The process of protein folding involves: Select one: a. formation of a number of final structures simultaneously, with the incorrect structures removed by proteolysis. b. a progressive random search through multiple distinct complete conformations that eventually results in the native protein. c. a rapid "collapse'" of the extended polypeptide chain into the fully folded native conformation. d. progressive stabilization of correct secondary structural intermediates, and unfolding of incorrect structures, until the final structure is attained. e. an increase in free energy until the proper final structure is reached

The correct answer is: d. progressive stabilisation of correct secondary structural intermediates, and unfolding of incorrect structures, until the final structure is attained.

Explanation:

Protein folding is a sequential process and not a random one. It is necessary that a protein must fold in such a way that in the end it is able to attain the particular conformation which is required for its function. The folding of an amino acid sequence is a process that is thermodynamically favourable. This is because in a folded protein the hydrophobic amino acids escapes from interacting with water molecules and hence gets stabilised. A folded protein has lower energy and is more stable than a linear polypeptide chain. The folding of an amino acid linear chain into a protein takes place in the following way: The folding of the amino acid sequence begins during the process of translation of the mRNA (messenger Ribonucleic Acid) by Ribosome. (Co-translational folding) This is because initially the linear amino acid sequence forms a secondary structure (alpha helix, beta sheet) which is formed between the nearby amino acid residues. These structures form progressively and sequentially and each structure after formation is checked for its correct conformation before the next structure (tertiary structure) is formed. If an incorrect folding has occurred, the folded polypeptide chain opens up and refolds until correct orientation is attained. Folding, opening and refolding process continues until the correct conformation of the protein is attained.