The use of gas–phase substrates to study enzyme catalysis at low hydration

Abstract

Although there are varying estimates as to the degree of enzyme hydration required for activity, a threshold value of ca. 0.2 g of water per gram of protein has been widely accepted. The evidence upon which this is based is reviewed here. In particular, results from the use of gas-phase substrates are discussed. Results using solid-phase enzyme-substrate mixtures are not altogether in accord with those obtained using gasphase substrates. The use of gaseous substrates and products provides an experimental system in which the hydration of the enzyme can be easily controlled, but which is not limited by diffusion.All the results show that increasing hydration enhances activity. The results using gas-phase substrates do not support the existence of a critical hydration value below which enzymatic activity is absent, and suggest that enzyme activity is possible at much lower hydrations than previously thought; they do not support the notion that significant hydration of the surface polar groups is required for activity. However, the marked improvement of activity as hydration is increased suggests that water does play a role, perhaps in optimizing the structure or facilitating the flexibility required for maximal activity.

Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life

“…Wyatt et al ., ) and may facilitate the acquisition and retention of water, cannot efficiently absorb water from the vapour phase at equilibrium relative humidities of much less than ∼ 50%, equivalent to 0.500 a w (Fakes et al ., ). Some enzymes (especially some lipases) can remain catalytic below 0.500 a w , other enzymes can become highly inefficient as their hydration decreases, and others can lose their catalytic activity at water activities below the known limits for microbial multiplication (Dunn and Daniel, ; Kurkal et al ., ; Lopez et al ., ), though the implications of these findings for the physiological limits of cellular function at low water‐activity have yet to be established. There is evidence that DNA becomes disordered, and is therefore no longer transcribable, below a water activity of 0.550 (Falk et al ., ).…”

Enzyme hydration, activity and flexibility: A neutron scattering approach

“…The apparent hydration threshold for activity in some enzymes has led to suggestions that this is the critical hydration level at which 'loosening up' of the enzyme occurs to permit catalysis [20]. However, much of the data on which this threshold concept is based arise from experiments where activity is measured in dry enzymes using solid substrates so that the threshold may arise from a diffusional limitation on substrate and/or product rather than from any requirement for surface hydration [21,22].…”

Lone pair ··· π interactions between water oxygens and aromatic residues: Quantum chemical studies based on high‐resolution protein structures and model compounds

“…In protein structures, they are usually seen in different sites of the molecule and are found on surfaces, in crevices, at binding interfaces and at times buried in the interior 1. These water molecules are often integral part of the structures, and have been observed to play a critical role in protein folding, structure, activity, dynamics, and protein–ligand interaction 2–11. These properties are, in general, driven by water's hydrogen bonding capacity with main chain carbonyl, amide and other electronegative atoms of side chain.…”