Thursday, January 10, 2013

Ribosomes - Protein Assemblers of the Cell

Ribosomes are tiny organelles which are present in large numbers in both prokaryotic and eukaryotic cells (described here). They are important site for protein synthesis.
Structure of Prokaryotic Ribosome
The ribosomes are designated as per their rate of sedimentation as 70S for bacterial ribosomes and 80S for eukaryotic ribosomes (where S stands for rate of sedimentation).Both prokaryotic and eukaryotic ribosomes are composed of two different subunits. these subunits comprises of proteins and rRNAs. Both of the ribosomes have similar structure; however, they differ in some details like different number of proteins and types of rRNAs.
The prokaryotic 70S ribosome is made up of large subunit as 50S and small subunit as 30S.
The small subunit 30S consists of 21 proteins and 16S rRNA whereas the large subunit is consists of 34 proteins and 23S and 5S rRNAs.
Structure of Eukaryotic Ribosome

Eukaryotic 80S ribosomes in eukaryotes is made up of  60S as large subunit and 40S as small subunit. the large subunit, 60S is made up of 28S rRNA, 5.8S rRNA, 5S rRNA and approximately 45 proteins. Whereas the small subunit, 40S is comprises of 18S rRNA and approximately 30 proteins.

One of the striking feature of ribosome is that they can be formed in vitro by self assembly of their RNA and protein constituents. This was first described by Masayasu Nomura when he purified the constituents of ribosome and mixed them in appropriate conditions and the functional ribosome was re-formed.

Translation of mRNA by a Ribosome into a Peptide Chain.
As already mentioned, the ribosome are the site for protein synthesis; which is the process of translating mRNA into proteins. The mRNA comprises a series of codons (triplets of nucleotides) that will ultimately form the amino acids to make the protein.
The ribosome uses this mRNA as template and translates each codon by pairing it with appropriate amino acid which is provided by aminoacyl-tRNA (aminoacyl-tRNA contains a complementary anti-codon). The ribosome contains three RNA binding sites as A, P and E. The A site binds an aminoacyl-tRNA; the P site binds a peptidyl-tRNA and E site binds a free tRNA before it exits the ribosome as can be seen in the diagram.