Isoelectric focusing definition

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... 

The terms cellulolytic enzymes and cellulases now enjoy a workable definition. This has been made possible through the separation of the components of the cellulolytic complex, as well as some clarification of their mechanism of action. New viscometric, isoelectric focusing, gel filtration, and other techniques for the determination of the enzymic components have been devised. Detailed observations on the molecular sites of hydrolytic reactions, the susceptibility of the enzymic reactions to metallic ions, proteolytic enzymes, and other chemicals also provided considerable insight into the nature of cellulase. 

The isoionic point, for a protein solution not too low in concentration, is experimentally defined as that pH which does not change when a small amount of pure protein is added to the solution (46). This definition also applies to a protein which has been focused isoelectrically (7,26). If more protein is added to the zone, the pH does not change. The buffering at the site of focusing is considered to be completely dominated by the... 

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