Transamination characteristics

In a number of nonenzymatic reactions catalyzed by pyridoxal, a metal ion complex is formed—a combination of a multivalent metal ion such as cupric oi aluminum ion with the Schiff base formed from the combination of an amino acid and pyridoxal (I). The electrostatic effect of the metal ion, as well as the electron sink of the pyridinium ion, facilitates the removal of an a -hydrogen atom to form the tautomeric Schiff base, II. Schiff base II is capable of a number of reactions characteristic of pyridoxal systems. Since the former asymmetric center of the amino acid has lost its asymmetry, donation of a proton to that center followed by hydrolytic cleavage of the system will result in racemic amino acid. On the other hand, donation of a proton to the benzylic carbon atom followed by hydrolytic cleavage of the system will result in a transamination reaction—that is, the amino acid will be converted to a keto acid and pyridoxal will be converted to pyridoxamine. Decarboxylation of the original amino acid can occur instead of the initial loss of a proton. In either case, a pair of electrons must be absorbed by the pyridoxal system, and in each case, the electrostatic effect of the metal ion facilitates this electron movement, as well as the subsequent hydrolytic cleavage (40, 43). 

In individuals with PKU, a secondary, normally little-used pathway of phenylalanine metabolism comes into play. In this pathway phenylalanine undergoes transamination with pyruvate to yield phenylpyruvate (Fig. 18-25). Phenylalanine and phenylpyruvate accumulate in the blood and tissues and are excreted in the urine—hence the name phenylketonuria. Much of the phenylpyruvate, rather than being excreted as such, is either decarboxylated to phenylacetate or reduced to phenyllactate. Phenylacetate imparts a characteristic odor to the urine, which nurses have traditionally used to detect PKU in infants. The accumulation of phenylalanine or its metabolites in early life impairs normal development of the brain, causing severe mental retardation. This may be caused by excess phenylalanine competing with other amino acids for transport across the blood-brain barrier, resulting in a deficit of required metabolites. 

A basic group of the enzyme catalyzes the proton transfer characteristics of transaminations. Bruice found that imidazole buffer can catalyze transamination reactions in model systems [6], We conjectured that transamination rates should be improved by attaching a basic side arm to pyridoxamine. Thus we synthesized a series of simple pyridoxamine derivatives carrying basic groups at the end of flexible chains of various lengths [7-9]. We measured the transamination rates of these pyridoxamine derivatives in the presence of Zn(OAc)2 at pH 4.00 in methanol. 

Phenylketonuria, caused by a deficiency of phenylalanine hydroxylase, is one of the most common genetic diseases associated with amino acid metabolism. If this condition is not identified and treated immediately after birth, mental retardation and other forms of irreversible brain damage occur. This damage results mostly from the accumulation of phenylalanine. (The actual mechanism of the damage is not understood.) When it is present in excess, phenylalanine undergoes transamination to form phenylpyruvate, which is also converted to phenyllactate and phenyl-acetate. Large amounts of these molecules are excreted in the urine. Phenylacetate gives the urine its characteristic musty odor. Phenylketonuria is treated with a low-phenylalanine diet. 

When transaminated, the three branched-chain amino acids (valine, leucine, and isoleucine) form compounds that have the characteristic odor of maple symp. An enzyme known as branched-chain a-keto acid dehydrogenase converts these compounds into CoA esters. People who do not have this enz3me have the genetic disease known as maple syrup urine disease—so-called because their urine smells like maple syrup. 

Thus, we have prepared triethoxysilanes which contain radicals R [such as n-C i (O I, S II), CeHs (O III, S IV), (CH2)3Si(OC2Hs)3 (O V, S VI), CNSCeftt (O VH, benzo-thiazole radical)]. It should be noted that V and VI were synthesized earlier [35-38] using the transamination reaction. In this case V was obtained as an oil with a yield of 12.2%. Irrespective of solvents (n-hexane or benzene), reaction [5] results in formation of a substance which crystallizes as a white solid (melting point 61-63°C) in quantitative yield. The spectral characteristics of this product are consistent with those described in [38], except... 

Pong Bi Bi reaction mechanism. As they contain PLP, the mechanism is almost certainly similar to that known for the animal aminotransferases (Fig. 1). Details of this mechanism are discussed by Braunstein (1973) and by Metzler (1977). The apoenzyme moiety determines substrate specificity and confers high catalytic efficiency, as well as suppressing side reactions and eliminating the metal requirement characteristic of nonenzymatic transamination. Initially the amino acid binds to an anchoring site on the enzyme. Condensation then takes place between the amino acid and the enzyme pyridoxal-lysine imine to form an aldimine. Following further rearrangements, a ketimine is produced. Ketimine formation is then followed by a hydrolysis to... 

The effects of this inhibitor on the metabolism of A. fumigatus were very different from those of the other compounds used, since formation of all the characteristic acetate-derived phenols was strongly suppressed (Packter and Collins, 1974). Treatment of cultures gave a completely altered pattern and led unexpectedly to the secretion of atypical shikimic acid-derived phenols, including 2,3-dihydroxybenzoic acid, 4-hydroxyphenylpyruvic acid, and 4-hydroxybenzyl cyanide. In addition, the inhibitor was metabolized directly by transamination to fluorophenylpyruvic acid. 

In certain organs the onset of transamination is definitely coincident with critical stages of functional differentiation, e.g., in muscle — with the time of appearance and accumulation of phosphocreatine and establishment of rapid contraction (Koshtoyants and Ryabinovskaya, 103), in heart — with acquisition of definitive heart rate, in kidney — with the beginning of active excretory function. In the early period of postnatal life transamination values gradually increase, and only at the age of one month do they approach the levels characteristic of adult rabbits. 

Amino acid oxidase oxidatively deaminates the amino acid at the left (one of many) the enzyme specifically catalyzes this particular reaction as described extensively in Chapt. VIII-7. Another possible reaction, decarboxylation, does not occur. The catalysis of CO2 loss requires a different enzyme, and a third reaction, transamination, requires a third enzyme assisting in the exchange of functional groups (the keto group of oxaloacetic acid with the amino group). Obviously each of the three enzymes possesses a characteristic reaction specificity this is true for all enzymes. 

As a consequence of the enzyme block, the three a-keto acids accumulate in the tissues and body fluids. The transamination step is reversible, hence the concentrations of leucine and isoleucine in the body fluids rise to about 10 to 15 times normal and that of valine to four to five times normal. The a-keto acids also undergo other reactions, including reduction to the O-hydroxy acids, some of the products being excreted and giving the urine the characteristic odour resembling maple syrup. 

Branched-chain keto aciduria or maple syrup urine disease (McKusick 24 860) is an autosomal recessive disease first described by Menkes et al. (1954) in which, because of the reversible nature of the preceding transamination step, a characteristic branched-chain amino aciduria also occurs (Westall etal, 1957). In the classical form of the disease, the metabolism of all three branched-chain amino acids is affected, leading to accumulation of the three corresponding keto acids (Menkes, 1959 MacKenzie and Woolf, 1959 Dancis etaL, 1959), with the keto acid of isoleucine, 2-keto-3-methylvalerate, apparently being responsible for the characteristic maple syrup smell in the patients urine (Snyderman ra/., 1964). 

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