7.3 - Translation

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tRNA activating enzymes

each tRNA molecule is recognized by a tRNA-activating enzyme that attaches a specific amino acid to the tRNA, using ATP for energy

activation involves attaching an amino acid to the 3' terminal to the tRNA

there are 20 d/t tRNA-activating enzymes for each of the 20 amino acids

energy from ATP is needed for the attachment of amino acids

once the amino acid is attached, the amino acid is activated by the formation of a bond between the enzyme and adenosine monophosphate (AMP)

the activated amino acid is then covalently attached to the tRNA

energy from this bond is later used to link the amino acid to the growing polypeptide chain during translation

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Initiation, Elongation, Termination

Initiation of translation

to start, the mRNA molecule binds to the small ribosomal subunit at an mRNA binding site

an initiator tRNA molecule carrying methionine binds at the start codon "AUG"

the large ribosomal subunit then binds to the small one

then,

the initiator tRNA is in the P site

the next codon signals another tRNA to bind

it occupies the A site

a peptide bond is formed between the amino acids in the P and A site

Elongation of translation

the ribosome moves the tRNA in the P site to the E site, freeing it

this allows a tRNA with the appropriate anticodon to bind to the next codon in the vacant A site

btw, peptide bonds are forming between the amino acids

there are only 2 tRNA molecules on the ribosome at a time, on the P and A

Termination of translation

the process continues until a stop codon is reached

the free polypeptide is released

btw, the direction of movement along the mRNA is 5' end to the 3' end

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Where in the cell is the protein made?

this is determined by the final destination of the protein

it can either be synthesized in the cytoplasm or the endoplasmic reticulum

whether the ribosome free in the cytosol or bound to the ER:

depends on the presence of a signal sequence on the polypeptide being translated

as the signal sequence is created, it becomes bound to a signal recognition protein that stops the translation until it can bind to a receptor on the surface of the ER

once this happens, translation begins again with the polypeptide moving into the lumen of the ER as it is created

Free ribosomes

translation occurs more commonly in the cytosol

synthesized by free ribosomes are:

proteins destined to use in the cytoplasm, mitochondria, and chloroplasts

Bound ribosomes

synthesizes by ribosomes bound to the ER are:

proteins destined for use in the ER, the Golgi apparatus, lysosomes, the plasma membrane or outside the cell

The coupling of transcription and translation in prokaryotes

in eukaryotes:

once transcription is complete, the transcript is modified before leaving the nucleus

there is a delay between transcription and translation due to compartmentalization

in prokaryotes:

as soon as the mRNA is transcribed, translation begins

Polysomes

structures visible in an electron microscope

they represent multiple ribosomes attached to a single mRNA molecule

because transcription and translation occur in the same compartment in prokaryotes, multiple polysomes are visible associated with one gene

in eukaryotes, polysomes occur in both the cytoplasm and next to the ER

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Protein Structures

the biological activity of a protein is related to its primary, secondary, tertiary and quaternary structure

exposure to high temperatures or changes in pH can lead to alterations in the structure, disrupting its biological activity

this is also known as denaturation

(permanent loss of structure) - more about this in topic 2.5

Primary structure

the sequence of amino acids in a polypeptide is its primary structure

Secondary structure

defined by the pattern of hydrogen bonds between the amino hydrogen and carboxyl oxygen atoms in the peptide backbone

Tertiary structure

the overall three-dimensional shape of the protein

different types of interactions,

positively charged R-groups will interact with negatively charged R-groups

avoid contact with water, because of hydrophobic amino acids

polar R-groups will form H bonds with other polar R-groups

Quaternary structure

when polypeptides fit together when there's more than one chain

it also refers to the addition of non-polypeptide components