Post translational modifications of proteins

What does this title actually entail?

• Post → after

• Translational → The ribosome reading the mRNA and synthesising the polypeptide

• Modifications → Chemical changes done on the protein

So it will be discussing the chemical changes made to proteins after they have been synthesised by ribosomes

What can modifications influence?

Protein structure and folding, protein activity and function, protein stability and degradation, protein location within the cell and interactions with other molecules

What is the first amino acid added during translation in eukaryotes and bacteria? and why?

In eukaryotes its methionine and in bacteria its N - formylmethionine. This is because translation begins with the start codon, usually AUG which codes for methionine

What is a Nascent protein?

An immature, growing chain of amino acids that is currently being synthesised by a ribosome before it folds into its functional three dimensional shape

What is N - terminal acetylation?

The addition of an acetyl group (CH3CO-) to the free amino group at the beginning of the polypeptide

NatA

This recognises proteins where an initial methionine has been removed, exposing a small amino acid such as alanine, serine or glycine

NatB

Recognises particular N - terminal sequences where methionine is retained, especially Met - Asp, Met - Glu, and Met - Asn

NatC

Recognises proteins with retained N - terminal methionine followed by certain hydrophobic amino acids

NatD

Has specialised specificity for the N - terminal regions of histones H2A and H4

NatE

Additional substrate specificity and can acetylate certain N - terminal sequences, including some retained with methionine

NatF

Is associated with membranes, for example the golgi apparatus and it acetylates certain membrane - associated proteins

NatG

Is plant - specific and is associated with chloroplasts

Why is N - terminal acetylation considered as a co - translational modification?

Because the NAT enzymes work co - translationally, meaning they modify the polypeptide while it is still being synthesised by the ribosome

Where are the NAT enzymes located in yeast, mammalian and plant cells?

Yeast → NAT is associarted with ribsomes and they can acetylate proteins as they are being synthesised

Mammalian cells → NAT are found in different cellular compartments including ribosomes and the golgi

Plant cells → additional NAT, Nat G associated with chloroplasts

A newly synthesised eukaryotic polypeptide beings with Met - Ala - Gly - Seri - Leu and the inital methionine is removed enzymatically by methionine aminopeptidase. Which amino acid is now exposed at the N - terminus and why does the removal of methionine expose a new terminus?

Alanine is now exposed so becomes the new N - terminal residue so the protein can now become a substrate for NatA because this is the enzyme that commonly recognises proteins where the initial methionine has been removed, exposing amino acids such as alanine, serine and glycine

Why does the N - terminal acetylation neutralise the positive charge of the amino group?

The acetyl group converts the free amino group into an amide as before acetylation is - NH3+ but after acetylation its - NH - CO - CH3.

This creates an amide bond, the electronic anvironment has changed and is no longer positively charged under physiological conditions