In response to my last article, The Taq behind PCR, one of our readers, Bonnie Barrilleaux, asked whether DNA could naturally survive at temperatures that would denature it. It also begged the question; how do proteins stay intact and functioning at these high (55°C and up) temperatures? It turns out, cells do a lot of simple chemical tweaks to stay alive, indeed thrive, when it's very, very hot.
These biochemical tweaks allow for life to thrive at well above 100°C, far above what scientists as recently as 2000 were estimating as the limits of life. Not only have these thermophiles helped give us PCR, they have also shown potential for improving manufacturing processes (no need to protect against heat!), and even opened up a new kingdom, Archaea.
So, starting backward with proteins (more specifically, enzymes), if you're a cell in a hot spring or hydrothermal vent, how do you adapt?
Change your amino acids: charged residues (like arginine and lysine) form more ion pairs, which create weak bonds to hold protein molecules together. These residue changes don't change enzyme function, either.
Make more hydrogen bonds: lactate dehydrogenase in the thermophile Chlorobium tepidum has more polar amino acids that form hydrogen bonds within subunits of the enzyme.
Get hydrophobic: if hot water can't get into the center of the structure, it can't denature anything. Proteins in heat-thriving Thermotoga maritima appear to have more densely packed cores than proteins in their more moderate temperature-dwelling cousin.
Reel in those surface loops: Thermophilic bacteria have been found with reductions in loop content, shortened loops, and amino acid substitutions that add more weak bonds between loops and the rest of the protein structure.
Embrace disulfide bonding: a covalent tertiary bond, these also help stabilize protein structure. Thermophile proteins have been found to have the highest levels of disulfide bonds
Yes, at room temperature, they would. In fact, many of these heat-loving cells were overlooked decades ago because they couldn't be cultured at room temperature. But at their home temperatures, snug in their black smoker vent or Yellowstone hot spring, their function is comparable to proteins more accustomed to 32°C.
These little changes have helped open the doors to a huge range of niches in which to survive. But some researchers are investigating the possibility that our 37°C, pH7, and O2 (or CO2) world may have been the odd one out. Thermophiles, whether thriving in cold or hot conditions, may have occupied the widest-available habitats during the earliest days of life on Earth.
So, who's sweating now?
–Don't miss part two, where I talk about how thermophile DNA/RNA survives the heat…
Kumar, S., et al. (2007) Temperature-dependent molecular adaptation features in proteins. In Gerday, C. and Glansdorff, N., eds., Physiology and Biochemistry of Extremophiles. Washington, D.C.: ASM Press.
Beeby, M., et al. (2005) The genomics of disulfide bonding and protein stabilization in thermophiles. PLoS Biology 3(9): 1549-1558.
Dalhus, B., et al. (2002) Structural basis for thermophilic protein stability: structures of thermophilic and mesophilic malate dehydrogenases. J. Molec. Biol. 318: 707-721.