Question

Endorphin is a fully functional protein however its amino acid sequence is much shorter than the coding region of the DNA and shorter than the mature mRNA sequence. Explain why. Give three other examples of post-translational modifications that might affect the activity of endorphin.

Answer 1

Endorphins are peptide hormones. Hence they are secreted outside the cell and act on distant targets from where they are synthesized.

The coding region of the DNA is comprised of the start and the stop codons. It also has exons and introns. Exons are the ones that code for the protein while the introns are spliced out during RNA maturation. This matured RNA or mRNA has only exons in it. However, it still has the start and stop codons.

The protein is translated by the ribosomal machinery which reads the codons present on the mRNA.

Now, since the protein is to be secreted out, it goes through the endomembrane system as it passes through the Endoplasmic reticulum, Golgi and vesicles before it exits the cell.

To ensure this facilitated transport, the protein has a localization signal attached to it. This localization signal is recognized by the proteins in each of these compartments. However, before the protein becomes functional, this localization signal is cleaved. As a result, the amino acid length of the protein decreases.

There are several other modifications that can happen to the protein after it has been translated. These modifications provide the functionality to the protein. Some modifications include Glycosylation (addition of a carbohydrate chain), Ubiquitylation (addition of Ubiquitin chain), Phosphorylation (addition of phosphates), SUMOylation, arginylation, acetylation,