Protein Biosynthesis (Translation)

 

Protein Biosynthesis (Translation)

2. Introduction

• Protein biosynthesis is the process by which cells manufacture proteins according to the genetic information encoded in DNA.
• It is the second step of gene expression.
• The information flow follows the Central Dogma:

DNA → RNA → Protein

The actual synthesis of proteins occurs on ribosomes. Every protein is synthesized by joining amino acids together through peptide bonds.

3. Importance of Protein Biosynthesis

Why is protein synthesis essential?

• Growth
• Tissue repair
• Enzyme production
• Hormone synthesis
• Antibody production
• Muscle contraction
• Transport of molecules
• Cell signaling
• Regulation of metabolism

4. Components Required for Protein Biosynthesis

A. mRNA (Messenger RNA)

Functions

• Carries genetic information from DNA.
• Acts as the template.
• Contains codons.

DNA
↓

mRNA
↓

Protein

B. tRNA (Transfer RNA)

Characteristics

• Smallest RNA
• Cloverleaf structure
• Adapter molecule

Important Parts

Acceptor arm

• Carries amino acid

Anticodon loop

• Recognizes codons

TΨC loop

• Ribosome binding

DHU loop

• Aminoacyl synthetase recognition

Variable arm

• Varies in length

tRNA acts as an adaptor molecule.

C. rRNA

Largest proportion of cellular RNA.

Functions

• Structural component of ribosome.
• Catalytic activity.
• Formation of peptide bond.
• Peptide bond formation is catalyzed by rRNA (Ribozyme).

D. Ribosomes

Structure

Made of rRNA + proteins.

Prokaryotes

70S

50S + 30S

Eukaryotes

80S

60S + 40S

Ribosomal Sites

A Site

Aminoacyl site

Incoming tRNA

P Site

Peptidyl site

Growing chain

E Site

Exit site

Empty tRNA leaves

Mnemonic- APE

A = Arrival

P = Peptide

E = Exit

5. Genetic Code Relevant to Translation

Properties

·       Triplet code

·       Degenerate

·       Unambiguous

·       Comma-less

·       Universal

·       Non-overlapping

·       Continuous

Start Codon

AUG

Codes for Methionine

Stop Codons

UAA

UAG

UGA

No amino acid

Mnemonic

U Are Away

U Are Gone

U Go Away

6. Amino Acid Activation (Charging of tRNA)

First step before translation.

Amino acid

•  

ATP

•  

tRNA

↓

Aminoacyl-tRNA

Enzyme

Aminoacyl tRNA synthetase

ATP is converted into AMP.

This is the energy-investing step.

7. Mechanism of Translation

Protein synthesis occurs in four major stages.

Stage 1

Activation of amino acids

↓

Charging of tRNA

Stage 2

Initiation

In Prokaryotes

Small ribosomal subunit binds mRNA. Initiator tRNA binds AUG. Large subunit joins. Initiation complex forms.

Important Initiator- f-Methionine (formyl methionine)

In Eukaryotes

Initiator amino acid- Methionine (not formylated)

Initiation Factors

IF-1

IF-2

IF-3

(GTP dependent)

Points to remember

First amino acid in prokaryotes

↓

f-Methionine

First amino acid in eukaryotes

↓

Methionine

Stage 3

Elongation

Longest stage. Repeated cycle.

Step 1

Charged tRNA enters A site.

↓

Step 2

Codon–anticodon pairing.

↓

Step 3

Peptide bond forms.

↓

Step 4

Ribosome moves one codon.

↓

Step 5

Empty tRNA exits via E site.

↓

Repeat.

Energy Required

GTP

Catalyst of peptide bond

Peptidyl transferase

(rRNA)

8. Translocation

i. Ribosome moves

5'

↓

3'

on mRNA.

ii. Protein grows

N-terminal

↓

C-terminal

9. Termination

Occurs when stop codon appears.

Stop Codons

UAA

UAG

UGA

Release factors bind.

Protein released.

Ribosome dissociates.

Translation ends.

10. Post-Translational Modifications

Protein may undergo

• Folding
• Cleavage
• Phosphorylation
• Glycosylation
• Acetylation
• Methylation
• Formation of disulfide bonds

11. Cellular Location

Prokaryotes

Cytoplasm

Eukaryotes

Free ribosomes

↓

Cytosolic proteins

Rough ER

↓

Secretory proteins

Golgi

↓

Modification

12. Differences Between Prokaryotic and Eukaryotic Translation

Feature	Prokaryotes	Eukaryotes
Ribosome	70S	80S
Initiator amino acid	f-Methionine	Methionine
Transcription & Translation	Simultaneous	Separate
mRNA	Polycistronic	Mostly monocistronic
Site	Cytoplasm	Cytoplasm (after nuclear export)
Initiation factors	Few	Many
mRNA processing	Absent	Present

13. Energy Requirement

ATP

Used for amino acid activation.

GTP

Used during

• Initiation
• Elongation
• Translocation
• Termination

14. Inhibitors of Protein Synthesis

Prokaryotes

Streptomycin

30S

Blocks initiation

Tetracycline

30S

Blocks aminoacyl tRNA binding

Chloramphenicol

50S

Blocks peptidyl transferase

Erythromycin

50S

Blocks translocation

Eukaryotes

Cycloheximide

80S

Diphtheria toxin

Blocks elongation factor

Ricin

Damages 60S ribosome

15. Protein Targeting

Proteins synthesized on

Free ribosomes

↓

Remain inside cell

Proteins synthesized on

Rough ER

↓

Secreted

↓

Membrane proteins

↓

Lysosomal proteins

16. High-Yield Facts

·       Translation occurs on ribosomes.

·       rRNA acts as a ribozyme.

·       AUG is both start codon and codes for methionine.

·       Stop codons do not code for amino acids.

·       Ribosome moves 5' → 3'.

·       Protein grows N → C.

·       A site receives charged tRNA.

·       P site holds growing peptide.

·       E site releases empty tRNA.

·       Peptide bond is catalyzed by rRNA.

·       Translation requires GTP.

·       First amino acid in bacteria is f-Methionine.

·       First amino acid in eukaryotes is Methionine.

16.  18. One-Page Revision Sheet

Flowchart

DNA
↓ (Transcription)
mRNA
↓ (Translation at Ribosome)
Amino Acid Activation (ATP)
↓
Initiation (AUG + Initiator tRNA)
↓
Elongation (A → P → E cycle)
↓
Termination (UAA/UAG/UGA)
↓
Protein Folding & Modification
↓
Functional Protein

19. Mnemonics

• APE → Arrival, Peptide, Exit (ribosomal sites).
• AUG → Always Usual Go (Start codon).
• UAA, UAG, UGA → U Are Away, U Are Gone, U Go Away (Stop codons).
• N → C → Protein synthesis always proceeds from the amino (N) terminus to the carboxyl (C) terminus.