Hum. My explanation does pose a bit of a question though. Many of the "facts" behind what I said are dragged up from "A"-level Biology which I did rather along time ago: Animal enzymes have evolved to work best at "body temperature" whereas there is no such constraint on plant enzymes so they keep working faster and faster until the enzyme gets too hot and is destroyed. Where does that leave enzymes in cold-blooded creatures?
But I got a criticism recently for bandying about this info because some plant enzymes will start working optimally again if cooled (i.e. they haven't been destroyed). Is that right? So plant amylase enzymes are destroyed at about optimum performance because "optimum" has it sailing too close to the wind?
It just so happens that I work at the interface between protein chemistry and evolution! An enzyme work by binding to a substrate and lowering the energy barrier needed to be passed in order for the substrate to turn into the product, then letting go of it. The more thermal energy there is (i.e. higher temperature), the more likely the substrate is to pass the barrier and get converted. Plus, the faster the substrate diffuses to the binding site, and the quicker the product is let go, freeing up the enzyme to convert another molecule. This is more than enough to counter the fact that the enzyme isn't as good at clinging on to the substrate at higher temps. So, higher temp yields more turnover, in general. However, the enzyme itself also responds to higher temperature, eventually unfolding and losing its function altogether. For most enzymes, as long as they don't get so hot they cross-react irreversibly with other things or themselves, or get caught up in amorphous aggregates, then they can refold when the temp comes down again, because of something called
Anfinsen's dogma. Evolution acts only on temperatures that the organism encounters. For humans that is a pretty narrow range, while for plants, bacteria, yeast, fungi and cold-blooded creatures, it's generally a lot wider. As the protein hasn't evolved to behave in a specific way when it's outside of that temperature, what happens is somewhat random, within what is possible by protein chemistry. 60C is certainly outside of what barley or it's ancestors ever experienced frequently enough for it's behaviour at that temp to be subject to natural selection. The protein may retain some residual activity, and that may go up or down with increasing temperature depending on how perturbed by thermal energy the part that grabs onto the substrate gets. Eventually, it will unfold, cross-react and lose function, but again the point at which that happens depends on how that particular protein sequence just so happens to behave when at a temperature it wasn't evolved to deal with. The fact that alpha and beta amylase respond the way they do is down to the happenstance of nature.
