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Biochemistry

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Purines, Pyrimidines and Nucleic Acid Metabolism

Question
41 out of 59
 

Ribosome has following enzyme activity



A Peptidyl transferase

B Amine acyl t RNA synthetase

C Peptidase

D Carboxylase

Ans. A Peptidyl transferase

The α - amino group of the new aminoacyl- tRNA in the A site carriers out a nucleophilic attack on the esterifies carboxyl group of the peptidyl-tRNA occupying the P site (Peptidyl or polypeptide site). At initiation, this site is occupied by aminoacyl-tRNA met. This reaction is catalyzed by a peptidyltransferase, a component of the 28S RNA of the60S ribosomal subunit. This is another example of ribozyme activity and indicates an important – and previously unsuspected – direct role for RNA in protein synthesis.

Fig. Bacterial protein synthesis and the site of action of antibiotics

a. The messenger RNA (mRNA) attaches to the 30S ribosome. The initiation complex of mRNA starts protein synthesis and polysome formation.

b. The nacent peptide chain is attached to the peptidyl (P) site of the 50S ribosome. The next aminoacid (a) is transported to the acceptor (A) site of the ribosome by its specific tRNA which is complementary to the base sequence of the next mRNA codon (C).

c. The nacent peptide chain is transferred to the newly attached aminoacid by peptide bond formation. The elongated peptide chain is shifted back from the 'A' to the 'P' site and the ribosome moves along the mRNA to expose the next codon for aminoacid attachment. Finally the process is terminated by the termination complex and the protein is released.

d. Aminoglycosides bind to several sites at 30S and 50S subunits as well as to their interface-freeze initiation, interfere with polysome formation and cause misreading of mRNA code.

e. Tetracyclines bind to 30S ribosome and inhibit aminoacyl tRNA attachment to the 'A' site.

f. Chloramphenicol binds to 50S subunit-interferes with peptide bond formation and transfer of peptide chain from 'P' site.

g. Erythromycin and clindamycin also bind to 50S ribosome and hinder translocation of the elongated peptide chain back from 'A' site to 'P' site and the ribosome does not move along the mRNA to expose the next codon. Peptide synthesis may be prematurely terminated.

Purines, Pyrimidines and Nucleic Acid Metabolism Flashcard List

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