Non-ribosomal peptide synthesis: why Glutathione cannot be produced by the ribosome?

Question

Case: I am writing a summary for a class in protein structure and function, and was asked to describe some different ways that peptides are synthesized (that does not involve the ribosome). I understand that glutathione is a tri-peptide consisting of L-cystine with a "gamma" bond to L-glycine which is then bound to glutamic acid.

Problem: Why is it that the ribosome cannot produce this peptide? What is special about the peptide-gamma-bond?

My reasoning: The peptide is too short, meaning that the large ribosome complex cannot bind to mRNA of this length as it need many different binding spots in order for the large and small subunit to come together. Also, the "gamma" bond is very special, and the ribosome can only produce peptides with the "Standard" peptide bonds.

• Is this correct, and is there something else I am missing?

• Does the "gamma" bond relate to the fact the glutathione is not that easily degraded by peptidases in the cytosol?

Answer 1

It's definitely not the length. Although ribosomes can't make really really small peptides because they need something to hold onto and in any case proteins start with methionine(usually), there are workarounds. There's no reason why a tripeptide couldn't be coded for by a gene directly.

The 'traditional' method of producing very short peptides is producing a longer protein and cleaving off a chunk of it, which works great for signal peptides and a number of other systems. Very small peptides are more efficiently made by enzymes like this but in principle there's no reason to besides efficiency and complexity. (You need a special protease for each small peptide etc etc etc)

However, this gamma bond is a different beast entirely, and no ribosome could ever produce it, even in the middle of a longer protein. Amino acids 'snap' together in a very specific way. The amino group bonds to the carboxyl group of the previous amino acid, making a chain. The side groups of an amino acid aren't involved. Like Legos that snap together only one way and can be any color or have any designs on the side. The difference here is glutamate has a carboxyl group as part of its side chain. It's like a Lego with holes on two sides. The NRPS here connects the cysteine to the side chain of the glutamate instead of the 'bottom' where it would go if a ribosome-synthesized glutamate-cysteine bond.

This gamma bond also explains the increased longevity of the product. Proteases aren't designed for things that look like this. The gamma bond doesn't 'fit right' in the protease active sites and therefore activity is slower.

Answer 2

From I've read about glutathione, I think the answer to your question is :

"Because there is no glutathione gene ", and hence, no glutathione mRNA.

Concerning the gamma bond, this wikipedia article quotes :

This peptide coupling is unique in that it occurs between the amino moiety of the cysteine and the terminal carboxylic acid of the glutamate side chain (hence the name gamma-glutamyl cysteine)

citing this paper as a reference.

Also, it seems like the gamma bound is indeed the reason why is not degraded in the cytosol.

I don't know how you perceive wikipedia, but to me the english version is a good way to gather general informations about an unknown subject, before going more deeply in details with scientific litterature.