Abiotic Stress Adaptation: Protein Folding Stability and Dynamics - Abiotic Stress: Plant Responses and Applications in Agriculture - page 8

ic co-solvents increase the fraction of more dense species in the hydration water thereby
destabilizing protein structure [28]. Molecules that stabilize the surface low-density water
and increase the surface tension will stabilize the protein structure (kosmotropes).
Co-solvent effects that alter the water structure are not the sole driving force for increasing or
decreasing protein stability. It also has to be considered that the interaction of a co-solvent with
the protein surface may not be favorable and thus would destabilize a protein. Due to the fact
that unfolded or denatured states comprise a higher solvent and co-solvent accessible surface
area, the equilibrium tends to shift to the more compact folded state known as the "excluded
volume effect". Among all the interactions that may stabilize or destabilize proteins, a main
driving force for protein folding is "hydrophobic interactions". Hydrophobic forces will also be
affected in the presence of co-solvents, partly depending on the ability of a solute to be exclud‐
ed or incorporated in the hydration shell of a certain protein [29].
Increases in temperature, pressure or osmotic stress alter the properties of protein conforma‐
tion and the hydration state. The free energy change resulting from folding or unfolding de‐
pends on the combined effects of the exposure of the interior and non-polar groups and
their interaction with water, including changes in water-water interactions.
3.2. Dynamics in enzymatic activity
The ability to maintain protein performance under abiotic stress depends on intrinsic stabili‐
ty, chaperon activity, protein turnover and extrinsic stabilization through co-solvents (com‐
patible solutes). Molecular motion as well as protein flexibility and dynamics is highly
linked to enzymatic activity, which is clearly dependent on the particular environment of a
protein [30].
Hydration status and temperature are the main factors that contribute to the catalytic mech‐
anism. Hydration is necessary for enzyme catalytic function since dry enzymes are less func‐
tional, and below a threshold hydration level enzymes are inactive. Protein hydration may
also be necessary for diffusion of substrate and product [31]. Temperature is a fundamental
environmental stress, as flexibility and functionality of enzymes are highly temperature de‐
pendent. Low temperatures result in decreased catalytically activity, which is metabolically
not favorable. Increases in the thermal energy will increase enzyme molecules that have the
required energy for conformational changes into catalytically active enzymes, showing an
increased catalytic rate ( k cat ). High temperatures, on the other hand, can cause the structure
to become so loose that substrates and co-factors can no longer bind [32]. Extreme tempera‐
tures cause complete denaturation. Osmolyte (glycerol, sorbitol, xylitol, glucose, fructose,
saccharose, proline, glycine betaine, myo -Inositol, pinitol, quercitol) protection of enzymes
against heat-induced loss of activity has been extensively studied in vitro [33-35]. The partic‐
ular properties of a protein and the nature of the added osmolyte strongly influence protein
thermal stability and enzyme activity. The ability to protect enzymes from heat induced ac‐
tivity loss varies between different osmolytes but preserving enzymes under heat stress
seems to be a general feature for these osmolytes. Loss of enzymatic activity under high
temperature treatment does occur but is always slower and at higher temperatures when
compared to proteins without protective additives. Enzymatic activity tests demonstrate the