Translation

Cellular Roles and Diversity of Proteins

• Enzymatic Catalysis:

  • Proteins function as enzymes that accelerate and regulate metabolic chemical reactions.

• Membrane Transport:

  • Carrier proteins facilitate the translocation of specific ions and polar molecules across phospholipid bilayer membranes.

• Immunological Defense:

  • Antibodies specifically bind to antigens to neutralize pathogens and defend against disease.

• Structural Support:

  • Structural proteins provide mechanical scaffolding and physical integrity to individual cells and biological tissues.

• Signal Transduction:

  • Peptide and protein hormones transmit biological information and physiological signals between organ systems.

• Systemic Oxygen Transport:

  • Specialized transport proteins such as haemoglobin bind and carry oxygen throughout the vascular system.

• Mechanical Contraction:

  • Contractile proteins enable cellular motility and generate mechanical tension allowing muscular contraction.

Compartmentalization of Gene Expression: Transcription to Translation

• Nuclear Transcription:

  • Occurs within the eukaryotic cell nucleus.

  • RNA polymerase binds to a DNA template strand to synthesize a complementary single-stranded messenger RNA ($\text{mRNA}$) molecule.

• Cytoplasmic Export:

  • Following synthesis and processing, the $\text{mRNA}$ exits the nucleus via nuclear pores into the cell cytoplasm.

• Cytoplasmic Translation:

  • Occurs in the cell cytoplasm where ribosomal complexes assemble around the $\text{mRNA}$.

  • Amino acids are sequentially delivered by transfer RNA ($\text{tRNA}$) adaptor molecules and polymerized into a growing polypeptide chain.

• Core Components Required for Translation:

  • Amino Acids: The $20$ fundamental monomeric building blocks utilized to assemble cellular proteins.

  • Messenger RNA ($\text{mRNA}$): A single-stranded RNA template carrying genetic codons transcribed from genomic DNA.

  • Transfer RNA ($\text{tRNA}$): Adaptor RNA molecules each carrying a specific trinucleotide anticodon complementary to an $\text{mRNA}$ codon, covalently bound to the corresponding amino acid.

  • Ribosomes: Ribonucleoprotein complexes comprising large and small subunits that coordinate $\text{mRNA}$ decoding, $\text{tRNA}$ positioning, and peptide bond synthesis.

  • Aminoacyl-tRNA Synthetases: A specialized family of enzymes responsible for catalyzing the esterification of specific amino acids to their cognate $\text{tRNA}$ molecules prior to ribosomal entry.

The Triplet Genetic Code

• Mathematical Derivation of Codon Length:

  • Cellular proteins are constructed from $20$ distinct standard amino acids.

  • A singlet nucleotide code ($4^1$) yields only $4$ possible combinations, which is insufficient.

  • A doublet nucleotide code ($4^2$) yields only $16$ possible combinations, which remains insufficient.

  • A triplet nucleotide code ($4^3$) yields $64$ unique combinations, which provides ample informational capacity to encode all $20$ amino acids.

• Reading Frame and Strand Polarity:

  • DNA Template Strand: Read in the $3' \rightarrow 5'$ direction (e.g., $3'\text{-ACCAAACCGAGT-}5'$ paired to non-template coding strand $5'\text{-TGGTTTGGCTCA-}3'$).

  • mRNA Transcript: Synthesized antiparallel in the $5' \rightarrow 3'$ direction: $5'\text{-UGGUUUGGCUCA-}3'$.

  • Translation into Polypeptides: Codons are parsed non-overlappingly from the $5'$ end to the $3'$ end into discrete amino acids:

  • Codon $5'\text{-UGG-}3'$ codes for Tryptophan ($\text{Trp}$.

  • Codon $5'\text{-UUU-}3'$ codes for Phenylalanine ($\text{Phe}$.

  • Codon $5'\text{-GGC-}3'$ codes for Glycine ($\text{Gly}$.

  • Codon $5'\text{-UCA-}3'$ codes for Serine ($\text{Ser}$.

The Genetic Code Dictionary and Codon Organization

• Start Codon and Translational Initiation:

  • $\text{AUG}$ functions universally as the initiation (start) codon to begin translation.

  • $\text{AUG}$ simultaneously specifies the amino acid Methionine ($\text{Met}$.

• Termination (Stop) Codons:

  • Three codons signal termination of translation: $\text{UAA}$, $\text{UAG}$, and $\text{UGA}$.

  • Stop codons do not encode an amino acid; rather, they serve as binding targets for protein termination factors.

