Isoelectric Point Isoelectric Point Amphoteric molecules called zwitterions contain both positive and negative charges depending on the functional groups present in the molecule. The net charge on the molecule is affected by pH of its surrounding environment and can become more positively or negatively charged due to the loss or gain of protons (H+). The pI is the pH value at which the molecule carries no electrical charge or the negative and positive charges are equal. Surfaces naturally charge to form a double layer. In the common case when the surface charge-determining ions are H+/OH-, the net surface charge is affected by the pH of the liquid in which the solid is submerged. The pI value can affect the solubility of a molecule at a given pH. Such molecules have minimum solubility in water or salt solutions at the pH that corresponds to their pI and often precipitate out of solution. Biological amphoteric molecules such as proteins contain both acidic and basic functional groups. Amino acids that make up proteins may be positive, negative, neutral, or polar in nature, and together give a protein its overall charge. At a pH below their pI, proteins carry a net positive charge; above their pI they carry a net negative charge.
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Proteins can, thus, be separated according to their isoelectric point (overall charge) on a polyacrylamide gel using a technique called isoelectric focusing, which uses a pH gradient to separate proteins. Isoelectric focusing is also the first step in 2-D gel polyacrylamide gel electrophoresis. Classification of Amino Acids Amino acids are classified into different ways based on polarity, structure, nutritional requirement, metabolic fate, etc. Generally used classification is based on polarity. Amino acid polarity chart shows the polarity of amino acids. Based on polarity, amino acids are classified into four groups as follows, 1. Non-polar amino acids 2. Polar amino acids with no charge 3. Polar amino acids with positive charge 4. Polar amino acids with negative charge Classification based on polarity Amino acid are classified based on those with non-polar R group, uncharged polar R group, charged polar R group. The non-polar side chain amino acids are called hydrophobic and the amino acid with uncharged polar side chain is called hydrophilic. It also contain the acid side chains and basic side chains. Non Polar Amino Acids Non Polar Amino Acids have equal number of amino and carboxyl groups and are neutral. These amino acids are hydrophobic and have no charge on the 'R' group. The amino acids in this group are alanine, valine, leucine, isoleucine, phenyl alanine, glycine, tryptophan, methionine and proline.
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Essential Amino Acids :- Of the 20 amino acids in proteins, humans are able to synthesize only 11, called non-essential amino acids, the other 9 called essential amino acids, must be obtained in diet. The division between essential and nonessential amino acids is not clear cut however Tryosine for instance is sometimes considered non-essential because humans can produce it from phenylalanine, but phenylalanine itself is essential and must be obtained in the diet. Arginine can be synthesized by humans, but much of the arginine in proteins also comes from the diet. Essential amino acids are not produced by the body. We can get them by eating complete protein foods or from a combination of incomplete vegetables. The nine essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, tryptophan, and valine. The thirteen non-essential amino acids are alanine, arginine, aspartic acid, cysteine, cystine, glutamic acid, glutamine, glycine, hydroxyproline, proline, serine, and tyrosine.
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