Sulfhydryl-disulfide interactions

There is another phenomenon, regarded as a deteriorative change in the protein of soy milk, caused also by the evaporation of water. This is a film formation on the surface of soy milk, which occurs when heated soy milk is kept open to the air. This phenomenon is observed not only in heated soy milk but also in heated cow s milk. Film formation of soy milk occurs only when the soy milk is heated above 60°C and there is evaporation of water from the surface of the soy milk. The mechanism of protein insolubilization is basically the same as that of soy milk powder produced from heated soy milk (10. When water is removed from the surface of heated soy milk by evaporation, the molecular concentration of protein near the surface increases locally and the exposed reactive groups of the denatured molecules come close enough to interact intermolecularly both by hydrophobic interactions and through the sulfhydryl/disulfide interchange reaction to form a polymerization (film) on the surface. The upper side of the film contains more hydrophobic amino acids because of orientation of the hydrophobic portions of the unfolded molecules to the atmosphere rather than into the aqueous solution. 

Since reductive S-alkylation of disulfide bonds introduces unnatural amino acid side chains into the protein and, therefore, cannot serve as a useful model for nutritional and toxicological studies, we initiated systematic studies of effects of thiols on the inhibitory process. Such thiols are expected to interact with inhibitor disulfide bonds via sulfhydryl-disulfide interchange and oxidation reactions (Friedman, 1973). 

DeLucia, A. J., M. G. Mustafa, M. Z. Hussain, and C. E. Cross. Ozone interaction with rodent lung. III. Oxidation of reduced glutathione and formation of mixed disulfides between protein and nonprotein sulfhydryls. J. Gin. Invest. 55 794-802, 1975. 

Larger proteins often contain more than one polypeptide chain. These multi-subunit proteins have a more complex shape, but are still formed from the same forces that twist and fold the local polypeptide. The unique three-dimensional interaction between different polypeptides in multi-subunit proteins is called the quaternary structure. Subunits may be held together by noncovalent contacts, such as hydrophobic or ionic interactions, or by covalent disulfide bonds formed from the cysteine residue of one polypeptide chain being cross-linked to a cysteine sulfhydryl of another chain (Fig. 15). 

Grollmann U, Schnabel W (1980) On the kinetics of polymer degradation in solution, 9. Pulse radiolysis of polyethylene oxide). Makromol Chem 181 1215-1226 Hamer DH (1986) Metallothionein. In Richardson CC, Boyer PD, Dawid IB, Meister A (eds) Annual review of biochemistry. Annual Reviews, Palo Alto, pp 913-951 Held KD, Harrop HA, Michael BD (1985) Pulse radiolysis studies of the interactions of the sulfhydryl compound dithiothreitol and sugars. Radiat Res 103 171-185 Hilborn JW, PincockJA (1991) Rates of decarboxylation of acyloxy radicals formed in the photocleavage of substituted 1-naphthylmethyl alkanoates. J Am Chem Soc 113 2683-2686 Hiller K-O, Asmus K-D (1983) Formation and reduction reactions of a-amino radicals derived from methionine and its derivatives in aqueous solutions. J Phys Chem 87 3682-3688 Hiller K-O, Masloch B, Gobi M, Asmus K-D (1981) Mechanism of the OH radical induced oxidation of methionine in aqueous solution. J Am Chem Soc 103 2734-2743 Hoffman MZ, Hayon E (1972) One-electron reduction of the disulfide linkage in aqueous solution. Formation, protonation and decay kinetics of the RSSR radical. J Am Chem Soc 94 7950-7957... 

Figure 38 Representation of the equilibrium between the cyclic and acyclic forms of a hexapeptide. The N- and C-terminal blocking groups, CH3CO— and —NHCH3, respectively, are not shown.233 The standard free energy change for this process depends on the intrinsic chemistry to form a disulfide bond from two sulfhydryl groups, the tendency of Pro-Giy (or any other dipeptide in this position) to form a P turn, and the tendencies of residues X and Y to adopt the extended conformation (and interact with each other). Table 5 of Reference 234 illustrates the range of standard free energy changes for a family of such hexapeptides.

It is well known that blocking of the free sulfhydryl, Cys-34, with iodoacetamide, cysteine, or glutathione prevents the occurrence of mixed disulfides in aged albumin, as well as the occurrence of the albumin dimer (Peters, 1985). In the structure, Cys-34 is not in close proximity to any disulfide, consequently, it is difficult to imagine its intramolecular participation without substantial conformational change of the albumin molecule. However, it could perhaps participate in the formation of mixed disulfides by catalyzing the reaction via intermolecu-lar interaction, because Cys-34 is known to have an unusually low p/CsH (Pedersen and Jacobsen, 1980) (see Section III,C,1). 

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