Please help with this biochemistry concept The more detail t

Please help with this biochemistry concept. The more detail the better--thank you so much.

Do ribosomes have C-terminal to N-terminal exopeptidase proof-reading activity, similar to the 3 \'-5\' exonuclease activity found in DNA polymerases? Based on the mechanism of ribosome-mediated protein synthesis, why is this true?

Ans:

DNA, RNA, and protein synthesis is always directional:

A. New DNA is always made in the 5\' to 3\' direction. Therefore the template DNA strand is being copied from 3\' TO 5\' (i.e., from its 3\' end towards its 5\' end) since the two strands must be antiparallel.

B. RNA is always made in the 5\' to 3\' direction. Again the template DNA strand must be read from 3\' to 5\'.

C. Protein is made by reading mRNA in the 5\' to 3\' direction and making new protein from its amino end (N-terminus) towards its carboxyl end (C-terminus). Several ribosomes read a single mRNA at once, forming a polysome.

That protein is synthesized N-terminus to C-terminus. In 1963 H. Dintzis pulse-labeled red blood cells with radioactive amino acids for various times and then purified full length a -globin protein and fragmented it into small peptides. By this time V. Ingraham had determined enough of the amino acid sequence of this protein to know the order of each peptide from N-terminus to C-terminus. Dintzis showed that pulse label first showed up in full length protein in the C-terminal peptide meaning it must be synthesized last. As pulses got longer label began to appear in peptides closer and closer to the N-terminus.

Yes ribosomes do have C-terminal to N-terminal exopeptidase proof-reading activity and show the 5\' to 3\' direction of each strand of nucleic acid and the N to C terminus direction of the protein. i.e., Ribosomes read mRNA in the 5\'->3\' direction, not the 3\'->5\' direction.

To prove, one must know the Genetic Code. For example, AAA codes for Lys, AAC codes for Asn and CAA codes for Gln. In a cell-free protein synthesizing system, initiate protein synthesis by adding artificial mRNA, consisting of polyA with one C (cytosine) at the 3\' end. Analyze the N and C termini of the peptide fragments.

Nucleotide Sequence

Protein Sequence

Direction of Synthesis

5\'-AAAAAA(AAA)nAAC- 3\'

Lys-Lys-(Lys)n-Asn

5\' -> 3\'

3\'-CAAAAA(AAA)nAAA- 5\'

Gln-Lys-(Lys)n-Lys

3\' -> 5\'