Sulfenic acid in human serum albumin: Reaction with thiols, oxidation and spontaneous decay.

Human serum albumin (HSA) contains 17 disulfides and only one reduced cysteine, Cys34, which can be oxidized to a relatively stable sulfenic acid (HSA-SOH). This derivative has been previously detected and quantified. However, its properties are poorly understood. Herein, HSA-SOH formation from the exposure of HSA to hydrogen peroxide was confirmed using the sulfenic acid probe bicyclo [6.1.0]nonyne-biotin (BCN-Bio1), and by direct detection by whole protein mass spectrometry. The decay pathways of HSA-SOH were studied. HSA-SOH reacted with a thiol leading to the formation of a mixed disulfide. The reaction occurred through a concerted or direct displacement mechanism (SN2) with the thiolate (RS-) as nucleophile towards HSA-SOH. The net charge of the thiolate affected the value of the rate constant. In the presence of hydrogen peroxide, HSA-SOH was further oxidized to sulfinic acid (HSA-SO2H) and sulfonic acid (HSA-SO3H). The rate constants of these reactions were estimated. Lastly, HSA-SOH spontaneously decayed in solution. Mass spectrometry experiments suggested that the decay product is a sulfenylamide (HSA-SN(R')R″). Chromatofocusing analysis showed that the overoxidation with hydrogen peroxide predominates at alkaline pH whereas the spontaneous decay predominates at acidic pH. The present findings provide insights into the reactivity and fate of the sulfenic acid in albumin, which are also of relevance to numerous sulfenic acid-mediated processes in redox biology and catalysis.

The thiol of human serum albumin: Acidity, microenvironment and mechanistic insights on its oxidation to sulfenic acid.

Human serum albumin (HSA) has a single reduced cysteine residue, Cys34, whose acidity has been controversial. Three experimental approaches (pH-dependence of reactivity towards hydrogen peroxide, ultraviolet titration and infrared spectroscopy) are used to determine that the pKa value in delipidated HSA is 8.1±0.2 at 37°C and 0.1M ionic strength. Molecular dynamics simulations of HSA in the sub-microsecond timescale show that while sulfur exposure to solvent is limited and fluctuating in the thiol form, it increases in the thiolate, stabilized by a persistent hydrogen-bond (HB) network involving Tyr84 and bridging waters to Asp38 and Gln33 backbone. Insight into the mechanism of Cys34 oxidation by H2O2 is provided by ONIOM(QM:MM) modeling including quantum water molecules. The reaction proceeds through a slightly asynchronous SN2 transition state (TS) with calculated Δ‡G and Δ‡H barriers at 298K of respectively 59 and 54kJmol-1 (the latter within chemical accuracy from the experimental value). A post-TS proton transfer leads to HSA-SO- and water as products. The structured reaction site cages H2O2, which donates a strong HB to the thiolate. Loss of this HB before reaching the TS modulates Cys34 nucleophilicity and contributes to destabilize H2O2. The lack of reaction-site features required for differential stabilization of the TS (positive charges, H2O2 HB strengthening) explains the striking difference in kinetic efficiency for the same reaction in other proteins (e.g. peroxiredoxins). The structured HB network surrounding HSA-SH with sequestered waters carries an entropic penalty on the barrier height. These studies contribute to deepen the understanding of the reactivity of HSA-SH, the most abundant thiol in human plasma, and in a wider perspective, provide clues on the key aspects that modulate thiol reactivity against H2O2.

Modulation of the reactivity of the thiol of human serum albumin and its sulfenic derivative by fatty acids.

The single cysteine residue of human serum albumin (HSA-SH) is the most abundant plasma thiol. HSA transports fatty acids (FA), a cargo that increases under conditions of diabetes, exercise or adrenergic stimulation. The stearic acid-HSA (5/1) complex reacted sixfold faster than FA-free HSA at pH 7.4 with the disulfide 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and twofold faster with hydrogen peroxide and peroxynitrite. The apparent pK(a) of HSA-SH decreased from 7.9±0.1 to 7.4±0.1. Exposure to H(2)O(2) (2mM, 5min, 37°C) yielded 0.29±0.04mol of sulfenic acid (HSA-SOH) per mole of FA-bound HSA. The reactivity of HSA-SOH with low molecular weight thiols increased ∼threefold in the presence of FA. The enhanced reactivity of the albumin thiol at neutral pH upon FA binding can be rationalized by considering that the corresponding conformational changes that increase thiol exposure both increase the availability of the thiolate due to a lower apparent pK(a) and also loosen steric constraints for reactions. Since situations that increase circulating FA are associated with oxidative stress, this increased reactivity of HSA-SH could assist in oxidant removal.