Aspartic acid isoelectric point

The side chains of aspartic acid and glutamic acid contain acidic carboxyl groups. These amino acids have acidic isoelectric points around pH 3. An acidic solution is needed to prevent deprotonation of the second carboxylic acid group and to keep the amino acid in its neutral isoelectric state. 

Some ammo acids have side chains that bear acidic or basic groups As Table 27 3 indicates these ammo acids are characterized by three values The third pK reflects the nature of the side chain Acidic ammo acids (aspartic and glutamic acid) have acidic side chains basic ammo acids (lysine arginine and histidine) have basic side chains The isoelectric points of the ammo acids m Table 27 3 are midway between the pK values of the zwitterion and its conjugate acid Take two examples aspartic acid and lysine Aspartic acid has an acidic side chain and a pi of 2 77 Lysine has a basic side chain and a pi of 9 74... 

Thus if a mixture containing alanine aspartic acid and lysine is subjected to electrophoresis m a buffer that matches the isoelectric point of alanine (pH 6 0) aspartic acid (pi = 2 8) migrates toward the positive electrode alanine remains at the origin and lysine (pi =9 7) migrates toward the negative elec trode (Figure 27 3b)... 

FIGURE 27 3 Application of electrophoresis to the separation of aspartic acid alanine and lysine according to their charge type at a pH corresponding to the isoelectric point (pi) of alanine... 

The isoelectric points of the amino acids in Table 27.3 are midway between the pK values of the zwitterion and its conjugate acid. Take two exanples aspartic acid and lysine. Aspartic acid has an acidic side chain and a pi of 2.77. Lysine has a basic side chain and a pi of 9.74. 

Protein mixtures were well resolved on poly(aspartic acid)-silica columns using 0.05 mol/1 phosphate buffer, pH 6.0 and a gradient of sodium chloride from 0 to 0.6 mol/1. The columns displayed a high capacity and selectivity. Figure 3 shows the separation of several standard proteins with isoelectric points ranging from 4.7 to over 11. Peaks are sharp and show minimal tailing. The poly(aspartic acid) coating was quite stable the columns lasted for hundreds of hours of use without decrease in efficiency and capacity. 

When assessing the charge on a structure at physiologic pH, the isoelectric point (pi) is a useful reference. The pi is the pH at which the structure carries no net charge. For instance, Figure 1-8-3 shows the structure of aspartic acid at four different pH values. 

Enzyme Properties. The two isolated veratryl alcohol oxidases had very similar properties (Table I). The difference in isoelectric points might be accounted for by aspartate content all other amino acid contents except glycine were the same within experimental error (5%). The specific activities (veratryl alcohol as substrate) were significantly different, but both enzymes contained a flavin prosthetic group (25) and converted one molecule of oxygen to one molecule of hydrogen peroxide during alcohol oxidation. 

I 21.10 Predict whether the isoelectric points for the following a-amino acids are considerably acidic, slightly acidic, or basic (a) alanine, (b) lysine, (c) aspartic acid, (d) cystine, (e) tyrosine. (See Table 21-1 and Problem 21.6.) ... 

A more strongly acidic solution is needed to repress this ionization. Aspartic acid s isoelectric point is strongly acidic (pH = 2.7). (d) Cystine is a diaminodicarboxylic acid and behaves like a monoaminomonocarboxylic acid. The isoelectric point is slightly acid (pH = 4.6). (e) Tyrosine is a monoaminomonocarboxylic acid containing a phenolic OH, which however is too weakly acidic to ionize to any significant extent. The isoelectric point is slightly acidic (pH = S.6). 

Many enzymes exist within a cell as two or more isoenzymes, enzymes that catalyze the same chemical reaction and have similar substrate specificities. They are not isomers but are distinctly different proteins which are usually encoded by different genes.22 23 An example is provided by aspartate aminotransferase (Fig. 2-6) which occurs in eukaryotes as a pair of cytosolic and mitochondrial isoenzymes with different amino acid sequences and different isoelectric points. Although these isoenzymes share less than 50% sequence identity, their internal structures are nearly identical.24-27 The two isoenzymes, which also share structural homology with that of E. coli,28 may have evolved separately in the cytosol and mitochondria, respectively, from an ancient common precursor. Tire differences between them are concentrated on the external surface and may be important to as yet unknown interactions with other protein molecules. 

Calculate the isoelectric point for histidine, aspartic acid, and arginine. Calculate the fractional charge for... 

Calculate the isoelectric point of aspartic acid (pKal = 2.05, pKa2 = 3.87, pKa3 = 10.00). 

The acid-base behavior of amino acids may also be illustrated via titration curves. If one started with aspartic acid hydrochloride, that is, aspartic acid crystallized from solution in hydrochloric acid, one would require 3 mol base to remove completely the protons from 1 mol aspartic acid. The titration curve obtained with structures at each step of the reaction series is shown in Figure 4.1. Note that the isoelectric point is attained after one proton equivalent has been removed from the molecule. At this point, aspartic acid contains one positive and one negative charge it is zwitterionic. 

As an example, consider a mixture of alanine, lysine, and aspartic acid in a buffer solution at pH 6. Alanine is at its isoelectric point, in its dipolar zwitterionic form with a net charge of zero. A pH of 6 is more acidic than the isoelectric pH for lysine (9.7), so lysine is in the cationic form. Aspartic acid has an isoelectric pH of 2.8, so it is in the anionic form. 

Calculate the isoelectric point (pH/) of aspartic acid, using the information from Prob. 3.6. SOLUTION... 

What is the structure of each amino acid at its isoelectric point (a) alanine (b) methionine (c) aspartic acid (d) lysine ... 

Calculate the isoelectric point (pJ) and the pH at which the maximum total number of charges are present (pH ) for (a) glycine, (b) aspartic acid, and (c) lysine. 

Using amphoteric, isoelectric buffers at pH close to their isoelectric points (p7), at which the electrolytes have a net charge of zero, is an efficient way to decrease the BGE conductivity and apply extremely high field strength. Thus the separation time can be reduced to the order of a few minutes, and high resolution is achieved as a result of minimal diffusion-driven peak spreading. Several acidic isoelectric buffers, such as cysteic acid (p7 1.85), iminodiacetic acid (p7 2.23), aspartic acid (p7 2.77), and glutamic acid (p7 3.22), all at 50 mM concentra-... 

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