Microorganisms that we encounter in our daily lives grow best under moderate con
ID: 508717 • Letter: M
Question
Microorganisms that we encounter in our daily lives grow best under moderate conditions: temperatures that are neither too hot nor too cold, typically between 20 and 45 degree C; pH values that are neither too acidic or too basic, typically between pH 3-9; and ionic strengths that are neither too low or too high, typically around 0.1M salt. Organisms that grow under these moderate conditions are known as mesophiles. However, microorganisms have been isolated from almost every conceivable environment, including those that we would consider extreme, and would be deadly to most life. Microorganisms that grow under extreme conditions are known as extremophiles. Extremophiles have been isolated from multiple locations including extremely cold (psychrophiles), extremely hot (thermophiles), extremely acidic (acidophiles), extremely alkaline (alkaphiles), and environments with extremely high salt concentrations (halophiles). Extremophiles use the same 20 common proteiogenic amino acids employed by mesophiles for protein synthesis. Extremophilic protein structures are stabilized by the same weak non-covalent interactions that stabilize the protein structures of mesophiles. Despite these similarities proteins isolated from mesophiles loose their activity when subjected to conditions under which proteins from extremophiles are found to be stable and functional. Why do mesophilic proteins loose activity at low temperatures? Given your answer to question 1 predict the characteristics that would allow proteins from psychrophilic organisms to maintain activity at low temperatures. Why do mesophilic proteins loose activity at high temperatures? Given your answer to question 2 predict the characteristics that would allow proteins from thermophilic organisms to maintain activity at high temperatures.Explanation / Answer
Ans. All enzymes require optimum condition of temperature for maximum activity. At optimal functional 3D-conformation, hundreds of amino acids in the protein are located precisely at correct distance with respect to each other- so, have optimum interaction of H-bonding, ionic interactions, Van der waals interactions, hydrophobic interactions and other non-covalent interactions among amino acid residues in the enzyme.
#1. Mesophilic proteins lose activity at low temperature because of their denaturation. It is noteworthy that denaturation at lower temperatures in reversible- that’s why enzymes remain functional when thawed back to room temperature from -200C.
At lower temperature, the kinetic energy of each atom reduces. It favors the formation of additional H-bonds, or few groups may come closer because greater number of H-bonds in vicinity bring them closer, etc.- it distorts the overall native 3D conformation of enzymes. For catalysis, the relative orientation of catalytic groups and their orientation at the active site is crucial. The distortion in active site makes the enzyme catalytically inactive. Inactivation of enzymes further arrests the metabolic pathways leading to arrest of cellular growth and metabolism.
#2. Hydrophobic residues generally don’t mediate the formation of H-bonds. A larger hydrophobic core is relatively more stable at lower temperature because it does not freeze (psychrophilic distortion) on low temperature- when compared to a smaller hydrophilic core in mesophilic proteins. So, a protein with relatively higher proportion of hydrophobic residues may prevent the psychrophilic protein from distortion at low temperature. Thus, these proteins are catalytically active at lower temperatures.
#3. Higher temperature denatures the protein. The denaturation at higher temperature is irreversible.
An elevated temperature denatures the enzyme (proteins) by cleaving peptide bonds and intra- and inter- chain disulphide (s-s) bonds (cleavage of covalent bond makes denaturation irreversible), breaking H-bonds, hydrophobic interactions, ionic interactions or non-covalent interactions. These distortions finally inactivate (denature) the enzymes. Inactivation of enzymes further arrests the metabolic pathways leading to arrest of cellular growth and metabolism.
#4. An increase in the number of polar and charge residues may stabilize the protein at higher temperature by increasing the number of H-bonds and salt-bridges. Moreover, greater number of disulfide bonds (strong, covalent bonds) may also be useful in keeping the regions of the molecules linked (and shaped) together without showing considerable distortion at higher temperature.