Protein Synthesis, Structure, and Translation

Fundamentals of Translation and Cellular RNA Classes

• Definition of Translation: Translation (protein synthesis) is the complex biochemical process by which the genetic information encoded within a linear sequence of codons in messenger RNA ($\text{mRNA}$) is converted into a linear sequence of amino acids to synthesize a polypeptide chain ($\text{mRNA} \rightarrow \text{proteins}$.

• Genetic Machinery: Translation is a complicated process requiring the coordinated action of numerous gene products, including protein factors, structural RNAs, and enzymes.

• Primary Classes of RNA Involved:

  • Messenger RNA ($\text{mRNA}$): Functions as the informational template and intermediate carrier of genetic code from genomic $\text{DNA}$ to the translational apparatus.

  • Functional RNAs: Non-coding RNA molecules that do not translate into proteins but perform crucial structural, catalytic, and adapter roles during translation:

  • Transfer RNA ($\text{tRNA}$): Acts as the adapter molecule reading mRNA codons and supplying cognate amino acids.

  • Ribosomal RNA ($\text{rRNA}$): Forms the catalytic and structural core of ribosomes.

• Relative Cellular Abundance and Stability:

  • Functional RNAs constitute the vast majority of total RNA within a cell.

  • In a typical, actively dividing eukaryotic cell, $\text{rRNA}$ and $\text{tRNA}$ account for approximately $95\%$ of all cellular RNA.

  • Both $\text{rRNA}$ and $\text{tRNA}$ are metabolically stable molecules that persist intact significantly longer than $\text{mRNA}$, which is subject to continuous turnover.

Amino Acid Chemistry and Polypeptide Synthesis

• Monomeric Units: Proteins are unbranched polymers built from repeating monomeric units called amino acids. Historically referred to as peptides, proteins are frequently termed polypeptides.

• Standard Amino Acids: There are $20$ naturally occurring standard amino acids specified by the universal genetic code, each defined by a distinct side chain ($\text{R}$-group) that confers unique biochemical characteristics.

• Universal Amino Acid Structure: Every amino acid possesses a central alpha-carbon ($\text{C}$) covalently bound to four groups:

  • A hydrogen atom ($-\text{H}$

  • A basic amino group ($-\text{NH}_2$

  • An acidic carboxyl group ($-\text{COOH}$

  • A variable side chain ($\text{R}$-group)

• Chemical Classification of the $20$ Amino Acids:

  • Acidic and Amide Side Chains:

  • Aspartate ($\text{Asp}$ / $\text{D}$): Carboxylic acid side chain; negatively charged at physiological $\text{pH}$.

  • Glutamate ($\text{Glu}$ / $\text{E}$): Carboxylic acid side chain; negatively charged at physiological $\text{pH}$.

  • Asparagine ($\text{Asn}$ / $\text{N}$): Uncharged polar amide derivative of aspartate.

  • Glutamine ($\text{Gln}$ / $\text{Q}$): Uncharged polar amide derivative of glutamate.

  • Basic Side Chains:

  • Lysine ($\text{Lys}$ / $\text{K}$): Positively charged primary aliphatic amine side chain.

  • Arginine ($\text{Arg}$ / $\text{R}$): Positively charged guanidino group.

  • Histidine ($\text{His}$ / $\text{H}$): Ionizable imidazole ring.

  • Aliphatic (Non-polar Hydrocarbon) Side Chains:

  • Glycine ($\text{Gly}$ / $\text{G}$): Simplest amino acid; possesses a single hydrogen atom as its side chain, conferring exceptional conformational flexibility.

  • Alanine ($\text{Ala}$ / $\text{A}$): Methyl side chain ($-\text{CH}_3$

  • Valine ($\text{Val}$ / $\text{V}$): Branched-chain aliphatic hydrocarbon.

  • Leucine ($\text{Leu}$ / $\text{L}$): Branched-chain aliphatic hydrocarbon.

  • Isoleucine ($\text{Ile}$ / $\text{I}$): Branched-chain isomer of leucine.

  • Aromatic Side Chains:

  • Phenylalanine ($\text{Phe}$ / $\text{F}$): Hydrophobic benzene ring attached to a methylene group.

  • Tyrosine ($\text{Tyr}$ / $\text{Y}$): Phenol ring possessing a reactive hydroxyl ($-\text{OH}$) group.

  • Tryptophan ($\text{Trp}$ / $\text{W}$): Bicyclic indole ring system.

