Post-Translational Protein Modifications

Once a protein has been synthesised, it often has to undergo a few modifications to carry out its function. This may help convert a protein from an inactive form (denoted by the prefix apo-) to an active form (holo-). Let's take a look at what some of those modifications are...

Proteolysis

In this case, I'm not referring to the complete breakdown of proteins. Instead, I'm referring to the cleavage of certain parts of the protein. An example of a protein that does this is insulin. Insulin starts off as preproinsulin which has a primary structure of N-(signal peptide)-(B chain)-(C peptide)-(A chain)- C. The signal peptide gets cleaved in the endoplasmic reticulum to form proinsulin. In the Golgi apparatus, the B- and A-chains form disulfide bonds with each other, and the C peptide is cleaved off to form insulin. Presence of the C peptide can be used to detect whether people are making enough insulin or not, which is useful for testing for diabetes.

Now for the million dollar question- why do this? Why waste so much extra energy synthesising the signal peptide and the C peptide if they're just going to get cleaved off? Well, it has been hypothesised that this way allows the disulfide bonds to form properly. Additionally, synthesising both chains on the same peptide means that you're definitely going to get equal amounts of A- and B-chains.

Phosphorylation

I feel like I've spoken about phosphorylation to death- after all, it is the most common post-translational modification. It's just adding phosphate groups to proteins. This process requires energy, unlike dephosphorylation, which doesn't.

An example of a protein that undergoes this modification is glycogen phosphorylase, which is involved in the breakdown of glycogen. When phosphorylated, glycogen phosphorylase b becomes glycogen phosphorylase a, which is the active form (the a/b terminology isn't very common- it's just that it was named like that ages ago and the name stuck). This phosphate group is added by an enzyme called phosphorylase kinase, which in turn can get phosphorylated by other stuff, which in turn gets phosphorylated by other stuff, all the way up to glucagon which is the hormone that kicks off the whole process.

Lipidation

Lipidation, as the name suggests, is the addition of a lipid group to a protein. I've written more about lipidation in a previous post about protein modification. As mentioned in that previous post, one of the functions of lipidation is to "anchor" a protein into the lipid-soluble cell membrane so that it isn't flying off into the cytoplasm. For example, the α-subunit of G-proteins is often myristoylated on its N-terminus for this very reason. (Just so you know, myristic acid always joins to a Gly residue at the N-terminus).

Aside from localising a protein to the cell membrane, lipidation can also have functional roles. Pyruvate dehydrogenase, which turns pyruvate into Acetyl CoA, relies on the lipoic acid attached to one of its lysine residues to be able to do its job. In this reaction, pyruvate first reacts with TPP (thiamine pyrophoshate) to produce hydroxyethyl TPP (the hydroxyethyl group is like an acetyl group, but with an -OH instead of an =O). Hydroxyethyl TPP then reacts to the oxidised lipoic group to form a reduced lipoic group joined to an acetyl group, which can then be joined to Coenzyme A (CoA) to form Acetyl-CoA. The electron carrier FAD can then oxidise the lipoic group again so that the cycle can start over. Or something. I'm not very good with nitty-gritty details.

Glycosylation

Glycosylation is the addition of sugars, as mentioned in a previous post. As mentioned before, there are two types. O-linking is done via a serine or threonine residue, and N-linking is done via an asparagine residue. O-linked oligosaccharides don't have any discernable consensus sequence, but the consensus for N-linking is Asparagine-X-Serine/Threonine, where X can be any residue.

There are three main types of N-linked saccharides, and they all share a common pentasaccharide core consisting of (Asparagine)-GlcNAc-GlcNAc-mannose triad, where GlcNAc is N-acetylglucosamine and the mannose triad is basically a V-shaped configuration of mannose (i.e. there is one mannose attached to the second GlcNAc, and then that mannose joins to two other mannoses). Each of the mannoses that make up the "V" can be joined to other stuff. If the "other stuff" consists entirely of mannose residues, then the N-linked saccharide type is "High Mannose." If the "other stuff" consists of mannose on one side and a variety of things on the other side (e.g. galactose, N-acetylglucosamine), then it's a "Hybrid Type." If the "other stuff" just has a whole lot of stuff, including sialic acids (Sia), then it's a "Complex Type."

Synthesis of N-linked saccharides occurs in and around the endoplasmic reticulum (presumably the rough ER as the diagram shows proteins being synthesised). The core is built up on the outside of the ER, then it is translocated inside the ER where it continues to grow. After synthesised, the carbohydrate chain attaches to the protein while the protein is still growing. This process can be interrupted by antibiotics such as tunicamycin.

There are a few different reasons why proteins are glycosylated. Firstly, glycosylation may assist with folding and stability. Secondly, if glycosylated proteins are on the surface, these sugar chains can play roles in recognition by other cells, such as immune system cells. For example, the well-known A and B antigens on the surface of red blood cells are sugars.