• Degeneracy of the Code:

  • The genetic code is degenerate (redundant), meaning individual amino acids are specified by multiple synonymous codons:

  • Phenylalanine ($\text{Phe}$): $\text{UUU}$, $\text{UUC}$

  • Leucine ($\text{Leu}$): $\text{UUA}$, $\text{UUG}$, $\text{CUU}$, $\text{CUC}$, $\text{CUA}$, $\text{CUG}$

  • Isoleucine ($\text{Ile}$): $\text{AUU}$, $\text{AUC}$, $\text{AUA}$

  • Methionine ($\text{Met}$): $\text{AUG}$

  • Valine ($\text{Val}$): $\text{GUU}$, $\text{GUC}$, $\text{GUA}$, $\text{GUG}$

  • Serine ($\text{Ser}$): $\text{UCU}$, $\text{UCC}$, $\text{UCA}$, $\text{UCG}$, $\text{AGU}$, $\text{AGC}$

  • Proline ($\text{Pro}$): $\text{CCU}$, $\text{CCC}$, $\text{CCA}$, $\text{CCG}$

  • Threonine ($\text{Thr}$): $\text{ACU}$, $\text{ACC}$, $\text{ACA}$, $\text{ACG}$

  • Alanine ($\text{Ala}$): $\text{GCU}$, $\text{GCC}$, $\text{GCA}$, $\text{GCG}$

  • Tyrosine ($\text{Tyr}$): $\text{UAU}$, $\text{UAC}$

  • Histidine ($\text{His}$): $\text{CAU}$, $\text{CAC}$

  • Glutamine ($\text{Gln}$): $\text{CAA}$, $\text{CAG}$

  • Asparagine ($\text{Asn}$): $\text{AAU}$, $\text{AAC}$

  • Lysine ($\text{Lys}$): $\text{AAA}$, $\text{AAG}$

  • Aspartate ($\text{Asp}$): $\text{GAU}$, $\text{GAC}$

  • Glutamate ($\text{Glu}$): $\text{GAA}$, $\text{GAG}$

  • Cysteine ($\text{Cys}$): $\text{UGU}$, $\text{UGC}$

  • Tryptophan ($\text{Trp}$): $\text{UGG}$

  • Arginine ($\text{Arg}$): $\text{CGU}$, $\text{CGC}$, $\text{CGA}$, $\text{CGG}$, $\text{AGA}$, $\text{AGG}$

  • Glycine ($\text{Gly}$): $\text{GGU}$, $\text{GGC}$, $\text{GGA}$, $\text{GGG}$

Deciphering the Genetic Code: The Nirenberg and Matthaei Experiment (1961)

• Hypothesis:

  • A triplet codon based on three-base codons specifies amino acids.

• Methodology:

  • A cell-free bacterial extract was prepared containing ribosomes, tRNAs, amino acids, and translation enzymes, but depleted of endogenous $\text{mRNA}$.

  • Synthetic homopolymer $\text{mRNA}$ molecules consisting of a single repeating nucleotide base were added to separate reaction tubes.

• Experimental Results:

  • Poly-U $\text{mRNA}$ ($\text{UUUUUUUUU...}$) produced a polypeptide consisting entirely of phenylalanine ($\text{Phe-Phe-Phe}$.

  • Poly-A $\text{mRNA}$ ($\text{AAAAAAAAA...}$) produced a polypeptide consisting entirely of lysine ($\text{Lys-Lys-Lys}$.

  • Poly-C $\text{mRNA}$ ($\text{CCCCCCCCC...}$) produced a polypeptide consisting entirely of proline ($\text{Pro-Pro-Pro}$.

• Conclusions:

  • $\text{UUU}$ is an $\text{mRNA}$ codon specifying phenylalanine.

  • $\text{AAA}$ is an $\text{mRNA}$ codon specifying lysine.

  • $\text{CCC}$ is an $\text{mRNA}$ codon specifying proline.

• Biochemical Caveat (Why the Experiment Theoretically Should Not Have Worked):

  • Canonical translational initiation in vivo strictly requires an $\text{AUG}$ start codon to establish the initiation complex.

  • The synthetic homopolymers completely lacked $\text{AUG}$ initiation codons; translation was enabled artificially in vitro by abnormally elevated magnesium ($\text{Mg}^{2+}$) concentrations within the cell-free bacterial extract, bypassing standard initiation requirements.

Transfer RNA (tRNA) Structure and Anatomy

• Cloverleaf Secondary Structure:

  • Composed of internal base pairing that creates hairpin loops and double-stranded helical stems.

• $3'$ Amino Acid Attachment Site:

  • Located at the extended single-stranded $3'$ end of the $\text{tRNA}$ molecule.

  • Carries the invariant terminal sequence where the specific amino acid is covalently linked via an ester bond.

• $5'$ Terminal End:

  • The phosphorylated $5'$ terminus of the $\text{tRNA}$ chain.

• Loop Domains:

  • D Loop: Contains the modified base dihydrouracil, aiding in recognition by specific enzymes.

