Question

A number of molecules have been identified that inhibit ribosomal function. Some are antibiotics others are toxins. List them and where they interfere with ribosomal function.

Answer 1

Ribosomes are smallest cell organelles found attached either on rough endoplasmic reticulum (RER) or floating freely in the cytoplasm .Ribosomes are also called as protein factories or engines of cell because these are the sites of protein synthesis.Proteins control either cellular structure or act as enzymes and control cellular functions.

There are a number of molecules that interrupt the normal functioning of ribosomes .Some antibiotics and toxins are also included in these molecules .

Antibiotics also called as antibacterial ,is a type of antimicrobial drug used in the treatment and prevention of bacterial infections . Antibiotics help us to fight several diseases but a number of antibiotics are found inhibiting ribosomal functioning e,g_

1) Tetracyclines bind to 30s subunit at the A site and prevent the attachment of tRNAs carrying amino acids .This means that the next bead on the polypeptide string can't be brought into the ribosome.

2) Chloramphenicol interacts with the 50s subunit of ribosomes and prevents the formation of peptide bonds .When Chloramphenicol is around ,the amino acid bead can't be linked together into a polypeptide string.

3) Streptomycin binds to 30s subunit and causes the ribosome to misread the genetic code which can lead a well_ordered polypeptide string into a jumbled _up mess that can't carry out its functions.

4) Erythromycins bind to 50s subunit and block the tunnel where the polypeptide string is supposed to exit.This clogs up the ribosomes and stops translation.

5)Neomycin also inhibits interaction between tRNA and mRNA etc.

There are two types of toxic proteins that kill eukaryotic cells by covalently modiglfying unique structural features of components that are essential for protein synthesis . Intoxication by these proteins results from the entry of a catalytic fragment into the cytoplasm .The catalytic component of the class of toxins_ Diptheria toxin and pseudomonas exotoxin A, ADP_ ribosylates and inactivates elongation factor 2 which is an essential participant in protein synthesis.This modification occurs at a unique post _translational histidine derivative ,diphthamide ,that is present in the ribosomal binding site of the elongation factor .The two toxins differ in their molecular organisation but appear to posses identical reaction mechanism and very similar active sites .

Other class of toxins _alpha _sarcin a member of a family of fungal toxins and sarcoma _ member of a group of closely related plant proteins collectively termed as ribosome inactivating protein. The catalytic component of this class of toxins inactivates large ribosomal subunits through two different hydrolytic alterations of 23_28s RNA .Alpha _sarcin act as a specific endonuclease whereas ricin as a specific N_glycosidase.These hydrolytic cleavages occur at a pair of adjacent nucleotides within a highly conserved sequence near the 3'end of 23_28s RNA .The covalent integrity of this region of RNA is essential to elongation factor _dependent ribosomal binding domain of these factors.