Aspartate degradation

Although the major route for aspartate degradation involves its conversion to oxaloacetate, carbons from aspartate can form fumarate in the urea cycle (see Chapter 38). This reaction generates cytosolic fumarate, which must be converted to malate (using cytoplasmic fumarase) for transport into the mitochondria for oxidative or anaplerotic purposes. An analogous sequence of reactions occurs in the purine nucleotide cycle. Aspartate reacts with inosine monophosphate (IMP) to... 

Mammals, fungi, and higher plants produce a family of proteolytic enzymes known as aspartic proteases. These enzymes are active at acidic (or sometimes neutral) pH, and each possesses two aspartic acid residues at the active site. Aspartic proteases carry out a variety of functions (Table 16.3), including digestion pepsin and ehymosin), lysosomal protein degradation eathepsin D and E), and regulation of blood pressure renin is an aspartic protease involved in the production of an otensin, a hormone that stimulates smooth muscle contraction and reduces excretion of salts and fluid). The aspartic proteases display a variety of substrate specificities, but normally they are most active in the cleavage of peptide bonds between two hydrophobic amino acid residues. The preferred substrates of pepsin, for example, contain aromatic residues on both sides of the peptide bond to be cleaved. 

Since biosynthesis of IMP consumes glycine, glutamine, tetrahydrofolate derivatives, aspartate, and ATP, it is advantageous to regulate purine biosynthesis. The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of PRPP, whose pool size depends on its rates of synthesis, utilization, and degradation. The rate of PRPP synthesis depends on the availabihty of ribose 5-phosphate and on the activity of PRPP synthase, an enzyme sensitive to feedback inhibition by AMP, ADP, GMP, and GDP. 

The degradation of the 2-acetamido-N-(L-aspart-4-oyl)-2-deoxy-/ -D-glucopyranosylamine linkage by alkali and hydrazine hydrate, although... 

Joshi AB, Sawai M, Kearney WR, et al. Studies on the mechanism of aspartic acid cleavage and glutamine deamidation in the acidic degradation of glucagon. /. Pharm. Sci. 2005 94 1912-1927. 

L and the D/L ratio approaches zero. After the death of the living organism, proteins start to spontaneously break down. An inter-conversion of the amino acids occurs from one chiral form (L) to a mixture of D- and L- forms following protein degradation this process is called amino acid racemisation. The extent of racemisation is measured by the ratio of D/L isomers and increases as a function of time and temperature. The longer the racemisation process continues the closer to 1 the ratio between the D- and L-forms becomes. If the D/L ratio is <1 it may be possible to use it to estimate age. The D/L ratio of aspartic acid and isoleucine are the most widely used for this dating technique [104]. Dates have been obtained as old as 200 000 years. However, it has been used mainly to date samples in the 5000 100 000 year range. Recent studies [ 105] mention an estimation of the method accuracy to be around 20%. 

Further insights into the influence of pH on the reactivity at aspartic acid residues are provided by a study of the model peptide Val-Tyr-Pro-Asp-Gly-Ala (Fig. 6.28,a) [93], At pH 1 and 37°, the tm value for degradation was ca. 450 h, with cleavage of the Asp-Gly bond predominating approximately fourfold over formation of the succinimidyl hexapeptide. At pH 4 and 37°, the tm value was ca. 260 h due to the rapid formation of the succinimidyl hexapeptide, which was slowly replaced by the iso-aspartyl hexapeptide. Cleavage of the Asp-Gly bond was a minor route. At pH 10 and 37°, the tm value was ca. 1700 h, and the iso-aspartyl hexapeptide was the only breakdown product seen. In Sect. 6.3.3.2, we will compare this peptide with three analogues to evaluate the influence of flanking residues. 

At pH 10 and 70°, the hexapeptides had tm values for degradation of ca. 30-40 h, with the exception of Val-Tyr-Pro-Asp-Val-Ala, the stability of which was much greater (t1/2 ca. 800 h). Here, the hexapeptides containing Asp-Gly or Asp-Ser were clearly much more reactive than the Asp-Val hexapeptide. Interestingly, the D,L-iso-aspartic hexapeptide was the only product formed from the Asp-Gly hexapeptides, and it was the major product from the Asp-Ser hexapeptide (Fig. 6.28,a and c). Formation of the D-Asp hexapeptide was observed for the Asp-Ser hexapeptide, and it was the major one for the Asp-Val hexapeptide, presumably because base-catalyzed epimerization had ample time to occur given the very slow rate of other breakdown reaction. 

We now examine the degradation of two selected bioactive peptides containing aspartic acid residues flanked by activating residues. 

Asparagine residues (and glutamine residues, see below) are sites of particular instability in peptides. As will be exemplified below, rates of degradation at asparagine residues are markedly faster (tenfold and even much more) than at aspartic acid residues. As reported, the tm values for the internal asparagine in a large number of pentapeptides ranged from 6 to 507 d under... 

As in the case of degradation at aspartic acid residues, the major structural factors that influence the reactivity of asparagine and glutamine residues... 

Z. Shahrokh, G. Eberlein, D. Buckley, M. V. Paranandi, D. W. Aswad, P. Stratton, R. Mischak, Y. J. Wang, Major Degradation Products of Basic Fibroblast Growth Factor Detection of Succinimide and iso-Aspartate in Place of Aspartate15 , Pharm. Res. 1994, 11, 936-944. 

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