PROTEINSYNTHESIS Proteins are macromolecules of biological importance Chemically

PROTEIN-SYNTHESIS • Proteins are macromolecules of biological importance. • Chemically proteins are polymers od amino acids. • They are linked together in linear fashion forming a long polypeptide chain. • Only 20 amino acids are biologically important and participate in the formation of polypeptide chains of proteins. • All proteins required by the cell are synthesized by the cell itself.

• The process includes arrangement of a definite number of amino acids in a definite sequential order. • The sequence of amino acids in the polypeptide chains determined by the sequence of nucleotide in the chain of DNA. • A sequence of three nitrogenous bases in DNA codes for a specific amino acid of polypeptide chain, (Triplet code).

Components of Protein Synthesis Machinery • Amino acids • DNA • RNAs 1. r. RNA 2. m. RNA 3. t. RNA • Ribosomes • Enzymes

Mechanism of Protein Synthesis: Steps : 1. Transcription 2. Activation of Amino acids 3. Attachment of Amino acids with t. RNA 4. Translation (a) Initiation of polypeptide chain (b) Elongation of polypeptide chain 5. Termination of polypeptide chain 6. Modifications of Released polypeptide Chain

1. Transcription of m. RNA from DNA: • In presence of DNA dependent RNA polymerase enzyme, the genetic message encoded in DNA is transcribed into m. RNA. • The two strands of specific DNA molecule uncoil and one of these two strands acts as a template, from which exact sequence of nucleotides is transmitted to m. RNA molecule.

2. Activation of Amino acids: • Amino acids in the cytoplasm occur in dormant stage and cannot take part in protein synthesis. • The activation of amino acids facilitated through ATP, forming highly reactive amino acid phosphate-adenyl complexes k/as aminoacyl adenylates. • This activation process is governed by specific enzymes. (Amnio Acyl RNA Synthatase). • The enzyme finds its own amino acid & also its own t. RNA.

3. Attachment of Activated Amino acids with t. RNA: • The enzyme bound activated amino acids (amino adenylates) become attached with their t. RNA molecules. • The attachment is under the control of enzymes which are k/as aminoacyl transfer RNA synthetases. • The particular varieties of amino acids join specific t. RNA molecules. • For 20 amino acids atleast 20 different t. RNA molecules & 20 different enzymes are required.

4. Translation: • Translation means the process in which sequence of nucleotides in m. RNA is translated into the sequence of amino acids of polypeptide chain. (a) Initiation of polypeptide chain: • This process is brought about by amino acid methionine, which is coded by AUG. • Every message begins with the codon AUG i. e. AUG is called initiation codon. • In E. coli two different t. RNA receive methionine. • 1. t. RNA mmet (non formylated) • 2. t. RNAfmet (formylated)

b. Elongation of polypeptide chain: • After the formation of initiation complex with 70 S ribosome the chain elongation starts… • (i) Binding of AA-t. RNA at site-A of larger subunit of ribosome: • P-site (peptidyl or donar site) • t. RNA carrying peptide chain is attached to this site. • A- site (aminoacy or acceptor site) • Aminoacyl t. RNA complex is attached to this site.

• (ii) Formation of peptide bond: • A peptide bond is formed between COOH group of peptidyl t. RNA at site –P and the NH 2 group of aminoacyl t. RNA of site-A. • After the formation of peptide bond, the t. RNA from P-site is released and the polypeptide chain is transferred to t. RNA present on A-site. • (iii) Movement of peptidyl t. RNA from A-Site to Psite: • As soon as the t. RNA from P-site is released the peptidyl-t. RNA from A-site shifts back to site –P. • This process requires energy & transfer factor enzyme translocase.

• During this process ribosome shifts along the m. RNA in 5’-3’ direction, so next codon on m. RNA is available at A-site. This requires energy & G factor (elongation factor). • As one ribosome move along the length of m. RNA, the initiation point on m. RNA become free & can form initiation complex with 30 s subunit of another ribosome. • Number of ribosomes get attached to a single m. RNA molecule. • This complex is k/as polyribosome –complex. • During the protein synthesis process number of ribosomes could seen attached to a single m. RNA.

• 5. Termination of Polypeptide chain: • Termination of polypeptide occurs when the ribosome carrying peptidyl t. RNA reaches the termination codon (UAA, UAG or UGA) present on m. RNA. • Releasing factor R 1 & R 2 are needed for chain termination. • The termination process is catalyzed by the enzyme peptidyl transferase. • The free ribosome now dissociated into 2 subunits with the help of F 3 (dissociation factor).