Translation and Post-Translational Modification (1B) - MCAT Biological and Biochemical Foundations of Living Systems

Charges tRNA with its correct amino acid using ATP. This enzyme ensures accurate amino acid attachment to tRNA, enabling proper codon recognition during protein synthesis.

mRNA is read $5' \rightarrow 3'$. Ribosomes decode mRNA in the 5' to 3' direction to match the synthesis polarity of nucleic acids.

A site. The A site accommodates the next charged tRNA, allowing codon-anticodon pairing and peptide bond formation.

P site. The P site secures the tRNA with the nascent chain, positioning it for transfer to the incoming amino acid.

E site. The E site temporarily holds the empty tRNA after peptide transfer, facilitating its release from the ribosome.

AUG; methionine (Met) (fMet in bacteria). AUG initiates translation by recruiting the initiator tRNA, specifying methionine as the first residue in the polypeptide.

UAA, UAG, UGA. These codons do not code for amino acids but signal the end of translation by binding release factors.

Release factor (RF/eRF). Release factors mimic tRNA to bind stop codons, triggering hydrolysis and polypeptide release from the ribosome.

Peptidyl transferase (rRNA ribozyme activity). This ribosomal RNA catalyzes the formation of peptide bonds between amino acids during elongation.

Prokaryotic $70S=50S+30S$; eukaryotic $80S=60S+40S$. Prokaryotic and eukaryotic ribosomes differ in size and subunit composition, reflecting evolutionary divergence.

Aligns mRNA on the $30S$ subunit via $16S$ rRNA pairing. The Shine-Dalgarno sequence base-pairs with 16S rRNA to position the start codon correctly in the P site.

Consensus around AUG that promotes start site recognition. The Kozak sequence optimizes initiation by enhancing ribosome binding and scanning efficiency around the AUG codon.

$5'$ mRNA cap (m$^7$G cap). The 5' cap recruits initiation factors and the 40S subunit, enabling scanning for the start codon.

Enhances translation and stabilizes mRNA via PABP interactions. The poly(A) tail binds poly(A)-binding proteins, which interact with initiation factors to circularize mRNA and promote reinitiation.

Polysome (polyribosome) formation. Polysomes increase protein synthesis efficiency by allowing concurrent translation of a single mRNA by multiple ribosomes.

$2$ high-energy bonds (ATP$\rightarrow$AMP counts as $2$) plus $2$ GTP. Charging tRNA hydrolyzes ATP to AMP (two bonds), and elongation requires two GTP for tRNA delivery and translocation.

eEF2 with GTP. eEF2-GTP hydrolysis powers the movement of the ribosome along mRNA by one codon during elongation.

eEF1A with GTP. eEF1A-GTP forms a ternary complex with aminoacyl-tRNA, ensuring accurate delivery and proofreading at the A site.

Rough endoplasmic reticulum (RER)-bound ribosomes. Secreted proteins have signal sequences that direct ribosomes to the RER for co-translational translocation into the ER lumen.

Binds signal peptide, pauses translation, targets ribosome to RER. SRP recognizes the N-terminal signal peptide, halts elongation, and docks the ribosome to the ER translocon for protein insertion.

Assist folding and prevent aggregation without changing sequence. Chaperones bind nascent polypeptides to promote proper folding and inhibit misfolding or aggregation without altering the amino acid sequence.

Covalent ubiquitin tagging; targets protein for proteasomal degradation. Ubiquitination marks proteins for degradation by attaching ubiquitin chains, recognized by the proteasome for proteolysis.

Phosphorylation of Ser/Thr/Tyr residues. Phosphorylation adds phosphate groups to specific residues, modulating protein function, activity, or localization through conformational changes.