P r o t e i n S y n t h e s i s

S U B M I T T E D B Y- V I J A Y K A P O O R B P T - 1S T Y E A R

INTRODUCTION

• Protein synthesis is the process in which cells build proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA, which is then used as input to

• The cistron DNA is transcribed into a variety of RNA intermediates. The last version is used as a template in synthesis of a polypeptide chain. Proteins can often be synthesized directly from genes by translating mRNA. When a protein must be available on short notice or in large quantities, a protein precursor is produced.

• Aproprotein is an inactive protein containing one or more inhibitory peptides that can be activated when the inhibitory sequence is removed by proteolysis during post translational modification. A preprotein is a form that contains a signal sequence (an N-terminal signal peptide) that specifies its insertion into or through membranes, i.e., targets them for secretion. The signal peptide is cleaved off in the endoplasmic reticulum.Preproproteins have both sequences (inhibitory and signal) still present.

• For synthesis of protein, a succession of tRNA molecules charged with appropriate amino acids have to be brought together with an mRNA molecule and matched up by base-pairing through their anti-codons with each of its successive codons. The amino acids then have to be linked together to extend the growing protein chain, and the TRNAs, relieved of their burdens, have to be released. This whole complex of processes is carried out by a giant multimolecular machine, the ribosome, formed of two main chains of RNA, called ribosomal RNA (rRNA), and more than 50 different proteins. This molecular juggernaut latches onto the end of an mRNA molecule and then trundles along it, capturing loaded tRNA molecules and stitching together the amino acids they carry to form a new protein chain.

Protein synthesis

1.DNA unwinds

2. mRNA copy is made of one of the DNA strands.

3. mRNA copy moves out of nucleus into cytoplasm.

4. tRNA molecules are activated as their complementary amino acids are attached to them.

5.mRNA copy attaches to the small subunit of the ribosomes in cytoplasm. 6 of the bases in the mRNA are exposed in the ribosome.

6.A tRNA bonds complementarily with the mRNA via its anticodon.

7.A second tRNA bonds with the next three bases of the mRNA, the amino acid joins onto the amino acid of the first tRNA via a peptide bond.

8.The ribosome moves along. The first tRNA leaves the ribosome.

9.A third tRNA brings a third amino acid

10.Eventually a stop codon is reached on the mRNA. The newly synthesised polypeptide leaves the ribosome.

OVERVIEW

T ra n sc r ip t io n 1(m a k in g a m RN A co py o f D N A )

• The part of the DNA molecule (the gene) that the cell wants the information from to make a protein unwinds to expose the bases.

• Free mRNA nucleotides in the nucleus base pair with one strand of the unwound DNA molecule.

Transcription 2

• The mRNA copy is mode with the help of RNA polymerase. This enzyme joins up the mRNA nucleotide to make a mRNA strand.

• This mRNA strand is a complementary copy of the DNA (gene).

• The mRNA molecule leaves the nucleus via a nuclear pore into the cytoplasm.

t R N A–p ic k u p t h e ir s p e c if ic a m in o a c id s f r o m t h e c y t o p la s m

m R N A a t t a c h e s t o s m a l l r i b o s o m a l s u b u n i t

Translation

Translation. mRNA used to make polypeptide chain (protein)

1.

• First the mRNA attaches itself to a ribosome(to small subunit).

• Six bases of mRNA are exposed.

• A complementary tRNA molecule with its attached amino acid base pairs via its anticodon UAC with the AUG on the mRNA in the first position P.

• Another tRNA base pairs with the other three mRNA bases in the ribosome at position A.

• The emzyme peptidal transferase forms a peptde bond between the two amino acids.

• The first tRNA(without its amino acid) leaves the ribosome.

Translation 2

• The ribosome moves along the mRNA to the next codon.

• The second tRNA molecule moves into position P.

• Another tRNA molecule pairs with the mRNA in position A bringing its amino acid.

• A growing polypeptide is formed in this way until a stop codon is reached.

End of Translation

• A stop codon on the mRNA is reached and this signals the ribosome to leave the mRNA.

• A newly synthesized protein is now complete.

T r a n s l a t i o nm R N A t o P o l y p e p t i d e

Protein Synthesis InhibitorsProtein Synthesis Inhibitors

• Many of the antibiotics utilized for the treatment of bacterial infections as well as certain toxins function through the inhibition of translation. Inhibition can be effected at all stages of translation from initiation to elongation to termination.

Several Antibiotic and Toxin inhibitors of Translation

Inhibitor Comments

Chloramphenicol

inhibits prokaryotic peptidyl transferase

Streptomycin inhibits prokaryotic peptide chain initiation, also induces mRNA misreading

Tetracycline

inhibits prokaryotic aminoacyl-tRNA binding to the ribosome small subunit

Neomycin similar in activity to streptomycinErythromycin inhibits prokaryotic translocation through the ribosome large subunit

Fusidic acidsimilar to erythromycin only by preventing EFG from dissociating from the large subunit

Puromycinresembles an aminoacyl-tRNA, interferes with peptide transfer resulting in premature termination in both prokaryotes and eukaryotes

Diphtheria (diptheria) toxin

protein from Corynebacterium diphtheriae which which causes diphtheria (diptheria); catalyzes ADP-ribosylation and inactivation of eEF-2; eEF-2

contains a modified His residue known asdiphthamide (dipthamide), it is this resudue that is the target of the toxin

ADP-ribosylated diphthamide (dipthamide) residue

Ricinfound in castor beans, catalyzes cleavage of the eukaryotic large subunit Rrna

Cycloheximide

inhibits eukaryotic peptidyltransferase