  • T Loop (T$\psi$C Loop): Contains ribothymidine and pseudouridine, facilitating ribosomal binding.

  • Anticodon Loop: Contains the three-base anticodon sequence positioned at the loop apex.

• Antiparallel Codon Recognition:

  • The $\text{tRNA}$ anticodon pairs with the $\text{mRNA}$ codon in an antiparallel orientation.

  • For example, an $\text{mRNA}$ codon reading $5'\text{-GAG-}3'$ base-pairs with a $\text{tRNA}$ anticodon reading $3'\text{-CUC-}5'$.

Mechanism of Aminoacyl-tRNA Synthetase

• Enzyme Function:

  • Aminoacyl-tRNA synthetases comprise a family of enzymes that ensure high translational fidelity.

  • Each individual synthetase enzyme specifically recognizes both a distinct amino acid and its matching (cognate) $\text{tRNA}$.

• Step-by-Step tRNA Charging Reaction:

  • Step 1 (Substrate Binding):

  • A specific amino acid and an $\text{ATP}$ molecule enter the active site of the aminoacyl-tRNA synthetase enzyme.

  • Step 2 (Amino Acid Activation):

  • $\text{ATP}$ is cleaved into $\text{AMP}$ and pyrophosphate ($\text{PP}_i$).

  • $\text{AMP}$ is covalently attached to the carboxyl group of the amino acid, forming an aminoacyl-AMP intermediate:       $\text{Amino Acid} + \text{ATP} \rightarrow \text{Aminoacyl-AMP} + \text{PP}_i$

  • Subsequent enzymatic hydrolysis of pyrophosphate into two inorganic phosphates ($\text{PP}_i \rightarrow 2\,P_i$) renders the activation reaction exergonic and irreversible.

  • Step 3 (Esterification of tRNA):

  • The cognate uncharged $\text{tRNA}$ enters the active site.

  • The $\text{AMP}$ is displaced as the amino acid is transferred to the $3'$ hydroxyl attachment site of the $\text{tRNA}$, producing an activated aminoacyl-tRNA (charged $\text{tRNA}$.

  • Step 4 (Release of Product):

  • The charged aminoacyl-tRNA is released from the active site and is ready to enter the ribosomal elongation cycle.

Ribosomal Architecture and Binding Sites

• Subunit Organization:

  • Small Ribosomal Subunit: Contains the $\text{mRNA}$ binding site and coordinates base-pairing fidelity between codons and anticodons.

  • Large Ribosomal Subunit: Catalyzes peptide bond formation and contains the exit tunnel through which the nascent polypeptide chain exits the ribosome.

• The Three tRNA Binding Sites:

  • A Site (Aminoacyl-tRNA Binding Site):

  • Binds the incoming aminoacyl-tRNA carrying the next amino acid to be added to the polypeptide chain.

  • P Site (Peptidyl-tRNA Binding Site):

  • Holds the $\text{tRNA}$ covalently attached to the growing polypeptide chain.

  • E Site (Exit Site):

  • Accommodates deacylated (uncharged) $\text{tRNA}$ following peptide bondformation immediately prior to its dissociation from the ribosome.

The Three Stages of Translation

Initiation

• Step 1 (Small Subunit and Initiator tRNA Binding):

  • The small ribosomal subunit binds to the $\text{mRNA}$ transcript at the $\text{mRNA}$ binding site.

  • The initiator $\text{tRNA}$ carrying Methionine ($\text{Met}$), possessing the anticodon $3'\text{-UAC-}5'$, base-pairs with the $\text{AUG}$ start codon ($5'\text{-AUG-}3'$) positioned in the P site.

• Step 2 (Completion of the Initiation Complex):

  • The large ribosomal subunit joins the small subunit assembly.

  • The assembly is powered by the hydrolysis of guanosine triphosphate ($\text{GTP}$):     $\text{GTP} \rightarrow \text{GDP} + P_i$

  • The completed translation initiation complex is formed with the initiator $\text{tRNA}$ situated directly inside the P site, leaving the adjacent A site empty.

Elongation

• Step 1 (Codon Recognition):

  • An incoming aminoacyl-tRNA carrying an amino acid matches its anticodon to the exposed $\text{mRNA}$ codon in the A site.

  • Binding is driven by the hydrolysis of $\text{GTP}$ ($\text{GTP} \rightarrow \text{GDP} + P_i$).

• Step 2 (Peptide Bond Formation):

  • The ribosome's peptidyl transferase activity catalyzes peptide bond formation between the amino end of the new amino acid in the A site and the carboxyl end of the polypeptide chain in the P site.

  • The entire polypeptide chain is transferred from the P-site $\text{tRNA}$ onto the amino acid