Heats of binding protons to globular proteins

Abstract

SynopsisThe heats of binding of protons m b to three globular proteins, lysozyme, chymotrysinogen A, and oxidized cytochrome c, from pH 11-2 or lower a t 25°C were determined by flow microcalorimetry. In addition, the acid-base titrations of cbymotrypsinogen 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 Linderstrom-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 AHb = -18 kcal/ mol and pK, = 9.4, was detected in previous work [Watt, G . D. & Sturtevant, J. M. (1969) Biochemistry 8,45671. The normal heats of protonation for carboxyl, c-amino, phenolic, a-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. AHb 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 AHb observed for oxidized cytochrome c in the alkaline pH region.

Analytical titration curves of glycosyl hydrolase Cel45 by combined isoelectric focusing — electrophoresis

“…[7]. This is not surprising, since several other proteins show the same behavior [8±10] and conformational changes at different pH values have been observed [11,13,15]. Experimentally, the Cel45-core does not show any major deviation from theory to justify any large conformational change in the range of the studied pH values.…”

Thermodynamic and Biophysical Analysis of the Membrane-Association of a Histidine-Rich Peptide with Efficient Antimicrobial and Transfection Activities

“…For α-helix formation of synthetic peptides in organic solvents h was found to be h = −0.7 kcal/mol per amino acid residue (see Figure 3b in ref 33) whereas in aqueous solution h is larger with h = −1.1 kcal/mol. 34,35 The total folding enthalpy ΔH helix of LAH4-L1 can thus be estimated as ΔH helix = 16.6 × (−0.7) = −11.65 kcal/mol per mol LAH4-L1 at 25°C.…”

The role of carboxyl groups in the function of endo‐β‐1, 4‐glucanase from Schizophyllum commune

“…With the 8-1,4 endoglucanase from A . niger, Hurst et al [l,21 proposed the catalytic amino acids to include a carboxylate anion (pK 4.0 -4.5) and a protonated carboxyl group (pK 5.0-5.5) that act in a manner similar to hen eggwhite lysozyme. The pH dependence of inactivation of three fungal endoglucanases by an affinity label prompted Legler and Bause [5] to implicate an essential acid-catalyst in the function of these enzymes.…”