  • Hydroxyl- or Sulfur-Containing Side Chains:

  • Serine ($\text{Ser}$ / $\text{S}$): Contains a polar primary aliphatic hydroxyl group ($-\text{CH}_2\text{OH}$

  • Threonine ($\text{Thr}$ / $\text{T}$): Contains a secondary hydroxyl group.

  • Cysteine ($\text{Cys}$ / $\text{C}$): Contains a reactive, oxidizable sulfhydryl (thiol, $-\text{SH}$) group.

  • Methionine ($\text{Met}$ / $\text{M}$): Non-polar thioether side chain ($-\text{CH}_2-\text{CH}_2-\text{S}-\text{CH}_3$

  • Cyclic Side Chain:

  • Proline ($\text{Pro}$ / $\text{P}$): Contains a secondary cyclic pyrrolidine amine where the side chain bonds covalently to both the alpha-carbon and the nitrogen atom, imposing rigid conformational constraints on the polypeptide backbone.

• Peptide Bond Formation:

  • Amino acids are covalently linked together via amide bonds designated as peptide bonds.

  • A peptide bond is synthesized via a condensation (dehydration) reaction wherein the nucleophilic alpha-amino group ($-\text{NH}_2$) of one amino acid attacks the electrophilic alpha-carboxyl carbon ($-\text{COOH}$) of another, releasing one molecule of water ($\text{H}_2\text{O}$).

  • The condensation of three amino acids ($\text{aa}_1 + \text{aa}_2 + \text{aa}_3$) forms a tripeptide featuring two peptide bonds with the concomitant release of two water molecules ($2\,\text{H}_2\text{O}$

  • Directionality: Every polypeptide chain possesses an inherent polarity, extending from a free amino-terminal end ($\text{N}$-terminus) to a free carboxyl-terminal end ($\text{C}$-terminus).

Hierarchical Levels of Protein Structure

• Primary Structure ($1^\circ$):

  • Defined as the linear sequence of amino acids linked covalently by peptide bonds.

  • In a representative tripeptide such as $\text{Gly}-\text{Ala}-\text{Leu}$, glycine provides the free amine terminal end ($\text{N}$-terminus, $-\text{NH}_3^+$) and leucine provides the free carboxyl terminal end ($\text{C}$-terminus, $-\text{COO}^-$

  • Sequence Permutations: For a tripeptide assembled from three distinct amino acids ($\text{Gly}$, $\text{Ala}$, $\text{Leu}$

  • If each amino acid is utilized exactly once without repetition, the number of distinct sequence permutations is $3! = 3 \times 2 \times 1 = 6$ ($\text{Gly-Ala-Leu}$, $\text{Gly-Leu-Ala}$, $\text{Ala-Gly-Leu}$, $\text{Ala-Leu-Gly}$, $\text{Leu-Gly-Ala}$, $\text{Leu-Ala-Gly}$

  • If amino acids may be repeated at any position, the theoretical number of permutations is $3^3 = 27$.

• Secondary Structure ($2^\circ$):

  • Refers to local, regular geometric conformations of the polypeptide backbone stabilized by intramolecular and intermolecular hydrogen bonds between backbone amide groups (specifically between the carbonyl oxygen $\text{C}=\text{O}$ and the amide proton $\text{N}-\text{H}$

  • Additional stabilizing factors include local electrostatic forces and van der Waals interactions.

  • Discovered and modeled structurally by Linus Pauling and Robert Corey in the 1950s.

  • Alpha Helix ($\alpha$-helix): A tightly coiled, right-handed helical rod where each carbonyl oxygen forms a hydrogen bond with the backbone amide proton located four residues ahead ($i + 4$

  • Beta-Pleated Sheet ($\beta$-pleated sheet): Extended polypeptide strands aligned laterally in parallel or anti-parallel orientations, held by hydrogen bonds between adjacent strand segments.

  • The high tensile strength and rigid structural architecture of silk fibroin are direct consequences of an abundance of stacked $\beta$-pleated sheets.

• Tertiary Structure ($3^\circ$):

  • The complete three-dimensional folding of a single polypeptide chain into its native, biologically active conformation, determined strictly by interactions among amino acid side chains ($\text{R}$-groups):

  • Salt Bridges (Ionic Interactions): Electrostatic attractions resulting from the mutual neutralization of an acidic carboxylate side chain (e.g., glutamic acid / glutamate) and a basic amine side chain (e.g., lysine).

  • Disulfide Bonds: Strong covalent linkages ($-\text{CH}_2-\text{S}-\text{S}-\text{CH}_2-$) formed through the oxidation of sulfhydryl ($-\text{SH}$) groups on adjacent cysteine residues. These bonds can create an intramolecular loop within a single polypeptide chain or covalently cross-link two separate polypeptide chains.

