PROTEIN SYNTHESIS

Page 1: Protein Synthesis

Overview of Protein Synthesis

  • Protein synthesis is the cellular process by which proteins are constructed.

  • Involves nucleic acids, enzymes, and 20 amino acids (8 of which are essential and derived from food).

  • Proteins can be synthesized either in the cytoplasm or on the Rough Endoplasmic Reticulum (RER).

  • DNA contains genes, which are segments coding for polypeptides.

Stages of Protein Synthesis

1. Transcription

  • Process of synthesizing mRNA from a segment of DNA.

  • Steps:

    1. DNA unwinds (DNA Helicase enzyme).

    2. Template strand (strand containing the gene) is selected.

    3. RNA Polymerase binds to the promoter region.

    4. The template strand is read in the 3’–5’ direction, while mRNA is synthesized in the 5’–3’ direction.

    5. Free nucleotides (with uracil replacing thymine) are linked via phosphodiester bonds.

    6. DNA reforms (only 12 bases are exposed at any time).

    7. Transcription continues until a stop sequence is reached, completing the pre-mRNA molecule.

Page 2: Post-Transcriptional Modifications

Post-Transcription Processes

  • Before the pre-mRNA becomes mature mRNA, it undergoes three modifications:

    1. Capping:

      • Addition of a methylated guanine nucleotide at the 5’ end to prevent degradation and aid ribosome binding.

    2. Polyadenylation:

      • Addition of 100-200 adenine bases at the 3’ end signaling molecule integrity.

    3. RNA Splicing:

      • Removal of introns (non-coding regions) and retention of exons (coding regions).

  • Mature mRNA is exported through nuclear pores into the cytoplasm.

2. Translation

  • The process of translating the mRNA message into a polypeptide.

  • Begins when mRNA binds to a ribosome, starting at the AUG (start codon).

  • Codons consist of three bases that correspond to specific amino acids.

    • Total of 64 possible codons for 20 amino acids allows for redundancy (multiple codons for some amino acids).

Page 3: Translation Process

tRNA and Amino Acid Addition

  • tRNA molecules bring amino acids to the ribosome; each tRNA has an anticodon complementary to the mRNA codon.

  • AUG codon corresponds to the first amino acid, methionine, which initiates all proteins.

  • Enzyme aminoacyl tRNA synthetase attaches specific amino acids to tRNA.

  • Peptide bonds form between amino acids facilitated by peptidyl transferase (a form of aminoacyltransferase).

  • Ribosome movement allows continuous addition of amino acids (15/second).

  • Up to 50 ribosomes may bind to a single mRNA molecule forming a polysome, producing multiple copies of the polypeptide simultaneously.

Termination of Translation

  • Translation continues until a stop codon (UAA, UAG, UGA) is encountered.

  • A release factor triggers the detachment of the polypeptide from the ribosome.

  • Newly synthesized polypeptide undergoes folding to achieve functional protein structure (primary, secondary, tertiary, quaternary).

Post-Translational Modifications

  • Proteins can undergo further modifications:

    1. Cleavage:

      • Hydrolyzing the polypeptide to create oligopeptide units.

    2. Addition of Chemical Groups:

      • Includes methyl, acetyl, sulfate, and glycans (glycosylation).

    3. Intramolecular Bond Formation:

      • E.g., formation of disulphide bridges.