Protein prototropic groups

This review deals first (Part II) with recent work on the acid-base dissociations of native proteins. This is followed (Part III) by a discussion of unreactive prototropic groups in certain native proteins, with emphasis on conditions for their detection, and their relation to protein denaturation and the structural changes which accompany it. 

In some cases the quantities p(/ Ci t), and m yielded by analysis of titration curves may furnish special clues to the structure of particular proteins. For example, if ordinarily dissociable groups participate in formation of intramolecular bonds abnormal values for their intrinsic dissociation constants may result, or their number may even appear to be smaller than that found by amino acid assay. Examples will be furnished in the later sections on stoichiometry and on unreactive prototropic groups. 

Rather than resort to purely empirical selection of suitable values of ni and p(/Cint)< for equation 1 it is more usual to begin by fitting experimental data with values of m chosen to conform with the numbers of prototropic groups determined by several more direct and specific methods of examination of titration data. Even where the theoretical analysis of a titration curve is not attempted and exact values of p(Ki t), for each type of group are therefore lacking, the numbers of groups so determined may furnish valuable clues to the internal structure of the protein, especially when they are compared with the results of amino acid analyses. 

The stoichiometry of the hydrogen ion eqqilibria of proteins has been of particular interest in the detection of interactions of elements of the protein structure which may render inactive some of the prototropic groups. Sev-... 

If all the prototropic groups of native proteins were normally reactive to acids and bases, this normal reactivity would constitute a significant exception to the well-founded generalization that the functional groups (sulfhydryl, disulfide, phenoxyl) of native proteins are usually less reactive in native proteins than in proteins denatured by any means (Neurath et al., 1944), just as the susceptibility of proteins to hydrolysis by proteolytic enzymes is usually increased by denaturation. 

Smaller and less easily interpreted pH drifts have been reported by Kearney and Singer (1951) during inactivation of partially purified i-amino acid oxidases of snake venom. Reference ivill be made later to other proteins that exhibit other less direct indications of the existence of labile masking of certain prototropic groups. 

Electrostatic Component of AGads A major contribution to AGads in colloidal systems arises from the interaction of electrostatic double layers that form about charged particles immersed in electrolyte solutions. Oxide and protein surfaces principally develop charges by ionizing prototropic groups on their surfaces (27). 

Pectinolytic enzymes are used for the clarification of fruit and vegetable juices. The mechanism of clarification is as follows the core of the turbidity causing particles consists of carbohydrates and proteins (35%). The prototropic groups of these proteins have a positive charge at the pH of fruit... 

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