Most proteins are stable at near neutral pH and near body te

Most proteins are stable at near neutral pH and near body temperature. Some proteins, however, are stable and functional at extreme conditions. The gastric protein, pepsin, optimally functions at pH 2, and has a lot of acid amino acid residues. Proteins isolated from thermophilic bacteria living in thermal pools (> 60 C) contain many cysteine residues. Explain how these residues help to maintain the structures and functions of the proteins at the extreme conditions.

Solution

A protein is made up of a set of amino acid residues, the 20 standard amino acids. The protein structure not just has its initial primary structure, but also more complex, secondary and tertiary and in some cases also quaternary structure, which stabilise and strengthen a protein and regulate it\'s survival in various conditions ranging from the pH values to the various temperatures endowed by the thermophilic bacteria.

In addition to the various residues in the protein, such as cys there are hydrogen bonds in the secondary structure and various other bonds such as hydrophobic interactions, electrostatic interactions and in some cases also covalent bonding which strengthen the protein. For eg: in case of cysteine, cysteine has a thrill group or sulfhydryl group which often forms a disulphide bond. Cysteine is one of the most unique amino acids because of the presence of this disulfide bond. Two cysteine residues joined together are also termed as cystine. The disulphide bond as compared to the other bonds is more difficult to break. Depending upon the number of disulphide bonds and the extensive coiling and complex structure of the protein, a proteins sustainability is determined.

On the other hand depending upon the majority of the type of residues percent in a protein, it\'s pH or pI is determined. A protein having more number of Amino acids such as aspartate and glutamate are which are acidic in nature, render an overall acidity to the protein thus making it more stable at an acidic pH such as pH 2 . On the other hand basic amino acids such as lysine , arginine and histidine which contain an R group having basic characteristic make them protein more stable at higher pH I.e. Above 7.