Originally Posted by apeltes
I think the consensus is that barley amylase is well-adapted to operate at germination temperatures. Although it works at mash temperatures, it doesn't work as well... but that's fine. We want to convert at the top of the temperature range to suppress other enzymes and to prevent infection.
Question: What would happen if malt were mashed at 80F? Would the amylase succeed in quickly converting the starch? Would something interfere, since mashing does not happen in a living seed/embryo? Is the enzyme too dilute once mixed with so much water?
Someone earlier mentioned it, but it may have been lost among the sea of people who didn't understand your question (which is an entirely reasonable one).
Enzymes usually run faster at higher temperatures, and barley's alpha amylase does too. They also fall apart faster at higher temperatures. That barley's alpha amylase (which, contrary to what countless people here have said, isn't identical to the alpha amylase in everything else) is stable at high temperatures is probably, in large part, just our good luck. It's not completely unexpected, though: lots of enzymes don't get denatured until fairly far outside the organisms standard temperature range. (We can also view this from the opposite point of view: if barley's alpha amylase wasn't stable at high temperatures, we'd be brewing with some other grain and asking the question about it.)
The second element of your question is why barley alpha amylase's peak efficiency isn't more closely aligned with the temperatures the organism would experience in nature. As was mentioned earlier, this is probably mostly because it is not advantageous for the plant to run its enzymes that quickly. Germination is a slow, coordinated process, in which starch is converted to sugar at a rate that is required by the organism. It does the barley seed no good to bury itself in a sea of sugar it is not yet ready to use -- it's probably a disadvantage, in fact, because of the increase in osmotic pressure and increased infection risk.
So, in summary: Why doesn't barley's alpha amylase run fast at the temperatures it experiences during germination? Because faster isn't better, in this case. Why, then, can we use it at higher temperatures at all? Probably mostly luck, but not a huge amount of it.