ChEMBL

Proteins often have small absolute free energies of folding (ΔG_fold) and are frequently marginally stable at room temperature – the power of this on biological systems is really familiar in the adaptive response of fever to an infection – the body raises it’s temperature a few degrees with the aim of denaturing pathogen proteins and exposing antigens, slowing down replication, etc.. Raising a human’s body temperature just a few degrees further is fatal (normal body temperature is ca. 37 C, temperatures above 40 C can be fatal). Similarly, lowering temperature a few degrees has serious effects on the ability for proteins to do their job, and subsequently on normal everyday life. So proteins are only required to be functional over a comparatively narrow range of operational temperatures, and where temperature differences occur there can be evolutionary selection for this trait (as an interesting example, sequencing studies have identified alleles in cod under selective pressure for temperature (link here)). This dependence on temperature is probably an extra significant factor for proteins with two (or more) conformations for which the distribution/equilibrium is small anyway and is then ‘biased’ by small molecule binding.

An important corollary to this is that ΔG_fold at lab/body temperature is not necessarily reflected in the T_m for a protein.

Some organisms regulate their body temperature (like me), others don’t (like Vini and Bruce my Pogona) and just have to exist at ambient temperatures – again typically there is a range over which normal everyday life is possible, too cold and ‘cold-blooded’ life becomes sluggish and dormant, too hot and they typically seek shelter and again become inactive.

This temperature dependence on function is implicit in almost all of biology, and is so obvious that it is often unspoken what temperature an assay is performed at – but temperature differences can easily give rise to differences in bioassay results – especially for enzymes where rates of reactions are very sensitive to temperature changes. As an example of some of the issues - here is the protocol for a panel of assays (just chosen at random) using the ubiquitous 'room temperature' which can be between 15 and 25 C). I guess all the controls have been done though, and in this case it doesn't matter.

One of the impacts that non-synonymous genetic mutations can cause are changes in protein stability, and in the limit a significantly destabilising coding mutation will lead to completely non-functional protein - for example, by the introduction of a charged residue in the hydrophobic core of a protein. Since this has the physical appearance of a mis-folded protein it will be recognised as such and tagged for clearance – so a double whammy - what minimal function remains is itself rapidly cleared. However, we know that the vast majority of amino-acid mutations are functionally neutral, which is fortunate for all of us.

This "temperature dependence of function" feature is not regularly explored in drug discovery, but can take one some interesting places if you let your mind drift across the literature – so for example it leads to the hypothesis that it may be possible to stabilise a destabilised protein with a small molecule – and this has been tested a number of times – the best example that springs to mind is for p53 a key tumour suppressor in cancer cell biology – where screening at different temperatures has been used to identify small molecule stabilisers of mutant p53. Importantly this paper showed it was possible to find compounds (e.g. CP-31398) that perturbed a mutant protein back towards it's native state.

A great example of how normal function can be restored to a mutant protein is with the recently launched breakthrough drug for Cystic Fibrosis - Ivacaftor (VX-770, tradename Kalydeco).

As one step further, you could imagine performing an in vivo screen where you look for a compound that narrows or perturbs the range at which an organism can effectively live by making it more temperature sensitive. Compounds with these properties could have great commercial application in various pest species. 

Understanding the impact on thermodynamics and dynamics of 'unstable'/temperature-sensitive systems such as these is a great opportunity and is currently an under-explored area for future drug design. If you are interested in a PhD studentship in this area, please get in touch, and see this page for application details.