Pyrroline-5-carboxylate, an intermediate in the biosynthesis and degradation of glutamate, proline, and ornithine, acts as a strong reversible inhibitor of glucosamine-6-phosphate synthase, competitive with respect to glutamine. Proton magnetic resonance spectroscopy shows that, under these conditions, pyrroline-5-carboxylate exists in rapid equilibrium with glutamate γ-semialdehyde (0.05%). The observed variation of Ki with pH is consistent with inhibition by this rare species. Glutamate γ-semialdehyde is expected to react reversibly with a cysteine residue at the active site, identified by earlier inactivation studies, to form an analogue of a tetrahedral intermediate in glutamine hydrolysis. The apparent Ki value of glutamate γ-semialdehyde is approximately 3 × 10-8 M.
Nitrogen ranks behind only carbon, hydrogen, and oxygen in itscontribution to the mass of living systems. Most of this nitrogen is bound up inamino acids and nucleotides. The biosynthetic pathways for amino acids shareseveral key intermediates with those of nucleotides which will be discussed inthe next chapter. Certain amino acids or parts of amino acids are incorporatedinto the structure of purines and pyrimidines, and in one case part of a purinering is incorporated into an amino acid (histidine). The two sets of pathwaysalso share much common chemistry, in particular a preponderance of reactionsinvolving the transfer of nitrogen or onecarbon groups.
All amino acids are derived from intermediates in glycolysis, the citric acidcycle, or the pentose phosphate pathway (Fig. 151). Nitrogen enters thesepathways by way of glutamate and glutamine. Some pathways are simple, others arenot. Ten of the amino acids are just one or several steps removed from thecommon metabolite from which they are derived. The biosynthetic pathways forothers, such as the aromatic amino acids, are more complex.
A useful way to organize these biosynthetic pathways is to group them into sixfamilies corresponding to their metabolic precursors (Table 151), and we usethis approach to structure the detailed descriptions that follow.
the main use of the urea cycle is in arginine biosynthesis, ..
Histidine is derived from three precursors (Fig. 158): PRPP contributesfive carbons, the purine ring of ATP contributes a nitrogen and a carbon, andglutamine supplies the second ring nitrogen. The key steps are condensation ofATP and PRPP, in which N-1 of the purine ring is linked to the activated C-1 ofthe ribose of PRPP (step 1); purine ring opening that ultimately leaves N-1 andC-2 of adenine linked to the ribose (step 3); and formation of the imidazolering, a reaction in which glutamine donates a nitrogen (step 5). The use of ATPas a metabolite rather than a high-energy cofactor is unusual but not wasteful,because it dovetails with the purine biosynthetic pathway. The remnant of ATPthat is released after the transfer of N-1 and C-2 is5-aminoimidazole-4-carboxamide ribonucleotide (AICAR), an intermediate of purinebiosynthesis that is rapidly recycled to ATP.
Dr. Namrata Chhabra | Biochemistry for Medics – Lecture …
We tested the hypothesis that diversion of ornithine into polyamine biosynthesis (by transgenic approach) not only plays a role in regulating its own biosynthesis from glutamate but also affects arginine and proline biosynthesis.
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Our findings suggest that: (1) the overall conversion of glutamate to arginine and polyamines is enhanced by increased utilization of ornithine for polyamine biosynthesis by the transgene product; (2) proline and arginine biosynthesis are regulated independently of polyamines and GABA biosynthesis; (3) the expression of most genes (28 that were studied) that encode enzymes of the interacting sub-pathways of arginine and GABA biosynthesis does not change even though overall biosynthesis of Orn from glutamate is increased several fold; and (4) increased polyamine biosynthesis results in increased assimilation of both nitrogen and carbon by the cells.
glutamate/aspartate transporters ..
The pathway of arginine biosynthesis in Streptococcus bovis was studied by radioactive tracer techniques. Cells were grown anaerobically with 14CO2 in a synthetic medium containing NH4+ as the sole nitrogen source except for the trace present in nitrogen-containing vitamins. The protein fraction isolated from the labeled cells was acid-hydrolyzed, and 14C-arginine was isolated from the protein hydrolysate by ion-exchange chromatography. The carboxyl carbon of the isolated arginine was removed with arginine decarboxylase, and the guanidino carbon was removed by simultaneous arginase-urease degradation. By manometric measurement and liquid scintillation counting of the CO2 released by enzymatic degradation, 50% of the label was found in the carboxyl carbon and 50% in the guanidino carbon. Specific radioactivity determinations indicated that growth on 14CO2 resulted in twice as much label in arginine as with aspartate, glutamate, or lysine. These results are consistent with a glutamate → ornithine → citrulline pathway of arginine biosynthesis in S. bovis and provide further evidence for the synthesis of glutamate via the tricarboxylic acid cycle reactions from citrate through α-ketoglutarate.