ISSN:
0006-3525
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The heats of binding of protons ΔHb to three globular proteins, lysozyme, chymotrysinogen A, and oxidized cytochrome c, from pH 11-2 or lower at 25°C were determined by flow microcalorimetry. In addition, the acid-base titrations of chymotrypsinogen A and oxidized cytochrome c were investigated under conditions similar to those used in the calorimetric experiments. The results of the calorimetric experiments and the acid-base titrations for lysozyme and chymotrypsinogen A in the neutral and alkaline pH regions are in accord with the Linderström-Lang model with a pH-independent electrostatic factor. However, in order to interpret the results on oxidized cytochrome c in the same pH region, it is necessary to assume the protonation of two unidentified groups; one of these groups, with ΔHb = -18 kcal/mol and pKa = 9.4 was detected in previous work [Watt, G. D. & Sturtevant, J. M. (1969) Biochemistry 8, 4567]. The normal heats of protonation for carboxyl, ∊-amino, phenolic, α-amino, and imidazole groups on globular proteins as deduced from the study are 0, -10.5, -6.3, -10.0, and -6.3 kcal/mol, respectively. ΔHb of chymotrypsinogen A between pH 4.5 and 1.3 is +30 kcal/mol. This value, which is undoubtedly too large to be accounted for by the protonation of normal carboxyl groups, leads to the conclusion that this protein undergoes a pH-induced conformational change in this pH region. The same idea can be applied to explain the abnormal ΔHb observed for oxidized cytochrome c in the alkaline pH region.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bip.1976.360150613
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