Covalent Modification of Proteins

In addition to folding, there are other modifications that happen to proteins after translation is completed. In prokaryotic organisms, for example, the N-formyl group is removed from most proteins by an enzyme called a deformylase. This can happen as soon as the N-terminus emerges from the ribosome.

Bacterial proteins destined for secretion (translocation across the cell membrane) contain highly hydrophobic stretches of amino acids (called signal sequences or leader sequences) at their amino terminal region. Examples are shown in Table 27.5. After the protein has passed through the membrane, the leader sequence is cleaved off at the point shown by the arrow in the table.

The currently accepted model for translocation is shown in Figure 27.30. Steps are as follows:

> The protein to be translocated (called a pro-protein) is complexed in the cytoplasm with a chaperone (for example, the Sec B protein). The complex keeps the protein from folding prematurely, which would prevent it from passing through the secretory pore, which consists of two transmembrane proteins, Sec E and Sec Y. Sec A is an ATPase that helps drive the translocation. After the pro-protein is translocated, the leader peptide is cleaved by a membrane-bound protease and the protein can fold into its active three-dimensional form.

See also: Polypeptide Chain Folding, Covalent Modifications to Regulate Enzyme Activity (from Chapter 11)