Enzyme activity, vibration, and hydrogen exchange all show compartmental capacities that correlate with predictions of the SHM. Furthermore, the SHM relates fast conformational fluctuations in the picosecond range to hydration of the protein backbone. As globular lysozyme consists of two domains with both a-helix and b-sheet components, it leads to the hypothesis that fast fluctuations are vibrations of tightly bound protein secondary structures. Additionally, backbone water bridges participate in tight hydrogen bonding between amide and carbonyl groups, which predicts a monotonic increase in the proton exchange rate as the number ofbackbone amide protons associated with water increases with each bridge until all sites are fully occupied at hB = 0.217 g/g. The slow relaxation process starts to increase above hB = 0.217 g/g where the SH model predicts DWC form around the water bridges. DWC cover charge sites on the immobilized water bridges and remaining polar sites on the side chains of the protein. The dielectric water allows a range ofspatial configurations with nearly the same free energy that promotes vibrational motion of the protein tertiary structure. No further increase in slow vibration or enzymatic activity occurs above hPr = 0.677. Thus the correlations of enzymatic activity, proton exchange, and vibrational motion of the molecule are consistent with the SH model water compartments. The conclusion from these enzyme dynamic flexibility studies is that hydration changes the shape of the protein and liberates large-scale motion necessary for functional enzymes. Dry enzymes are not functional, as water is required to perfect the correct lock/key shape, provide enzyme motional freedom, and release the completed product from the enzyme in a timely manner.
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