Glutamate-l-semialdehyde

Scheme . Possible mechanisms for glutamate l-semialdehyde aminomutase...

In plants, algae, cyanobacteria and some other bacterial species 5-aminolevulinate (ALA) is synthesized from glutamate (1-6). This reaction sequence involves ligation of glutamate to a tRNA species and subsequent reduction to glutamate-l-semialdehyde (GSA) (7). The conversion of GSA to ALA is then catalysed by GSA-aminotransferase (E.C. S.4.3.8.) (2,8). Dependent on the requirement of pyridoxamine-phosphate (PAMP) or pyridoxal-phosphate (PALP) for activity, either 4,5-diaminovalerate (8-10) or 4,5-dioxovalerate (DOVA) (5,11,12) are proposed as intermediates. 

Figure 5. An example of a suicide substrate gabaculine and the postulated mechanism of inactivation of glutamate-l-semialdehyde aminotransferase.

L-glutamic-y-semialdehyde) B.flavum L-Glutamine pro sulfite is effective 13.2 144... 

This enzyme [EC 1.2.1.41], also known as y-glutamyl-phosphate reductase and glutamate-5-semialdehyde dehydrogenase, catalyzes the reaction of L-glutamate 5-semialdehyde with orthophosphate and NADP+ to produce L-y-glutamyl 5-phosphate and NADPH. 

Enzymes catalyze almost every metabolic reaction in extant cells. A few tmusually facile reactions, such as cyclization of L-glutamate 7-semialdehyde to form pyrroline-5-carboxylate in the proline biosynthesis pathway and decarboxylation of 2-amino-3-oxo-4-phosphonooxybutyrate in the pyridoxal phosphate (PLP) synthesis pathway, do not require acceleration to satisfy the demands of the cell. For all other reactions, catalysis is required because the rates of nonenzymatic reactions are very slow. Modern enzymes are marvelous catalysts. They accelerate reactions by up to 20 orders of magnitude, prevent side reactions of reactive intermediates, and catalyze stereoselective and stereospecific reactions. Further, they are often exquisitely regulated by small molecule ligands. 

V-acetyl-L-glutamate-5-semialdehyde NADP oxidoreductase (phosphorylating)... 

Chemicals glutamate-1-semialdehyde fGSA) was prepared by ozonolysis of vinyl 7-aminobutyric acid (13). L-glutamate-U- C (275 mCi mmole" ) and H-glutamate (22 Ci mmole" ) were obtained from Amersham, UK. cofactors and 5-aminolevulinate were obtained from Sigma (Poole, UK), 3-amino-2,3-dihydrobenzoic acid hydrochloride (gabaculine) was from Fluka AG (Switzerland). 

The nature of the intermediates of the Cg-pathway has been a matter of controversy for a long time. Although the involvement of glutamate-1-semialdehyde (G-l-SA) (1) is widely accepted, the role of 4,5-dioxovalerate (DOVA) (2) as a catalytic component in the Cg-pathway is still challenged (3). Well established is yet the activation of glutamate for the formation of 5-aminolevulinate (ALA) by a tRNA as demonstrated for barley (4), Chlamydomonas (5), Chlorella (6), Methanobacterium (7), Euglena (8), Cyanidium (9), Synechocystis (9) and spinach (9). 

Pro is glucogenic. It does not participate in a-helix formation, and therefore has spedal importance in protein tertiary structure (see oteins). Pro is biosynthesized chiefly from L-glutamate via glutamic-y-semialdehyde. Some may ako be formed from exogenous ornithine via pyrroline carboxylic add. It is a nonessential amino add in mammals, but is essential for Ae growA of chickens AzetiAne-2-carboxylic acid is a Pro antagonist. 

The direct reduction of glutamate 5-phosphate (Scheme 12.3) with the nicotinamide adenine diphosphate (NADPH)/NADP+-dependent glutamate-5-semialde-hyde dehydrogenase (EC 1.2.1.41) as shown in Scheme 12.5 produces glutamate 5-semialdehyde. The aldehyde spontaneously undergoes cyclization to (5)-3,4-dihydro-2//-pyrrole-2-carboxylate, which is then reduced to L-proline (Pro, P).The reduction is accomplished again with the phosphorylated nicotinamide adenine dinucleotide being oxidized while in the presence of the enzyme pyrroline-5-car-boxylate reductase (EC 1.5.1.2). 

Scheme 12.5. The formation of glutamate 5-semialdehyde from glutamate 5-phosphate and its conversion to L-proline (Pro, P). EC numbers and some graphic materials provided in this scheme have been taken from appropriate links in a URL starting with http // www.chem.qmul.ac.uk/iubmb/enzyme/.

There is a. ccTtain amount of evi(l(Mic( that tlu semialdehydo of glutamic acid is the intermediate formy the loss of the 5-araino group of ornithine. This evidence is the isolation of what was presumed to be the 2,4-dinitrophenylhydrazone of the semialdehyde upon partial oxidation of proline by the cyclophorase system. The glutamic acid semialdehyde can be expected to condense readily to pyrroline carboxylic acid, and reduction of this compound would yield proline (Fig. 2). 

Requena, J.S., Chao, C.-C., Levine, R.L., and Stadtman, E.R. (2001) Glutamic and aminoadipic semialdehydes are the main carbonyl products of metal-catalyzed oxidation of proteins. Proc. Natl. Acad. Sci. 98, 69-74. 

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