Question

A further level of protein structure is provided by specific modifications to the folded molecule. Which of the following modifications is not used by proteins? A.The addition of lipid molecules B.The addition of complex carbohydrate motifs C.The use of modified amino acids D.The addition of simple carbohydrate molecules E.The addition of nucleic acids

Answer 1

OPTION E. The modifications carried out on a protein synthesized by translation after complete folding of the protein are known as post-translational modifications. These modifications are carried out by enzymes that attach or detach groups and/or form or break bonds in the protein or transfer specific groups to the protein. This process is essential in making the protein fully functional or mature to serve its purpose in the organism. Not all proteins however, require such modifications.

The most fundamental level of modification involves addition of a new functional group to an amino acid in the existing protein thereby making a new amino acid side-chain. A common addition of such a group is the addition of phosphate groups to amino acids with the hydroxyl group, a process known as phosphorylation. Another such a modification is acetylation.

The next level of complexity in post-translational modifications is the use of modified amino acids. Three distinct unnatural amino acids are prevalent here - citrulline from arginine in the protein; glutamic acid and aspartic acid from glutamine and asparagine respectively and the dehydration of serine or threonine before or after phosphorylation to make an alkene side-chain.

Another set of post-translation modifications include processes like glycosylation which involves the attachment of simple carbohydrates such as glucose onto a protein's amino acid chain. This process is also extended to attach even complex polysaccharides like glycans to proteins.

Next, proteins can also be lipidated, which as the name suggests, is a process that involves attaching fatty acids and other lipid molecules to proteins.

Finally, proteins can be modified post-translationally even by the formation of disulfide bridges or by breaking any existing disulfide linkages. Moreover, it should be noted that all the post-translational modifications are covalent attachments and so are permanent changes.

Thus, it can be deduced that proteins do not undergo attachments to nucleic acids after folding. A reason for this is to ensure the integrity of transcriptional and translational machinery during their functioning with fail-safes that will not be triggered by stray proteins with nucleic acids attached. In fact, even amino acyl tRNA synthetases only procure their respective amino acids temporarily and not through post-translational modifications.