|MadSci Network: Biochemistry|
The answer is a very general one for two reasons. The answer is undoubtedly complex plus we have little actual data in whole organisms (as opposed to your experiment measuring a single isolated enzyme). First, consider increased temperature. At the body temperatures compatible with life (up to about 106-8 deg F), there are actually relatively few enzymes that will actually die. However, all are probably affected, some more than others. In addition, structural proteins will be changed slightly. Most important, in my opinion, at higher temperatures would be the membrane and its lipid components. Humans are optimized, for the most part, to function optimally at 98.6 deg F. If you heat our membranes to a higher temperature, they get progressively leakier. Since the ionic balance (homeostasis) across membranes is absolutely crucial for health and life, this leakage poses a major problem. Much of the leakiness is not from proteins, but from the lipid of the membrane. Basically, think of what happens if you take a cold stick of butter (lipid) out of the refrigerator and leave it out on the counter to warm to room temperature. It doesn't actually melt to a liquid, but it become very soft, cannot hold its shape well. Then think what this would translate to if the butter/lipid were only two molecules thick, as a membrane is. That softness would not help the cell function nor help it hold its shape. I doubt that at 106 deg F as in a fever many proteins become inactive. But organisms are very finely balanced machines. If there are 10,000 parts in a cell and 1000 of them change in activity all of a sudden by 5- 10%, and say 10 of those parts form an important linked sequence of reactions (a pathway), then the throughput for that pathway would be changed by (1.1)e+10 or about 250%. Think of this happening in several pathways. In addition, many proteins are unstable to temperature in this range, though not many. So, in some pathways, one of the linking enzymes does die, or decreases in activity so much, that the pathway becomes effectively blocked. The combination of this happening in several pathways at once could clearly be deleterious to the organism. Second, consider decreased temperature. This is easier. Basically, there are few proteins that die at cold temperatures, but live at higher temperatures. You can cool an organism down quite a lot before it dies, IF, repeat IF, the cooling and then subsequent warming are gradual. Even whole organisms (but only simple ones) can be frozen and revive when thawed. As an example, think of how your hands feel in the winter sometimes after being outside, even perhaps with gloves on. Your hands are cold and stiff, but they still function. The actual temperature of your fingers can and repeatedly often does get down to 40-50 deg F without any lasting harm at all. What has to stay warm is your core body temperature and your brain. The reason is your nervous system. At lower temperatures, nerves do not function as well, and your metabolism provides less energy (ATP) to run things. This further messes up the nerves because they need energy to maintain the crucial ion balances across the membrane so that nerve (electrical) impulses can be transmitted. If the cooling goes on too long, they cannot readily recover. This means that the nerves controlling the brain, and especially the heart and kidneys quit operating, thus risking heart stoppage. When that happens, the organism is highly unlikely to recover since the heart is unlikely to start spontaneoulsy when warmed, though it can. In terms of enzymes individually, yes, all of OUR enzymes will eventually die when heated too much. Some in the range of 110 deg F, some not until much higher, perhaps even 160-170 deg F. But there are organisms, some of them rather complex, that can grow at very high temperatures and/or very high pressures, such as the small animals, tube worms and a plethora of Bacteria and Archaea in deep sea vents, where the pressure is 300 atomspheres and the release of gas and even magma can heat the water to boiling. We use enzymes from such organisms in the laboratory precisely because they can survice very high temperatures. In my laboratory, I have a membrane ion transporter that I have cloned both from a mesophile, Salmonella typhimurium (mesophile = moderate temperature, meaning our temperature) and an extremophile Methanococcus jannaschii (extreme hot or cold or pressure, in this case both high temperature and high pressure). They are the same basic protein, transporting the same ion, and have considerable sequence identity. However, the protein from S. typh. starts to die at 50 deg Centigrade and is completely dead by 65 deg C, whereas the protein from M. jann. is still fully active at 70 deg C, the highest temperature we could go to.
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