Nitrogen transfer from aspartate

GENERAL FEATURES OF NITROGEN TRANSFER FROM ASPARTATE IN FORMATION OF C—N BONDS... 

The phosphate ester of the aldehyde form of vitamin B6, pyridoxal phosphate (pyridoxal-P or PLP), is required by many enzymes catalyzing reactions of amino acids and amines. The reactions are numerous, and pyridoxal phosphate is surely one of nature s most versatile catalysts. The story begins with biochemical transamination, a process of central importance in nitrogen metabolism. In 1937, Alexander Braunstein and Maria Kritzmann, in Moscow, described the transamination reaction by which amino groups can be transferred from one carbon skeleton to another.139 140 For example, the amino group of glutamate can be transferred to the carbon skeleton of oxaloacetate to form aspartate and 2-oxoglutarate (Eq. 14-24). 

One of the nitrogen atoms of urea comes from ammonia, the other is transferred from the amino acid aspartate, while the carbon atom comes from C02. Ornithine, an amino acid that is not in the standard set of 20 amino acids and is not found in proteins, is the carrier of these nitrogen and carbon atoms. Five enzymatic reactions are involved in the urea cycle (Fig. 1), the first two of which take place in mitochondria, the other three in the cytosol ... 

In terrestrial vertebrates, urea is synthesized by the urea cycle (Figure 23.16). The urea cycle, proposed by Hans Krebs and Kurt Henseleit in 1932, was the first cyclic metabolic pathway to be discovered. One of the nitrogen atoms of the urea is transferred from an amino acid, aspartate. The other nitrogen atom is derived directly from free NH4 +, and the carbon atom comes from HCO3 (derived by hydration of CO2 see Section 9.2). 

Many of the reactions described so far point to the concept of waste nitrogen. What happens to the ammonium ions discharged during catabolism of the various amino acids What happens to the amino groups transferred from various amino adds to oxaloacetic acid, yielding aspartate The ammonium ions and excess amino groups, carried in the form of aspartate, are handled in the following marmer. They are incorporated into a small, water-soluble molecule known as... 

The first step in the catabolism of most amino acids is the transfer of the o-amino group from the amino acid to a-ketoglutarate (tx-KG). This process is catalyzed by transaminase (aminotransferase) enzymes that require pyridoxal phosphate as a cofactor. The products of this reaction are glutamate (Glu) and the a-ketoacid analog of the amino acid destined for catabolic breakdown. For example, aspartate is converted to its a-keto analog, oxalo-acetate, by the action of aspartate transaminase (AST), which also produces Glu from a-KG. The transamination process is freely reversible, and the direction in which the reaction proceeds is dependent on the concentrations of the reactants and products. These reactions do not effect a net removal of amino nitrogen the amino group is only transferred from one amino acid to another. 

In addition to the transphosphorylation reactions discussed in Chapter 4, there are several general types of carbon and nitrogen transfer reactions which also occur in purine and pyrimidine nucleotide biosynthesis and interconversion. Among these are one-carbon and phosphoribosyl transfer reactions, amino group transfer from glutamine and aspartate, and amide syntheses. In most of these processes carbon-nitrogen bonds... 

To form the guanidino group of arginine the transfer of nitrogen from aspartate occurs so as to circumvent the participation of free NHg. In step (la) condensation of the acceptor carbon, in this case the isoureido carbon of citrulline, with the aspartate nitrogen gives rise to an intermediate which retains the aspartate carbon chain. In step (2a) chain detachment occurs so as to eliminate fumaric acid. Within a few years it was shown in other... 

FIGURE 16.27 A mechanism for the aspartic proteases. In the first step, two concerted proton transfers facilitate nucleophilic attack of water on the substrate carbonyl carbon. In the third step, one aspartate residue (Asp" " in pepsin) accepts a proton from one of the hydroxyl groups of the amine dihydrate, and the other aspartate (Asp" ) donates a proton to the nitrogen of the departing amine. 

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