  • Hydrophobic Interactions and Hydrogen Bonds: Non-polar side chains sequester away from water into the protein interior, while polar side chains form hydrogen bonds.

• Quaternary Structure ($4^\circ$):

  • The structural assembly and spatial arrangement resulting from non-covalent or covalent interactions between two or more independent polypeptide subunits (forming homodimers, heterodimers, or higher multimers).

  • Major Structural Morphologies:

  • Globular Proteins: Compact, spheroidal molecules highly soluble in aqueous environments, specializing in dynamic cellular processes (e.g., catalytic enzymes, transport proteins, antibodies). A classic example is hemoglobin, a heterotetramer composed of two $\alpha$ and two $\beta$ globin chains, each carrying an iron-containing heme group.

  • Fibrous Proteins: Highly elongated, mechanically strong, insoluble proteins that provide structural scaffolding in tissues (e.g., skin, hair, nails, tendons, cartilage).

• Functional Significance of Conformation:

  • Three-dimensional shape dictates the exact spatial layout of active sites, enabling specific and selective binding of substrates.

  • Protein Domains: Discrete, independently folding structural and functional units within a protein that mediate specialized activities (e.g., catalytic activity, $\text{DNA}$ binding, regulatory ligand recognition).

The Genetic Code: Discovery, Rules, and Organization

• Definition: The genetic code is the universal set of biochemical rules by which sequence information stored within nucleic acids ($\text{DNA}$ or $\text{mRNA}$) is translated into amino acid sequences by living cells.

• The RNA Tie Club:

  • Founded in 1954 by American biologist James D. Watson and physicist George Gamow.

  • Mission: Established to "solve the riddle of the RNA structure and to understand how it built proteins."

  • Motto: "Do or die; or don't try."

  • Membership: Restricted to $20$ core members (each assigned to represent one standard amino acid) and $4$ honorary members (each representing one of the four RNA nucleotides: $\text{A}$, $\text{U}$, $\text{G}$, $\text{C}$

  • Every member wore a proprietary woolen necktie bearing an embroidered helical insignia.

• Deciphering the Code:

  • Experimental biochemical approaches systematically cracked the codon dictionary throughout the 1960s.

  • The 1968 Nobel Prize in Physiology or Medicine was awarded to Har Gobind Khorana, Robert W. Holley, and Marshall W. Nirenberg for deciphering the genetic code and detailing its role in protein synthesis.

• Codon Definition: A codon is a contiguous sequence of three $\text{DNA}$ or $\text{RNA}$ nucleotides that directs the insertion of a specific amino acid or acts as a termination signal during protein translation.

• Stop Codons and Historical Nomenclature:

  • $\text{UAG}$: Named amber

  • $\text{UGA}$: Named opal (or umber)

  • $\text{UAA}$: Named ochre

  • Standard Stop Codon Mnemonics:

  • $\text{UGA}$: "U Go Away"

  • $\text{UAA}$: "U Are Away"

  • $\text{UAG}$: "U Are Gone"

• Key Organizational Principles of the Genetic Code:

  • Triplet Architecture: Theoretical calculations confirmed that single-nucleotide codons ($4^1 = 4$ combinations) or doublet codons ($4^2 = 16$ combinations) cannot independently encode $20$ distinct amino acids. A triplet system yields $4^3 = 64$ distinct permutations, providing ample capacity.

  • Non-Overlapping Sequence: Nucleotides within a single codon are dedicated exclusively to that specific codon; the translational machinery reads codons sequentially without sharing bases between successive triplets.

  • Three Possible Reading Frames: Any linear nucleotide sequence can theoretically be decoded in three alternative reading frames, dependent upon where translation initiates:

  • In the string $\text{GGGAAACCC}$:

    • Reading from position 1 yields codons: $\text{GGG}$, $\text{AAA}$, and $\text{CCC}$.

    • Reading from position 2 yields codons: $\text{GGA}$ and $\text{AAC}$.

    • Reading from position 3 yields codons: $\text{GAA}$ and $\text{ACC}$.

  • Degeneracy (Redundancy): Multiple synonymous codons can code for the same amino acid. For example, both $\text{GAA}$ and $\text{GAG}$ encode glutamic acid ($\text{Glu}$

  • Highly Ordered Organization: The codon matrix is organized so that structurally or chemically related amino acids share similar codon sequences (e.g., hydrophobic residues frequently feature a pyrimidine at the second nucleotide position), minimizing the deleterious consequences of point mutations.

