Final Touches: Post Translational Modification

If a protein is synthesized at the ribosome as a linear chain, then it is not yet functional. If it must undergo chemical changes or folding to become active, then those changes are categorized as post translational modification.

If a polypeptide chain is exposed to the crowded environment of the cytoplasm, then it might fold incorrectly or clump. If chaperone proteins bind to these chains to shield them and promote correct folding, then they are essential for protein stability.

If a protein requires strong covalent bonds to hold its 3D shape together, then it uses sulfur-sulfur links. If these links occur between cysteine amino acids, then they are called disulfide bridges.

If insulin is produced as a long, inactive "pro-insulin" chain, then it must be trimmed to work. If enzymes cut out a segment (the C-peptide) to create the active hormone, then the process is defined as proteolysis.

If a protein needs to fold without interference from other molecules, then it enters a hollow "isolation chamber." If this chamber is the chaperonin complex, then it is facilitating the folding phase of modification.

If a protein needs to stay attached to the oily plasma membrane, then it needs a chemical "anchor." If a fatty acid chain is covalently bonded to the protein to provide this anchor, then the process is called lipidation.

If the amino end of a protein is the N-terminus and the carboxyl end is the C-terminus, then any changes at the start of the chain are occurring at the N-terminus.

If a protein like collagen needs to be extra strong, then its proline and lysine residues must be modified. If an -OH group is added to these residues to allow for better cross-linking, then the modification is hydroxylation.

If a functional protein like hemoglobin consists of four separate subunits, then those subunits must be assembled after they are individual chains. If this assembly happens after translation, then it is a higher-level modification step.

If the cell can modify a single protein in five different ways to give it five different jobs, then the organism is more versatile. If this variety is achieved without adding more genes, then modification is the source of that functional complexity.

If a phosphate group is added to a serine, threonine, or tyrosine residue to change a protein's shape and function, then this chemical switch is identified as phosphorylation.

If carbohydrates are attached to a protein's surface, then they can act as labels or protective shields. If these labels are used for signaling or stability, then they fulfill these specific functions; however, they do not store genetic code.

If a protein is damaged or no longer needed, then the cell must identify it for disposal. If the cell attaches a ubiquitin chain to the protein, then the proteasome recognizes that tag and breaks the protein down.

If a reaction requires the transfer of a phosphate group from ATP to a protein substrate, then a specific catalyst is needed. If kinases are the enzymes that perform this transfer, then they are the drivers of phosphorylation.

If a protein is made on the rough ER, then it enters the ER lumen for initial folding. If it then travels to the Golgi apparatus for further glycosylation and sorting, then those two organelles are the primary modification centers.

If a cell needs to turn a signal on and then back off, then the modification must be reversible. If phosphatases can remove the phosphate groups that kinases added, then the process is not permanent.

If the environment of the cell is hot or chemically stressful, then the protein may tangle. If the protein requires chaperones or specific post translational modification steps to reach its 3D shape, then the sequence alone is not always sufficient.

If a modification like phosphorylation requires a hydroxyl (-OH) group to attach to, then amino acids with that group are the targets. Since serine, threonine, and tyrosine all have hydroxyl side chains, they are the most common targets.

If proteins clump together into insoluble, beta-sheet-rich fibers, then they interfere with brain function. If these fibers are identified as amyloids, then they are the result of failed post translational folding.

If "co-" means with and "post-" means after, then the timing relates to the state of the ribosome. If folding or capping starts while the chain is growing, it is co-translational; otherwise, it is post-translational.