Microbial cell membranes and DNA will rupture at high temperatures, while proteins undergo denaturation (a change in shape that leads to the loss of biological activity). For instance, bacteria and viruses are eliminated effectively when meat is heated to above 75°C. However, such temperatures are not sufficient to kill thermophiles. Some hyperthermophiles living near hot springs or hydrothermal vents in the sea floor can even survive and reproduce in boiling water. So why do these Archaea: thermophiles and hyperthermophiles linger in the forbidden zone of life?
Saturated Hydrocarbons and Ether Bonds in Cell Membranes
The fatty acids in phospholipids are unsaturated. Its hydrocarbon tail is bent 30° by cis carbon-carbon double bonds and is relatively short. When they form a phospholipid bilayer, the weak intermolecular attraction and gaps lead to a strong fluidity in cell membrane that can be immediately destroyed by high temperatures. Therefore, thermophiles and hyperthermophiles produce a more robust monolayer cell membrane than other organisms to withstand high temperatures. Two glycerol molecules are linked to two longer branched saturated fatty acids with about 40 carbon atoms via ether bonds. One molecule is straight, roughly as long as two phospholipids. They are tightly arranged to form a very thick monolayer cell membrane that does not split in the middle like a phospholipid bilayer. Poor fluidity and strong integrity are well-suited to endure high temperatures.
Hydrophobic Core of Proteins and Amino Acids
Archaea living in boiling water have more hydrophobic amino acids to form larger, more tightly packed hydrophobic cores in proteins, especially aromatic amino acids with larger hydrophobic parts. On the protein surface, an abundance of charged amino acids forms ionic bonds and salt bridges to maintain a stable shape. Cysteine is more prevalent in thermophiles, as it is used to build disulfide bonds that fold and stabilize proteins. Disulfide bond is slightly weaker than covalent bonds but much stronger than hydrogen bonds. These characteristics ensure that thermophilic proteins will not lose their activity even in boiling water.
Stable DNA🧬 in Thermophiles and Hyperthermophiles
G-C base pairs are more prevalent in thermophile DNA, as G and C have three hydrogen bonds, while A and T only have two. The cytoplasm is rich in metal ions and amine compounds by which the repulsion of negative phosphate groups in DNA is decreased, leading to a stable double helix. Certain special mechanisms restrict DNA from being directly exposed to aqueous environments: DNA is bound to heat-resistant histones; enzymes catalyze DNA into positive supercoils, a more twisted and tightly wound structure to prevent hydrogen bonds on the bases from breaking. Hyperthermophiles have evolved high-temperature-resistant DNA polymerases and repair enzymes that can replicate, transcribe, and repair DNA in extreme environments.