Water as a lubricant of life

The term lubricant of life was perhaps first used (in the sense being used here) by Barron et al. [24]. These authors observed by Raman spectroscopy fast dynamic events (in the picosecond timescale) where individual amino acid residues flicker between different secondary structure states. This flickering was attributed to fast dynamic events in bulk water and mediated through HBs of the amino acid residues with surrounding water molecules. These events driven by water could be the guiding force in many functions of proteins (folding and unfolding, enzyme kinetics, protein-DNA interaction), and hence water was termed the lubricant of life. 

As mentioned repeatedly, while we know a lot about the way in which proteins fold, comparatively little is known about the detailed role that water plays in these processes. The predominant interaction of water with proteins is through the formation of HBs. As discussed in previous chapters, the structure of the water surrounding a protein is continually changing, as HBs are broken and re-formed at a very rapid rate. This leads protein secondary structures such as alpha-helices and beta-sheets to inter-convert (or flicker ) among themselves on picosecond (10 s) timescales. It is the same dynamics that is reflected in SD at the protein surface. 

This flickering makes possible ehanges in protein strueture that oeeur during enzyme kineties or even folding. The ever mobile water moleeules reduee the frictional forces that otherwise would retard the relevant motions of protein side-chains. This effect has been ealled the lubrieant of life . 

Study of the low-frequeney Raman spectrum of poly-L-lysine on both sides of the helix-to-eoil transition using ultrafast spectroseopy reveals the existenee of a broad band that ean be assigned to HB stretching vibrations between the solvent and the peptide. This band is found to shift depending upon the loeal stmeture of the peptide and refleets ehanges in the strength of the interaetions between water and the biopolymer. The frequeney of the band shows that these dynamies oeeur on the seale of 15-30 ps. 

Another important problem where water aets as a lubricant is the motion of nonspeeifleally bound protein along a DNA chain. Proteins travel to find the specific binding site. Such a motion is greatly faeilitated by water molecules in the hydration layer. 

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