Counteracting Osmolyte Trimethylamine N-Oxide Destabilizes Proteins at pH below Its pK

Abstract

Earlier studies have reported that trimethylamine Noxide (TMAO), a naturally occurring osmolyte, is a universal stabilizer of proteins because it folds unstructured proteins and counteracts the deleterious effects of urea and salts on the structure and function of proteins. This conclusion has been reached from the studies of the effect of TMAO on proteins in the pH range 6.0 -8.0. In this pH range TMAO is almost neutral (zwitterionic form), for it has a pK a of 4.66 ؎ 0.10. We have asked the question of whether the effect of TMAO on protein stability is pH-dependent. To answer this question we have carried out thermal denaturation studies of lysozyme, ribonuclease-A, and apo-␣-lactalbumin in the presence of various TMAO concentrations at different pH values above and below the pK a of TMAO. The main conclusion of this study is that near room temperature TMAO destabilizes proteins at pH values below its pK a , whereas it stabilizes proteins at pH values above its pK a . This conclusion was reached by determining the T m (midpoint of denaturation), ⌬H m (denaturational enthalpy change at T m ), ⌬C p (constant pressure heat capacity change), and ⌬G D°( denaturational Gibbs energy change at 25°C) of proteins in the presence of different TMAO concentrations. Other conclusions of this study are that T m and ⌬G D°d epend on TMAO concentration at each pH value and that ⌬H m and ⌬C p are not significantly changed in the presence of TMAO.Many organisms are known to accumulate low molecular weight organic molecules (osmolytes) in their tissues in response to harsh environmental stresses. These osmolytes are generally categorized into three groups, namely amino acids and their derivatives, polyhydric alcohols, and methylamines (1). Molecules of the first two groups are "compatible osmolytes," which means that cells accumulate these osmolytes to high concentrations without significantly perturbing protein functions under physiological conditions (1-4). Molecules of the third group, which reverse the perturbations caused by urea, are known as "counteracting osmolytes" (2, 5). One such counteracting osmolyte is trimethylamine N-oxide (TMAO), 1 which is present in high concentrations in coelacanth (sharks) and marine elasmobranchs (rays) (6). The effect of TMAO on protein stability and enzyme activity has been widely studied. This osmolyte has been shown in vitro to do the following: (i) increase the melting temperature as well as the unfolding free energy of proteins (7-11); (ii) offset the destabilizing effects of urea (8, 10, 11); (iii) restore the enzyme activity that is lost upon urea treatment (12, 13); (iv) force the folding of unstructured proteins (4, 12-15); (v) favor the protein self-association and polymerization of microtubules (16 -18); (vi) correct temperature-sensitive folding defects (19); and (vii) interfere with the formation of scrape prion protein (20). TMAO has been shown in vivo to counteract the damaging effects of salts (21), hydrostatic pressure (22, 23), and urea (24, 25) on proteins.TMAO is a compou...

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