Amino semialdehyde

He Z, JC Spain (1997) Studies of the catabolic pathway of degradation of nitrobenzene by Pseudomonas pseudoalcaligenes JS45 removal of the amino group from 2-aminomuconic semialdehyde. Appl Environ Microbiol 63 4839-4843. 

Muraki T, M Taki, Y Hasegawa, H Iwaki, PCK Lau (2003) Prokaryotic homologues of the eukaryotic 3-hydroxyanthranilate 3,4-dioxygenase and 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase in the 2-nitrobenzoate degradation pathway of Pseudomonas fluorescens strain KU-7. Appl Environ Microbiol 69 1564-1572. 

GABA acts as an inhibitory transmitter in many different CNS pathways. It is subsequently destroyed by a transamination reaction (see Section 15.6) in which the amino group is transferred to 2-oxoglutaric acid, giving glutaric acid and succinic semialdehyde. This also requires PLP as a cofactor. Oxidation of the aldehyde group produces succinic acid, a Krebs cycle intermediate. 

This enzyme [EC 2.6.1.19] catalyzes the reversible reaction of y-aminobutyrate with a-ketoglutarate to yield succinate semialdehyde and glutamate. A number of enzyme preparations have been reported to also use )3-alanine, 5-aminopentanoate, and (i ,5)-3-amino-2-methylpropanoate as substrates. 

Gupta M, Polinsky M, Senephansiri H, Snead OC, Jansen EEW, Jakobs C, Gibson KM (2004) Seizure evolution and amino acid imbalances in murine succinate semialdehyde dehydrogenase (SSADH) deficiency. Neurobiol Dis 16 556-562... 

The essential amino acid lysine (2,5-diaminohexanoic acid) can be degraded via two pathways, viz. the so-called saccharopine pathway and the pipecolic acid (PA) pathway. Both pathways merge at the level of a-aminoadipic acid semialdehyde (AASA). It is generally accepted that the saccharopine pathway constitutes the major breakdown pathway. However, the PA pathway has attracted much attention since the discovery of the association between the presence of elevated PA levels and Zellweger syndrome almost 40 years ago. Mainly because the analysis of amino acids was the primary biochemical approach for studying presumed inborn errors of metabolism, PA in Zellweger syndrome was discovered even before it was realized that this disorder was based on a defect of peroxisomal functions. 

Tropoelastin molecules are crosslinked in the extracellular space through the action of the copper-dependent amine oxidase, lysyl oxidase. Specific members of the lysyl oxidase-like family of enzymes are implicated in this process (Liu etal, 2004 Noblesse etal, 2004), although their direct roles are yet to be demonstrated enzymatically. Lysyl oxidase catalyzes the oxidative deamination of e-amino groups on lysine residues (Kagan and Sullivan, 1982) within tropoelastin to form the o-aminoadipic-6-semialdehyde, allysine (Kagan and Cai, 1995). The oxidation of lysine residues by lysyl oxidase is the only known posttranslational modification of tropoelastin. Allysine is the reactive precursor to a variety of inter- and intramolecular crosslinks found in elastin. These crosslinks are formed by nonenzymatic, spontaneous condensation of allysine with another allysine or unmodified lysyl residues. Crosslinking is essential for the structural integrity and function of elastin. Various crosslink types include the bifunctional crosslinks allysine-aldol and lysinonorleucine, the trifunctional crosslink merodes-mosine, and the tetrafunctional crosslinks desmosine and isodesmosine (Umeda etal, 2001). 

FIGURE 3.4 The common pathway of the aspartate-derived amino acids in Corynebacteria. The mnemonic of the genes involved are shown in parentheses below the enzymes responsible for each step. Dotted lines indicate multiple enzymatic steps, and 16 is L-aspartic acid, 17 is L-aspartyl phosphate, 18 is L-aspartate semialdehyde, 19 is L-lysine, 20 is L-homoserine, 21 is L-isoleucine, 22 is L-threonine, and 23 is L-methionine. 

Figure 3. Lysyl oxidase. The enzyme, lysyl oxidase, appears to seek out lysyl residues in alanyl- and lysyl-rich regions in the pro fibrillar forms of elastin. The presence of an aromatic amino acid residue adjacent to lysine appears to block its oxidation. The product of oxidation is peptidyl a-aminoadipic-S-semialdehyde. Assays for the enzyme against elastin involve first the preparation of an elastin-rich pellet containing 3H-lysyl residues labeled in the 6 or 4,5 position. This is usually accomplished by incubating embryonic chick aortas in medium containing 3H-lysine plus f3-aminopropionitrile (BAPN) to inhibit endogenous lysyl oxidase activity. BAPN is then removed leaving behind an elastin-rich residue in which the profibrillar forms of elastin labelled with 3H-lysine are only partially crosslinked. When lysyl oxidase preparations are added to this residue the release of tritium represents the assay for activity. It has also been demonstrated that tropoelastin, when incubated with lysyl oxidase, forms a-aminoadipic-S-semialdehyde and eventually crosslinks as shown in Figure 4.

The quinone ring is derived from isochorismic acid, formed by isomerization of chorismic acid, an intermediate in the shikirnic acid pathway for synthesis of the aromatic amino acids. The first intermediate unique to menaquinone formation is o-succinyl benzoate, which is formed by a thiamin pyrophosphate-dependent condensation between 2-oxoglutarate and chorismic acid. The reaction catalyzed by o-succinylbenzoate synthetase is a complex one, involving initially the formation of the succinic semialdehyde-thiamin diphosphate complex by decarboxylation of 2-oxoglutarate, then addition of the succinyl moiety to isochorismate, followed by removal of the pyruvoyl side chain and the hydroxyl group of isochorismate. 

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