Sugar protein stability effects

The effect of "residual water" on either protein stability or enzyme activity continues to be a topic of great interest. For example, several properties of lysozyme (e.g., heat capacity, diamagnetic susceptibility (Hageman, 1988), and dielectric behavior (Bone and Pethig, 1985 Bone, 1996)) show an inflection point at the hydration limit. Detailed studies on the direct current protonic conductivity of lysozyme powders at various levels of hydration have suggested that the onset of hydration-induced protonic conduction (and quite possibly for the onset of enzymatic activity) occurs at the hydration limit. It was hypothesized that this threshold corresponds to the formation of a percolation network of absorbed water molecules on the surface of the protein (Careri et al., 1988). More recently. Smith et al., (2002) have shown that, beyond the hydration limit, the heat of interaction of water with the amorphous solid approaches the heat of condensation of water, as we have shown to be the case for amorphous sugars. 

As an example of practical use of the sugar stabilizing effect is given by the recipe of the angel cake, where the sucrose content is tuned so as to make the denaturation temperature of ovalbumin (the main protein of egg albumen) to coincide with the gelatinization temperature of the wheat starch, since these conditions allow the characteristic angel-cake structure to be built up on baking [238],... 

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

Microscopic structure of texturized water-extracted soy flour and texturized soy concentrate were quite similar to that of texturized soy flour. Scanning electron microgrpahs showed that water extraction of soy flours had little effect on morphological characteristics of texturized soy products (Figure 10). Solubility of soluble sugars was not affected by texturization, whereas solubility of proteins decreased sharply when soy flour was texturized (Table VII). It appears that soluble sugars did not interact with proteins during texturization. Based upon results of microscopy and solubility studies, it is reasonable to speculate that natural soluble carbohydrates are not required (do not play an important role) in development of texture or stabilization of structure. 

Ginsburg, A. Szczepanowski, R.H. Ruvinov, S.B. Nosworthy, N.J. Sondej, M. Umland, T.C. Peterkofsky, A. Conformational stability changes of the amino terminal domain of enzyme I of the Escherichia coli phosphoenolpyruvate sugar phosphotransferase system produced by substituting alanine or glutamate for the active-site histidine 189 implications for phosphorylation effects. Protein Sci., 9, 1085-1094 (2000)... 

The addition of polyhydroxyl compounds to enzyme solutions have been shown to increase the stabilities of enzymes, (13,16,19,20). This is thought to be due to the interaction of the polyhydroxyl compound, (e.g. sucrose, polyethylene glycols, sugar alcohols, etc), with water in the system. This effectively reduces the protein - water interactions as the polyhydroxy compounds become preferentially hydrated and thus die hydrophobic interactions of the protein structure are effectively strengthened. This leads to an increased resistance to thermal denaturadon of the protein structure, and in the case of enzymes, an increase in the stability of the enzyme, shown by retention of enzymic activity at temperatures at which unmodified aqueous enzyme solutions are deactivated. 

---