Aspartic acid metabolism

L-Methionine originates from L-homoserine, a product of L-aspartic acid metabolism (D 16). Key intermediates are 0-phospho-L-homoserine (plants) and 0-acyl-L-homoserine (microorganisms). Cystathionine is formed with L-cysteine as sulfur donor. It may be degraded to L-homocysteine, which, however, may also directly be formed with the participation of sulfide, a reaction resembling the formation of L-cysteine from 0-acetyl-L-serine (D 11). 5-Methyltetrahydrofolic acid acts as donor of the L-methionine methyl group (C 3.2). 

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

Neotame is an artificial sweetener designed to overcome some of the problems with aspartame. The dimethylbutyl part of the molecule was added to block the action of peptidases, enzymes that break the peptide bond between the two amino acids aspartic acid and phenylalanine. This reduces the availability of phenylalanine, eliminating the need for a warning on labels directed at people who cannot properly metabolize phenylalanine. 

Connections To amino acid metabolism by the requirement for glutamine and aspartate. 

The North American phenotype is associated with the presence of CRM. Possibly as a result, the clinical presentation is less devastating in early infancy, although the outcome is almost invariably fatal in later infancy or early childhood. These patients do not have the associated abnormalities of ammonia metabolism, and the serum aspartic acid concentrations are not as severely depleted. Only one patient has been described with the North... 

Sterol biosynthesis Bile acid biosynthesis C2rSteroid hormone metabolism Androgen and estrogen metabolism Nucleotide Metabolism Purine metabolism Pyrimidine metabolism Nucleotide sugar metabolism Amino sugar metabolism Amino Acid Metabolism Glutamate metabolism Alanine and aspartate metabolism Glycine, serine, and threonine metabolism... 

Aspartic acid has a side chain carboxyl group that will lose a proton and become an anionic carboxylate group under physiological conditions. Aspartic acid is the metabolic precursor to gamma (y)-aminobutyric acid (GABA), an important inhibitory neurotransmitter in the human central nervous system. 

All the template models produced have the same common features of a basic nitrogen atom at a distance of 5-7 A from the site of metabolism which is in general on or near a planar aromatic system. It is currently believed that aspartic acid residue 301 provides the carboxylate residues which binds the basic nitrogen of the substrates. 

Today aspartame is used in more than 6,000 food products. Aspartame is 160 times as sweet as sucrose based on mass equivalents. Approximately 16,000 tons are consumed annually on a global basis, with approximately 8,000 tons used in the United States and 2,500 tons in Europe. In the body aspartame is metabolized into its three components aspartic acid, phenylalanine, and methanol (Figure 11.1). Aspartic acid is a nonessential amino acid and phenylalanine is an essential amino acid. The condition called phenylketonuria (PKU) is a genetic disorder that occurs when a person lacks the enzyme phenylalanine hydroxylase and cannot process phenylalanine. This results in high phenylalanine blood levels that are metabolized into products one of these is phenylpyruvate, which contains a ketone group and... 

A small number of other biosynthetic pathways, which are used by both photosynthetic and nonphotosynthetic organisms, are indicated in Fig. 10-1. For example, pyruvate is converted readily to the amino acid t-alanine and oxaloacetate to L-aspartic acid the latter, in turn, may be utilized in the biosynthesis of pyrimidines. Other amino acids, purines, and additional compounds needed for construction of cells are formed in pathways, most of which branch from some compound shown in Fig. 10-1 or from a point on one of the pathways shown in the figure. In virtually every instance biosynthesis is dependent upon a supply of energy furnished by the cleavage to ATP. In many cases it also requires one of the hydrogen carriers in a reduced form. While Fig. 10-1 outlines in briefest form a minute fraction of the metabolic pathways known, the ones shown are of central importance. 

Metabolism. Aspartame is metabolised by the body into its two constituent amino acids and methanol. These hydrolysis products are handled by the body in the same way as the aspartic acid, L-phenylalanine and methanol from other commonly consumed foods. It adds nothing new to the diet. 

This transfer of reducing equivalents is essential for maintaining the favorable NAD+/NADH ratio required for the oxidative metabolism of glucose and synthesis of glutamate in brain (McKenna et al., 2006). The malate-aspartate shuttle is considered the most important shuttle in brain. It is particularly important in neurons. It has low activity in astrocytes. This shuttle system is fully reversible and linked to amino acid metabolism with the energy charge and citric acid cycle of neuronal cells. 

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