Isoelectric point amino acids, calculation

The deduced PGE protein consists of 378 amino acid residues. Based on the known composition of N-termini of PGI and PGII proteins it is assumed that the 39 N-terminal amino acids of PGE form the pre-prosequence of the protein. The molecular weight (35 584) and isoelectric point (pi = 3.6) of mature PGE were calculated based on deduced amino acid sequence. 

The amino acid compositions of each subunit and the native enzyme are shown in Table 5. There are distinct differences between the two subunits. As would be expected from the isoelectric points, the a subunit contains more acidic amino acids than the /3 subunit does, and the /3 subunit contains more basic amino acids than the a subunit does. The amino acid composition of the native enzyme obtained by actual analyses agrees quite well with that calculated from the compositions of the a and /3 subunits, based on the assumed subunit structure of (a ls- These results confirms the assumption that the native enzyme consists of 8 protomers, each of which is composed of a pair of two non-identical subunits (a2fi2)97 I0S. ... 

Isoelectric point — The isoelectric point (pi) is the pH at which a molecule carries no net electrical charge. For an acid with only one amino and one carboxyl group (e.g., an amino acid) the pi can be calculated from the pFCa s of this molecule ... 

The net charge on a protein is the algebraic sum of all its positive and negative charges. There is a specific pH for every protein at which the net charge it carries is zero. This isoelectric pH value, termed the isoelectric point, or pi, is a characteristic physicochemical property of every protein. The definition of pi for molecules as complex as proteins is more or less an operational one and is taken to be that pH at which a protein has zero electrophoretic mobility in an isoelectric focusing run. Nevertheless, it has been shown that the pis of some acidic proteins (up to about pH 7) can be calculated from their amino acid compositions.3 5... 

Calculate the isoelectric point for the following polyftmctional amino acids (a) histidine, (b) lysine, and (c) aspartic acid. (See Table 5-1.)... 

Calculate the isoelectric point for each amino acid. 

Table 28.1 also lists the isoelectric points (pi) for all of the amino acids. Recall from Section 19.14C that the isoelectric point is the pH at which an amino acid exists primarily in its neutral form, and that it can be calculated from the average of the pAT values of the a-COOH and a-NH3 groups (for neutral amino acids only). 

To calculate the isoelectric point of amino acids having other ionizable functional groups, we must also take into account the pKa of the additional functional group in the side chain. 

The electronic properties of amino acid side chains are summarized in Table 3, and they represent a wide spectrum of measures. The NMR data are derived experimentally (37). The dipole (38), C mull, inductive, field, and resonance effects were derived from QM calculations (15). The VHSE5 (39) and Z3 (25) scales were developed for use in quantitative structure-activity relationship analysis of the biologic activity of natural and synthetic peptides. Both were derived from principal components analysis of assorted physico-chemical properties, which included NMR chemical shift data, electron-ion interaction potentials, charges, and isoelectric points. Therefore, these scales are composites rather than primary measures of electronic effects. The validity of these measures is indicated by their lack of overlap with hydrophobicity and steric parameters and by their ability to predict biologic activity of synthetic peptide analogs (25, 39). Finally, coefficients of electrostatic screening by amino acid side chains (ylocal and Ynon-local) were derived from an empirical data set (40), and they represent a composite of electronic effects. 

In weak acid solution (about pH 5 6.0) the carboxyl group of a neutral amino acid (one amino group and one carboxyl group) is dissociated, and the amino group binds a proton to give a dipolar ion (zwitterion). The pH at which the concentration of the dipolar ion is a maximum is called the isoelectric point (pi) of that amino add. The isoelectric point of an amino add is calculated from the relationship... 

The isoelectric point (pi) of an amino acid is the pH at which the molecule has an average net charge of zero and therefore does not migrate in an electric field. The pi is calculated by averaging the pK values for the two functional groups that react as the zwitterion becomes alternately a monovalent cation or a monovalent anion. 

The gene encodes for a secreted protein, with the mature protein possessing 119 amino acids. The calculated molecular mass and isoelectric point of the recombinant form are very similar to those obtained by electrophoretic methods on the purified protein (Barde et al., 1982). A striking feature of the primary structure of mature BDNF is its similarity to NGF. There are 51 amino acids common to the various NGF s and to BDNF, including all six cysteine residues (Leibrock et al.,... 

Compute pI/MW (ExPASy Proteomic tools) is a tool that calculates the isoelectric point and molecular weight of an input sequence. Molecular weights are calculated by the addition of the average isotopic mass of each amino acid in the sequence plus that of one water molecule. The sequence can be furnished by the user in PASTA format, or by... 

The permeability coefficients of 20 amino acids at physiological pH (7.4) and at the pH of the isoelectric point were taken without alteration from Table 2. Based on pAT values calculated by SPARC and the pH reported in this article, we calculated that the ionic condition of the amino acids was zwitterionic for all but four measurements. The exceptions were aspartic acid at pH 7.4 and glutamic acid at pH 7.4 (which were both net anionic) and lysine at pH 7.4 and arginine at pH 7.4 (which were both net positive). 

Skoog, B., and Wichman, A. (1986) Calculation of the isoelectric points of polypeptides from the amino acid composition. Trends Anal. Chem. 5, 82-83. 

For aspartic and glutamic acids, the isoelectric point occurs at a pH where the net charge on the two carboxyl groups is -1 and balances the charge of +1 on the a-amino group. Calculate pi for these amino acids. 

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