Protein Translation I - Lecture 23

Three phases of the synthesis of proteins

Initiation, elongation, and termination

Initiation

The protein synthesis machinery assembles and the first aminoacyl tRNA is bound to mRNA in complex within the ribosome

elongation

In this phase, peptide bonds are formed between incoming amino acids that are bound to the aminoacyl tRNAs and ribosome

Termination

Once the entire sequence of amino acids has been added, the newly formed protein is released form the ribosome and the entire synthesis machinery disassembles

non-overlapping

no bases are shared between consecutive codons

comma-less

no bases exist in-between the consecutive codons

degenerate

several triplet codons can code for the same amino acid

what do you look out for when observing genetic code

AUG signals for Met and initiation of the start sequence

most amino acids have multiple codons

degeneracy of the genetic code is primarily found in the third base (wobble effect)

Wobble effect

Several different tRNAs can bind to and recognize more than one codons on the mRNA. This applies to the last base on the mRNA codon (3') and the first base of the tRNA anticodon (5') This occurs because the hydrogen bonding between these bases is less stringent than for the first two bases

aminoacyl tRNA synthetases

a high-specialized enzyme that catalyzes the activation of an amino acid and generation of aminoacyl tRNA

Charging reaction

Occurs over 2 steps, highly specific

First the amino acids forms a covalent bond to an adenine nucleotide (ATP) which activates the carbonyl carbon and forms amino acyl AMP

Second the activated amino acid is then transferred to the 3' end of a tRNA molecule with the concomitant release of AMP, forming a charged aminoacyl tRNA

free energy

will form ATP and provide the energy to form covalent links between the molecules

aminoacyl AMP

a mixed anhydride or carboxylic and phosphoric acid

Two discrete classes of aminoacyl tRNA synthetases

Class 1 and class 2

Class 1 discrete classes of aminoacyl tRNA synthetases

Class 1 charge the tRNA at the 2' hydroxyl group of the terminal tRNA nucleotide

Class 2 discrete classes of aminoacyl tRNA synthetases

Class 2 charge the tRNA at the 3' hydroxyl group of the terminal tRNA nucleotide

What reaction is required to move the 2' amino acid charged by Class I synthetases to the 3' hydroxyl of the tRNA

Transesterification

What two levels can selectivity be achieved

First the aminoacyl AMP remains bound to the synthetases during the charging of the tRNA; if the incorrect amino acid is charged to the AMP, it will not be transferred to the tRNA

Second specific binding will only occur only between a charged synthetase and cognate tRNA; indeed, binding sites within the synthetase recognize specific nucleotides of the tRNA molecule

ester linkage

forms from the charging of a tRNA with an appropriate amino acid