Chapter 9 Flashcards
Polypeptides
- made of 20 different amino acids
- linked by covalent peptide bonds
- sequence of AA determines the properties of a polypeptide: shape, hydrophobicity, specificity, interactions with other proteins and function
Structure of AA
central carbon, amino group, carboxyl group and an R chain
Three regions of ribosomes
- A-site (aminoacyl site): entry for charged tRNAs
- P-site (peptidyl site): holds the tRNA to which the polypeptide is attached
- E-site (exit site): exit of uncharged tRNA
Translation initiation in bacteria
- Shine-delgarno sequence helps the translation initiation complex identify the start codon
- The first amino acid is N-formylmethionine
Translation initiation in eukaryotes
- Kozak sequence helps identify the start codon
- starts with methionine
Translation termination
Happens when any one of the tree stop codons UAA, UAG, UGA enters the A-site of the ribosome
How does translation termination occur
- Release factor recruitment and binding to stop codon in the A-site
- polypeptide release occurs, eRF1 fills A-site to trigger release by hydrolysis of GTP
- Ribosome dissociation, RF ejection and mRNA release
Polycistronic mRNA in bacteria
- Multiple polypeptide-producing segments separated by an intercistronic spacer
- each has its own shine-Dalgarno site that participates in identification of the start of translation
The triplet code
- Three consecutive nucleotides correspond to one AA
- 64 different codons; 61 specific to AA, 3 stop codons
- Correspond to 20 AA (synonymous codons)
How many codons correspond to Isoleucine
3
How many codons correspond to leucine, or serine or arginine
6 each
How many codons correspond to tryptophan
1
Synonym order in genetic code
Codons assigned to the same amino acid are generally mutationally close to each other
Relatedness order in genetic code
Codons assigned to different AA with the same chemical properties are generally mutationally close to one another
Iso-accepting tRNAs
tRNAs that are charged with the same AA but that have different anticodons - degenerate code
Third-base wobble
Some tRNAs can recognize multiple mRNA codons that differ only in their 3rd position - degenerate code
Charging tRNA molecules
- Aminoacyl-tRNA synthetases catalyze the addition of AA to tRNAs
- Mutations in the tRNA including the anticodon positions prevent the tRNA synthetase from recognizing a tRNA
Posttranslational polypeptide processing
- Removal of amino acids after translation (often N-terminus leader region)
- Modification of AA: Phosphorylation by kinases, dephosphorylation by phosphorylases, Methylation, acetylation, hydroxyl addition, addition of carbohydrate side chain to form glycoprotein, proteolysis, lipidation, ubiquitination
- All influence folding, stability/degradation, localization, and activity
Cleavage of N-terminal AA
Removal of terminal AA
Chemical modification of internal AA
- one of the most common AA modifications is phosphorylation - serves to activate or inactivate a protein
- can also add methyl, hydroxyl or acetyl groups to AA
- Carbohydrate side chains are also added to some proteins to make glycoproteins
Polypeptide Cleavage
Cleavage of the polypeptide after translation generates a pro-peptide, which is the active form
Peptide signal sequence
- 15 to 20 AA at the N-terminus of some proteins directs them to their cellular destinations
- Translation on the surface of the ER shuttles proteins to the Golgi apparatus where they can be exported in vesicles
