Translation & the Proteome (DP IB Biology)

Initiation of Translation

• During translation, the specific sequence of messenger RNA (mRNA) is translated to produce a polypeptide chain consisting of amino acids

  • mRNA is a single stranded, linear, RNA molecule that transfers the information in DNA from the nucleus into the cytoplasm

• Translation is categorised into three stages: initiation, elongation and termination

• Translation occurs in the cytoplasm at complex molecules made of protein and RNA called ribosomes

  • Ribosomes have a two-subunit (large and small) structure that helps bind mRNA

  • Ribosomes have three tRNA binding sites termed “E” (exit), “P” (peptidyl) and “A” (aminoacyl)

    • At the A site the mRNA codon joins with the tRNA anticodon

    • At the P site the amino acids attached to the tRNA are joined by peptide bonds

    • At the E site the tRNA exits the ribosome

• Another key molecule in translation is transfer RNA (tRNA) that decodes mRNA
  

  • tRNA molecules are single stranded RNA molecules that fold to form a clover-shaped structure

    • The folded structure is held together by hydrogen bonds between bases at different points on the strand

    • tRNA molecules are the shortest of the RNA molecules, being only around 80 nucleotides in length

    • There are 20 different types of tRNA molecule, one for each of the amino acids involved in protein synthesis

  • tRNA molecules have a region that binds to a specific amino acid as well as a three-nucleotide region called an anticodon that is complementary to the codon on mRNA

  • The role of tRNA molecule is to carry a specific amino acid to the ribosome

Structure of tRNA

• In eukaryotic cells, the mRNA molecule leaves the nucleus through the nuclear pores

• Translation is initiated by the following process

  • A small ribosomal subunit attaches to the 5’ end of mRNA

  • An initiator tRNA molecule carrying the amino acid methionine binds to the small ribosomal subunit

    • The initiator tRNA occupies the “P” site on the ribosome

  • The ribosome moves along the mRNA until it locates a start codon (AUG)

  • The large ribosomal subunit binds to the small subunit

    • Elongation of the polypeptide can begin

• The initiator tRNA currently occupies the “P” site, the next codon on the mRNA signals for the corresponding tRNA to bind at the “A” site

  • The two amino acids (attached to the tRNAs) are linked with a peptide bond, forming a dipeptide

• Synthesis of the peptide chain now involves a repeated cycle of events

  • In the cytoplasm, free tRNA molecules bind to their corresponding amino acids and transport them to the ribosome

  • The ribosome shifts along the mRNA one codon (three bases) at a time

    • The initiator tRNA in the “P” site moves to the “E” site which releases it

    • The tRNA carrying the peptide chain moves from the “A” site to the “P” site

    • The next mRNA codon is exposed and a tRNA with the complementary anticodon binds to the unoccupied “A” site whilst its amino acid is linked to the polypeptide chain

• The cyclical process is repeated as new amino acids are added to the growing chain

Modification of Polypeptides

• Once the primary structure of the polypeptide has been synthesised during translation it is often not immediately usable by the cell

  • The polypeptide must be modified in order to be transformed into a functional protein

• Some examples of modifications include:

  • Protein folding into the secondary, tertiary and quaternary structures, including the formation of disulfide bonds in the tertiary and quaternary stages

  • Folding can require molecular chaperones that help to prevent incorrect folding

• The formation of insulin requires polypeptide modification

  • When insulin is first synthesised it is in the form of an 110 long polypeptide chain called pre-proinsulin, which is attached to the wall of the endoplasmic reticulum (ER)

  • It is then modified by an enzyme that removes a peptide called a signal peptide from the end, detaching it from the ER and transforming it to proinsulin

  • From there the proinsulin folds and disulfide bonds form between different sections of the polypeptide 

  • The proinsulin is packaged into vesicles at the Golgi apparatus

  • The proinsulin is then cleaved (during which a section called the C peptide is removed from the middle) resulting in two chains (A-chain and B-chain) attached together with two disulfide bonds

  • This is the final, mature form of insulin, ready to be secreted from the cell and used in the body

Recycling of Amino Acids

• Unneeded, damaged, or misfolded proteins can be recycled in the body into usable proteins

• This involves enzymes to break the peptide bonds in these proteins, and releasing the amino acids to be used in translation to synthesise new proteins

  • Proteases are enzymes that break down proteins in this way

  • This process is called proteolysis

• The proteasome is an organelle found in eukaryotic cells and acts as the location for proteolysis in the cell

• By containing the protease enzymes within an organelle it prevents other useful cellular proteins being broken down by mistake

• Proteins identified as being unneeded, damaged, or misfolded are tagged with a chemical called ubiquitin, which begins the process of them being broken down in the proteasome 

• This process is constantly taking place in the cell and is essential for sustaining a functional proteome