Protein synth

[chiddler]

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As a protein is synthesized from a ribosome, is the free end N or C?

I know amino acids are synthesized N-->C so to me it seems that the free end should be C. Book says it's N and i'm not understanding.

To illustrate:

 

[EnginrTheFuture]

Nice drawing... It is indeed the N terminus. When another amino acid is added to the protein in the ribosome, the amino group of the new amino acid is forced into a dehydration reaction with the carboxyl end of the protein chain. Not sure how else to explain why it is not the other way around without going into the mechanism but in order for the new amino acid to be added to the existing chain from inside the ribosome, it must be the carboxyl end waiting to be attacked, not the amino group.

This video shows translation visually, especially elongation. Note that the incoming N-terminus "attacks", in a sense, the C terminus causing it to grow from N (head) to C (tail)

http://www.youtube.com/watch?v=5bLEDd-PSTQ

 

[syoung]

As a protein is synthesized from a ribosome, is the free end N or C?

I know amino acids are synthesized N-->C so to me it seems that the free end should be C. Book says it's N and i'm not understanding.

To illustrate:

N-R1-C is the first one yes? Add a new AA and it is added on the C end
N-R1-C + N-R2-C = N-R1-R2-C.

To explain it the other way, N of the 2nd AA can nucleophilically attack the carbonyl carbon of the first AA.
i wish I had tumblr or imgur to upload photos...

 

[Buttafuoco]

Think about what's going on in the ribosome. You're adding to the Carbonyl of the last AA put on the protein. If you have Lehninger's biochem book, pg 1092 has some pretty good diagrams of what goes on.

This is an aminoacyl-tRNA. See how the AA is covalently attached by an ester linkage to the tRNA and has its amino group free?

So the polypeptide is the the P site bound to the last tRNA, then a new aminoacyl-tRNA comes into the A site and the lone pair on its amine N attacks the carbonyl that is binding the polypeptide to its tRNA to make an amide linkage aka peptide bond and the polypeptide is now covalently linked to the tRNA in the A site which is still held by its ester linkage.

 

[chiddler]

Oh! N-->C is not a clear enough description.

N of chain to C of new amino acid. Better description. I had it other way around.

Thanks.