25. Protein synthesis 2. Regulation, posttranslational modifications

Last updated on November 19, 2018 at 17:16

Summary

    • Protein synthesis is expensive, so it is regulated at the earliest possible points, often at initiation.
    • eIF2 and eIF4E are important points of regulation.
    • Many antibiotics act by inhibiting protein synthesis
    • The raw product of translation, the nascent polypeptide, is not necessarily the finished protein. Posttranslational modifications are needed
    • Examples are phosphorylation, carboxylation, methylation, sulfation and hydroxylation of amino acid residues
    • Glycosylation: attachment of carbohydrate side chains
    • Acylation, prenylation and farnesylation
    • Addition of prosthetic groups, like iron, heme or zinc
    • Proteolytic processing, or cleaving parts of the polypeptide off.
    • Hydroxylation of collagen requires vitamin C

Regulation

eIF2 is inactivated by phosphorylation. This happens in case of poor energy availability, heat shock (stress) or low heme concentration. It is also inactivated in case of viral infection by the function of interferons.

eIF-4E is the least available eukaryotic initiation factor, and is therefore an efficient regulation point. It catalyses the binding of the ribosome to the 5’ cap on mRNA. It is activated by phosphorylation at Ser209. Too high activation of eIF-4E will cause tumor development. Another protein, called 4E-binding protein (4E-BP), binds to eIF-4E and inhibits it. Insulin increases eIF-4E’s activity by the following pathway: Insulin -> PKB -> mTOR –| 4E-BP –| eIF-4E. Insulin activates PKB, which activates mTOR. mTOR inactivates 4E-BP, but 4E-BP itself inactivates eIF-4E. So mTOR inactivates something that inactivates eIF-4E, which results in activation of eIF-4E.

Antibiotics

There are several antibiotics we should know. Tetracycline works on bacteria and binds to the 30S subunit, inactivating it. Erythromycin also works on bacteria, but this blocks the 50S subunit instead. Puromycin works on both prokaryotes and eukaryotes, and causes premature termination of the protein synthesis by acting as an aminoacyl-tRNA. Neomycin works only on bacteria, by inhibiting binding of aminoacyl-tRNAs to the ribosome, never letting protein synthesis start. Fusidic acid inhibits the dissociation of EF-G from subunit 50S in bacteria. Lastly, ricin is a toxin which cleaves rRNA on the 60S subunit in eukaryotes.

Posttranslational modifications

The product of translation, called the nascent polypeptide chain, is often not ready to be a biologically active protein. Often, some modifications of the polypeptide chain are needed.

As outlined above, all polypeptide chains begin with Met (fMet in bacteria). This is often cleaved away. Signalling sequences are small sequences on either the N-terminal or C-terminal end of a polypeptide. They are needed to direct the polypeptide into the correct cellular compartments. Examples of such signalling sequences are PTS1 on the C-terminal end or PTS2 on the N-terminal end.

Very often, individual amino acid residues in the polypeptide chain are modified. Many types of covalent modifications are needed, like phosphorylation, carboxylation, methylation or hydroxylation. ADP-ribosylation, adenylylation and uridylylation are also important.

A good example of this posttranslational modification by hydroxylation is in collagen synthesis. In the conversion of pre-pro-collagen to pro-collagen, all lysine and proline residues on the peptide must by hydroxylated, to create strong cross-striations in the finished product. This hydroxylation requires vitamin C, which is why a lack of this vitamin will give you scurvy, which is first manifested by bleeding gums.

Glycosylation, the attachment of carbohydrate side chains, is especially important in extracellular protein. This is performed in Golgi and ER.

Prenylation and farnesylation is common on proteins that need to be anchored in a membrane.

Acetylation and deacetylation is very important in the case of histones, but also for transcription factors and chaperones.

Some protein need prosthetic groups to function. Hemoglobin need heme, ferritin needs iron, alcohol dehydrogenase needs zinc and so on.

The activation of insulin. The yellow bonds are disulphide bonds. Now you know what they look like.

 

Some polypeptide chains are inactive, and need to have parts cleaved off them to be active. For proinsulin to become insulin, a part of it needs to be cleaved off. The cleaved off part is what’s known as C peptide, while the remaining part of proinsulin is the mature insulin. Some other proteins that need cleavage to be activated are trypsinogen, pepsinogen and chymotrypsinogen.

Lastly, the formation of strong disulphide bonds may be necessary for certain proteins to protect their 3d structure. This is especially important for extracellular proteins, as the extracellular environment is oxidizing, which would compromise the proteins 3d structure if not for these bonds.


Previous page:
24. Protein synthesis 1. Participants and mechanism

Next page:
26. Protein folding, chaperones

Leave a Reply

Only the "Comment" field must be filled in. It is not compulsory to fill out your name; you can remain anonymous. Do not fill out e-mail or website; if you do, your comment will not be published.