Isothermal sorption measures on desiccation-tolerant biological systems give evidence of three hydration fractions: at low water content, a strong water-binding fraction; at intermediate water content, a weak waterbinding fraction; and at high water content, a loose water-binding fraction (Clegg, 1978). More recent and sensitive methods identify at least four fractions (Sun, 2000). Some of the biological systems used in rehydration studies rely on desiccation-tolerant seeds (soybean and cowpea), pollen, and brine shrimp cysts, as well as dehydration-intolerant seeds (red oak acorn) (Clegg, 1978, 1986; Clegg et al, 1978; Pagnotta and Bruni, 2006; Sun, 2000). Onset of biological function for dehydration-tolerant species relates to an increased level of hydration to liberate the organism from a dormant state (Bruni et al., 1989a,b; Clegg, 1982; Clegg et al., 1982; Vertucci and Roos, 1990). Survival but no metabolic activity was observed below 0.10 g/g in desiccation-tolerant species, whereas desiccation-intolerant species do not survive such dehydration. This correlates with concepts that the
SH model at hydration below hB «0.22 g/g removes structural water bridges and exposes protein molecule to high mechanical stress (Fullerton and Amurao, 2006). Enzyme activity was first observed at 0.20 to 0.30 g/g in brine shrimp cysts, which is slightly greater than hB «0.22 g/g and correlates with the earlier discussion of the hydration dependence of lyso-zyme enzymatic activity. Higher levels of metabolic activity and respiration occurred at hydration levels of greater than 0.40 g/g for seeds and at greater than 0.6 g/g for brine shrimp; this correlates with the hydration behavior of the lysozyme shown in Fig. 1.13.
This chapter reviewed physical evidence for the formation of categorical water compartments on proteins due to "irreducible separation'' of electrical charges on the protein backbone. Investigations of collagen and correlation of results on globular proteins led to the hypothesis of a stoi-chiometric hydration model applicable to all proteins. Comparison of predicted hydration values with measurement of hydration capacities for multiple proteins confirmed the utility of the SH model. Four methods were selected and presented to make accurate measurements of the hydration fractions on other proteins and tissues. The relatively uniform size of hydration fractions is directly due to the uniform character of protein backbones. Correlation of SH predictions with enzyme mobility and activity measurements as a function of hydration are in close agreement. Finally, measurements of biological activity of cells also show values consistent with SH predictions.
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