Lysine isoelectric point

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. 

RNase A is a basic protein with an isoelectric point (pi) of 9.45 and a net positive charge in neutral solution.35 However, the conversion of positively charged lysine side chains to polar, but neutrally charged, methylol adducts would be expected to lower the pi of formalin-treated RNase A. To explore this further, RNase A was treated with 5% formalin and analyzed by isoelectric focusing (IEF) gel electrophoresis. Figure 15.5a shows that the pi values were shifted into the pH 6.0-7.4 range. Figure 15.5b shows the results of IEF... 

The isoelectric point for lysine is that pH at which the compound is in an electrically neutral form, and this will be the average of pATa2 (the cation) and pAfa3 (the dipolar ion). For lysine, pATa2 = 8.95 and pATa3 = 10.52, so pi = 9.74. 

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.) ... 

Additional base is needed to repress this ionization. The isoelectric point of lysine is basic (pH = 9.6). 

The isoelectric points are different pH = 9.6 for lysine and pH = 5.97 for glycine. An aqueous solution of the mixture is placed between two electrodes, the pH adjusted to either 5.97 or 9.6, and an electric current applied. At pH 5.97, glycine doesn t migrate, while cationic lysine migrates to the cathode, where it is collected. At pH 9.6, lysine doesn t migrate, while anionic glycine migrates to the anode. 

Amino acids with two carboxylic groups or those with two amino groups behave slightly differently in that they are not entirely neutral, but may be acidic or basic. All amino acids therefore have a different isoelectric point (see Table 2.1.1). These differences in polarity form the basis of the separation of amino acids the neutral amino acids are in the middle part of the chromatogram and the dibasic amino acids elute late. In addition, the length of the aliphatic chain of the molecule makes the amino acid less polar, causing later elution (e.g., ornithine, which has five carbon atoms, elutes before its homolog lysine, which has six carbon atoms). 

H. Neutral lysine can be written HL. The other forms of lysine are H3L2, H2L+, and L. The isoelectric point is the pH at which the average charge of lysine is 0. Therefore, at the isoelectric point, 2[H3L2+] + [H2L+] = [L-]. Use this condition to calculate the isoelectric pH of lysine. 

H. We know that the isoelectric point will be near (pff2 + pA3) 9.95. At this pH, the fraction of lysine in the form H3L2+ is negligible. Therefore, the electroneutralify condition reduces to [H2L+] = [L ], for which the expression isoelectric pH = 2(pK2 + pK3) = 9.95 applies. 

Alterations in pH also can be responsible for the increase in solubility of loaded active agents. Glucose oxidase immobilized on sepharose beads were incorporated into ethyl vinyl acetate matrices along with insulin in the solid form. Glucose from blood enters these matrices, gets oxidized to glucuronic acid, and the decrease in pH increases the solubility of insulin, which diffuses out. The insulin in this case was modified by the addition of three extra lysine residues that ensured an isoelectric point of pH 7.4 for the molecule.33... 

Hemoproteinoids have peroxidatic and catalatic activity 26). The peroxidase activity of hematin is increased up to 50 times when the hematin is incorporated into proteinoids 26), The hemoproteinoids have been synthesized from various mixtures of amino acids containing 0.25-2.0% hematin. The isoelectric point of the lysine rich hemoproteinoids is about 8 and the molecular weights are a little below 20,000 by gel filtration 26). 

It has been suggested that the modification of the protein s isoelectric point could result in an alteration of its pharmacokinetic profile. Avidin acylation was performed by lysine amino group derivatization with succinyl anhydride or other anhydrides, which allowed the isoelectric point to be shifted to more acidic values, depending on the level of modification. Indeed, the protein anionization induced a reduction of accumulation in the liver, but resulted only in a limited prolongation of residence time in the circulation [30, 31]. 

Basic amino acids (histidine, lysine, and arginine) have isoelectric points at pH values of 7.6, 9.7, and 10.8, respectively. These values reflect the weak basicity of the imidazole ring, the intermediate basicity of an amino group, and the strong basicity of the guanidino group. A basic solution is needed in each case to prevent protonation of the basic side chain to keep the amino acid electrically neutral. 

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. 

---