Protein Synthesis Iswar Hazarika, Date:18/09/20121st yr. M.Pharm, Pharmacology

• Proteins are composed of amino acids – there are 20 different amino acids

• Different proteins are made by combining these 20 amino acids in different combinations

• Proteins are manufactured (made) by the ribosomes

•Function of proteins:1.Help fight disease

2.Build new body tissue

3.Enzymes used for digestion and other chemical reactions are proteins (Enzymes speed up the rate of a reaction)

4. Component of all cell membranes

Making a Protein—Transcription•First Step: Copying of genetic information from DNA to RNA called Transcription

Why? DNA has the genetic code for the protein that needs to be made, but proteins are made by the ribosomes—ribosomes are outside the nucleus in the cytoplasm.

DNA is too large to leave the nucleus (double stranded), but RNA can leave the nucleus (single stranded).

• Part of DNA temporarily unzips and is used as a template to assemble complementary nucleotides into messenger RNA (mRNA).

• mRNA then goes through the pores of the nucleus with the DNA code and attaches to the ribosome.

Making a Protein—Translation•Second Step: Decoding of mRNA into a protein is called Translation.•Transfer RNA (tRNA) carries amino acids from the cytoplasm to the ribosome.

These amino acids come from the food we eat. Proteins we eat are broken down into individual amino acids and then simply rearranged into new proteins according to the needs and directions of our DNA.

•A series of three adjacent bases in an mRNA molecule codes for a specific amino acid—called a codon.

•A triplet of nucleotides in tRNA that is complementary to the codon in mRNA—called an anticodon.

•Each tRNA codes for a different amino acid.

Amino acid

Anticodon

• mRNA carrying the DNA instructions and tRNA carrying amino acids meet in the ribosomes.

• Amino acids are joined together to make a protein.

Polypeptide = Protein

5’-ATGCCTAGGTACCTATGA-3’3’-TACGGATCCATGGATACT-5’

5’-AUGCCUAGGUACCUAUGA-3’

5’-AUG CCU AGG UAC CUA UGA-3’

N-MET-PRO-ARG-TYR-LEU-C

DNA

Transcription

decoded as

Translation

mRNA

Protein

Generalized tRNA

= UH2

Modified BasesFound in tRNAs

Alanine tRNA

tRNAs are activated by amino-acyl tRNA synthetases

Amino-acyl tRNA synthetase

Amino-acyl tRNA synthetases:

One synthetase for each amino acida single synthetase may recognize multiple tRNAsfor the same amino acid

Two classes of synthetase. Different 3-dimensional structuresDiffer in which side of the tRNA they recognize and how they bind ATP

Class I - monomeric, acylates the 2’OH on the terminal riboseArg, Cys , Gln, Glu, Ile, Leu, Met, Trp Tyr, Val

Class II - dimeric, acylate the 3’OH on the terminal riboseAla, Asn, Asp, Gly, His, Lys, Phe, Ser, Pro, Thr

Two levels of control to ensure that the proper amino acidis incorporated into protein: 1) Charging of the proper tRNA

2) Matching the cognate tRNA to the messenger RNA

and mitochondria

The association of the large and small subunits creates the structural features on the ribosome that are essential for protein synthesis

Three tRNA bindingsites:A site = amino-acyltRNA binding site

P site = peptidyl-tRNAbinding site

E site = exit site

In addition to the APE sites there is an mRNA binding groovethat holds onto the message being translated

Incorporation of the correct amino acyl-tRNA is determinedby base-pairing interactions between the anticodon of the tRNA and the messenger RNA

STEPS OF TRANSLATION:

1. Initiation

2. Elongation

3. Termination

Initiation of Translation

Initiation is controlled differently in prokaryotic and eukaryotic ribosomes

In prokaryotes a single transcript can give rise to multiple proteins

In prokaryotes, specific sequences in the mRNA around the AUG codon, called Shine-Delgarno sequences, are recognized by an intiation complex consisting of a Met amino-acyl tRNA, Initiation Factors (IFs) and the small ribosomal subunit

GTP hydrolysis by IF2 coincident with release of the IFs and binding of the largeribosomal subunit leads to formation of a completeribosome,on the mRNAand ready to translate.

Eukaryotic mRNAs have a distinct structure at the 5’ end

In contrast, Eukaryotesuse a scanning mechanismto intiate translation.

Recognition of the AUGtriggers GTP hydrolysisby eIF-2

GTP hydrolysis byeIF2 is a signal forbinding of the largesubunit and beginningof translation

Proper reading of theanticodon is the secondimportant quality controlstep ensuring accurateprotein synthesis

=EF-1

Elongation factors Introduce a two-step“Kinetic proofreading”

STEP 2: Elongation

A second elongation factorEF-G or EF-2, drives the translocation of the ribosome along the mRNA

Together GTP hydrolysisby EF-1 and EF-2 help driveprotein synthesis forward

STEP 3: Termination of translation is triggered by

stop codons

Release factor entersthe A site and triggersHydrolysis of the peptidyl-tRNAbond leading to release of the protein.

Release of the protein causesthe disassociation of the ribosome into its constituentsubunits.

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