manner The re-protonation occurs on the same face of the PLP-substrate aldimine on which the a-proton is abstracted. With only a few exceptions,27,28) each step of pyridoxal enzymes-catalyzed reaction proceeds on only one side of the planar PLP-substrate complex. However, in the amino acid racemase... 

Steps in the Transaminase Reaction Purified aspartate aminotransferase is capable of catalyzing the half-reaction of transamination (very slowly) in the crystal. This means that the conformational changes that occur during the reaction can be followed by X-ray diffraction crystallography. 

The a-ketoglutarate/L-glutamate couple serves as an amino group acceptor/donor pair in transaminase reactions. The specificity of a particular transaminase is for the amino group other than the glutamate. Two transaminases whose activities in serum are indices of liver damage catalyze the following reactions ... 

Tn amino acid production, we encounter an important problem in bio.synthesis—namely, stereochemical control. Because all amino acids except glycine are chiral, biosynthetic pathways must generate the correct isomer with high fidelity. In each of the 19 pathways for the generation ofchiral amino acids, the stereochemistry at the a-carbon atom is established by a transamination reaction that includes pyridoxal phosphate (PEP). Almost all the transaminases that catalyze these reactions descend from a common ancestor, illustrating once again that effective solutions to biochemical problems are retained throughout evolution. 

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). 

Transamination reactions dominate amino acid metabolism. In these reactions, catalyzed by a group of enzymes referred to as the aminotransferases or transaminases, a-amino groups are transferred from an a-amino acid to an a-keto acid ... 

In the liver, glutamate is formed as the reaction catalyzed by alanine transaminase is reversed. The oxidative deamination of glutamate yields a-ketoglutarate and NHj. 

Among the various enzymes capable of producing optically-active amino acids, transamination reactions, catalyzed by enzymes known as aminotransferases or transaminases, have broad potential for the synthesis of a wide variety of enantio-merically pure (R)- and (S)-compounds containing amine groups. Indeed, various examples of the use of aminotransferases for the production of d- and L-amino acids, both naturally-occurring and non-natural, have been published17 151. In addition, certain aminotransferases have been found to act on amines, and methods for the production of enantiomerically pure amines by transamination have been described116-211. This method allows for yields of up to 100% whereas routes based on hydrolases require external racemization to reach such yield levels. In this section we will focus on the application of aminotransferases. 

The pH-rate profile for the reaction catalyzed by the E. coli broad-range transaminase was determined using the immobilized transaminase with p-fluorophe-... 

The equilibrium of these reactions lies almost completely on the side of lactate. However, by trapping the pyruvate in a subsequent reaction catalyzed by the enzyme glutamate-pyruvate transaminase (GPT) in the presence of L-glutamate, the equilibrium can be displaced in favour of pyruvate and NADH (eqn. 2). 

In pathway A (Figure 1), observed in Pseudomonas MA-1, pyridoxal (2) is produced either from pyridoxamine (15) by a transamination reaction with pyruvate catalyzed by pyridoxamine pyruvate transaminase,or from pyridoxine (1) by an oxidation reaction catalyzed by the FAD-dependent... 

NADH can sometimes be used to follow enzyme reactions in which it is not directly involved by using it as a couphng agent in a secondary reaction with the product. For example, serum glutamic-oxaloacetic transaminase (GOT) catalyzes the reaction of a-ketoglutarate and asparate, and the product is reduced by NADH in the presence of another enzyme, malic acid dehydrogenase (MDH) ... 

ALT (also known as glutamic pyruvic transaminase, GPT) catalyzes the reaction... 

Describe the reactions catalyzed by the aminotransferases (transaminases) and state the major function of these reactions. 

Three nonessential amino acids (glutamate, alanine, and aspartate) are synthesized from a-keto acids via reactions catalyzed by transaminases. For example, alanine is produced from pyruvate and glutamate (which furnishes the amino group) ... 

Little is known about the NADH aitd NAD content of the cytosol. In order to estimate the NADH/NAD ratio, the cytosolic contents of malate, aspartate, glutamate and 2-oxoglutarate were determined by nonaqueous fractionation of spinach leaves (Table l). From these values the NADH/NAD ratio was calculated on the reasonable assumption that the reactions catalyzed by the cytosolic malate dehydrogenase and glutamate oxaloacetate transaminase are near to equilibrium. Introducing the equilibrium constants of these enzymes 2.8. 10 at pH 7.0 (4), Kqqt... 

Transamination is a reaction catalyzed by a family of enzymes called transaminases, which results in the exchange of an amine group of an amino acid with a ketone or a... 

Imidazolone propionate hydrolase catalyzes the enzymatic cleavage of the imidazole ring to yield formi-minoglutamate. The rat liver enzyme has been partially purified. In addition to the enzymic conversion, two nonenzymic spontaneous reactions yield N-formyl-isoglutamine and 4-oxoglutamic acid. In addition to the oxidative pathways for histidine, there exist three other pathways for its use protein synthesis, decarboxylation to yield histamine (see Inflammation), and transaminase. The activity of histidine pyruvic transamination in rat liver is three times that of histidase. The product of the transaminase reaction is imidazole pyruvic acid, which in turn is converted to imidazole acetic acid. 

Hydroxypyruvate is primarily derived from serine through a transamination catalyzed by an alanine hydroxypyruvate transaminase. It is believed that excessive amounts of hydroxypyruvate are converted to l-glycerate (the reaction catalyzed by lactic dehydrogenase) because of a lack of D-glyceric dehydrogenase which catalyzes the conversion of hydroxypyruvate to D-glyceric acid. 

Transaminases serve two vital roles. First, they provide a means of adjusting the relative proportions of amino acids to meet the particular needs of an organism. Second, they collect the nitrogen atoms of all amino acids into glutamate, because glutamate is the central source of nitrogen atoms for biosynthesis. An example of the reaction catalyzed by glutamate transaminase is presented here. 

First, there is the problem of transamination, shown in the first step, and for which a transaminase enzyme (EC 2.6.1.16) is needed. It appears that the process, in this case, is not reversible, and, in a backward reaction (catalyzed by a deaminase, EC 3.5.99.6), an equivalent of ammonia (but almost certainly not as NH3 ) is lost and the nitrogen is replaced by an -OH group. The transaminase obtains the nitrogen in the (effective) hydrolysis of an amide (glutamine). Furthermore, there is probably... 

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