Prototropic group

Derive a general equation that describes the average net charge (Z) on a prototropic group in terms of the solution pH and the pKa of the group. 

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. 

In most cases, however, recourse must be had to other methods to disentangle the dissociations of the several prototropic groups which are stronger bases than imidazole. 

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. 

The first direct indication that the prototropic groups conformed to the general rule resulted from end-group analyses by the 2,4-dinitrophenyl... 

Titrimetric Criteria for Liberation of Unreactive Prototropic Groups... 

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. 

Recently published data on the cleavage of horse-radish peroxidase (Maehly, 1952, 1953), although indicating other complexities, bear sufficient resemblance to the situation described above for ferrihemoglobin, to raise a strong presumption that cleavage is accompanied by the liberation of prototropic groups unavailable for combination with acid and base in the native enzyme. No direct titration data are available for comparison. 

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). 

These potentials are a result of charge carrier (electron and hole) migration in the solid phase attributable to the electrical fields entering the solid surface from an adjacent charged body. In our case, the virus develops electrical charge resulting from the ionization of prototropic groups in aqueous solutions. 

As already mentioned, the amino acid residues in enzymes have prototropic groups which have the potential to act as a general acid or as a general base. Of these, the imidazole ring of histidine is of special interest since it can perform both func-... 

Figure 2.31a Km is independent of pH in the range of 4 — 9. This means that the forms E"+ E", and E i. e. enzyme forms which are neutral, positively or negatively charged on the active site, can bind substrate. Figures 2.31b and c Km is dependent on one prototropic group, the pK value of which is below (Fig. 2.31b) or above (Fig. 2.31c) neutrality. In the former case, E and E are the active forms, while in the latter, E + and E are the active enzyme forms in substrate binding.

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