9.6Formation of Higher-order Structure and Post-translational Modification
The primary structure of a protein is determined by the mRNA code, and the distinctive higher-order structure of the protein needs to be formed by appropriate folding of the protein based on the primary structure. Many proteins can spontaneously fold into their higher-order structures to some extent. However, proteins called chaperones help formation of their higher-order structure (see Coluimn at the bottom, and see Chapter 12).
Some proteins function in the cytoplasm, while others are incorporated into membranes or translocated to function in organelles, including the nucleus and mitochondria. Signal sequences consisting of specific amino acid sequences are part of the primary structure of proteins, which determine a particular destination of a protein. This phenomenon will be explained in Chapter 12.
Synthesized proteins undergo a number of modifications, including the addition of sugar chains and lipid molecules, methylation, acetylation, and phosphorylation. These modifications play a major role in changing protein functions. Phosphorylation and dephosphorylation in particular are critical in the regulation of various protein functions including enzymatic activity in various cases (Chapter 14).
While the mRNA code determines the primary structures of proteins synthesized, special proteins were found to aid in the accurate folding of these proteins required for the formation of higher-order structures. These special proteins are termed chaperones. The term “chaperone” refers to elder women who help introduce young ones to the fashionable world. Heat shock proteins, which are synthesized on exposing cells to barely tolerable levels of heat, are the first chaperones discovered. These proteins unwind the higher-order structures of proteins partially denatured by heat and restore these structures. In addition, proteins that can restore the structure of proteins partially denatured by various stresses are broadly termed stress proteins. Eukaryotes possess chaperone-like proteins that unwind the three-dimensional conformation of proteins synthesized in the cytoplasm to form a thin string-like conformation and facilitate the passage of the protein through the membrane into the interior of organelles such as mitochondria. Following passage through the membrane, these chaperone-like proteins assist in restoring the protein to its proper higher-order structure. All proteins outlined here have the common function of aiding the process of temporarily unwinding and then helping to restor the higher-order structure of a protein.