  • Continuous and Unpunctuated (Comma-less): Protein synthesis initiates strictly at a designated start codon ($\text{AUG}$) and proceeds codon-by-codon without omissions or non-coding punctuation until reaching a termination codon.

Transfer RNA ($\text{tRNA}$) and Aminoacyl-tRNA Synthetases

• The Adapter Hypothesis:

  • Formulated by Francis Crick in 1958.

  • Posited that amino acids cannot directly recognize nucleotide sequences on an $\text{mRNA}$ template; instead, each amino acid attaches to an intermediate adapter molecule that physically base-pairs with the $\text{mRNA}$.

  • In its simplest form, this model predicted the existence of at least $20$ unique adapter molecules.

• Structural Anatomy of $\text{tRNA}$:

  • Secondary Structure (Cloverleaf Model): Arranged into four double-helical base-paired stems and three single-stranded terminal loops:

  • Acceptor Stem: Formed by base-pairing between the $5'$ and $3'$ ends of the strand, terminating in an invariant single-stranded $5'\text{-CCA-}3'$ sequence with a free $3'\text{-OH}$ group where the amino acid is covalently attached.

  • $\text{D}$ Loop (DHU Loop): Contains modified dihydrouridine ($\text{DHU}$ / $\text{UH}_2$) nucleotides.

  • Anticodon Loop: Located directly opposite the acceptor stem, the middle loop contains the three-nucleotide anticodon that pairs in an antiparallel orientation with the $\text{mRNA}$ codon.

  • $\text{T}\psi\text{C}$ Loop: Contains modified pseudouridine ($\psi$) and ribothymidine nucleotides.

  • Tertiary Structure: The cloverleaf folds into a compact, three-dimensional inverted L-shaped conformation via tertiary hydrogen bonding.

• Aminoacylation of $\text{tRNA}$ (Charging Process):

  • Aminoacylation is the enzymatic attachment of the correct amino acid to its cognate $\text{tRNA}$.

  • The amino acid's alpha-carboxyl group forms an ester bond with the $3'\text{-OH}$ group of the terminal adenosine on the $\text{tRNA}$ acceptor stem:     $\text{carboxyl (A.A.)} + \text{hydroxyl (tRNA)}$

  • Enzymatic Specificity: Catalyzed by aminoacyl-tRNA synthetases. Cells possess $20$ distinct aminoacyl-tRNA synthetases, at least one for each standard amino acid.

  • Two-Step Enzymatic Mechanism:

  • Step 1: Activation of Amino Acid:       $\text{A.A.} + \text{ATP} \xrightarrow{\text{aminoacyl-tRNA synthetase}} \text{A.A.}\sim\text{AMP} + \text{P}\sim\text{P}$       (The enzyme adenylates the amino acid carboxyl group with $\text{ATP}$, releasing inorganic pyrophosphate, $\text{PP}_i$

  • Step 2: Transfer of Aminoacyl to $\text{tRNA}$:       $\text{A.A.}\sim\text{AMP} + \text{tRNA} \xrightarrow{\text{aminoacyl-tRNA synthetase}} \text{A.A.}\sim\text{tRNA} + \text{AMP}$       (The activated amino acid is transferred to the $3'$ end of the $\text{tRNA}$, releasing $\text{AMP}$

  • Once charged, the aminoacyl-tRNA is competent to bind the functional ribosome-$\text{mRNA}$ complex.

Codon-Anticodon Recognition and the Wobble Hypothesis

• The Wobble Phenomenon:

  • While standard Watson-Crick base pairing governs the first two nucleotide positions of an $\text{mRNA}$ codon, the spatial alignment at the third position ($3'$ end of codon, corresponding to the $5'$ end of anticodon) exhibits conformational flexibility.

  • This flexibility permits non-standard hydrogen bonding alignments, enabling a single $\text{tRNA}$ species to recognize multiple synonymous codons.

• Codon-Anticodon Wobble Pairing Rules:

  • Anticodon $5'$ Base $G$: Can pair with Codon $3'$ bases $C$ or $U$.

  • Anticodon $5'$ Base $C$: Pairs exclusively with Codon $3'$ base $G$.

  • Anticodon $5'$ Base $A$: Pairs exclusively with Codon $3'$ base $U$.

  • Anticodon $5'$ Base $U$: Can pair with Codon $3'$ bases $A$ or $G$.

  • Anticodon $5'$ Base $I$ (Inosine): A deaminated purine base that can form stable wobble base pairs with Codon $3'$ bases $U$, $C$, or $A$.

Ribosome Architecture and Catalytic Centers

• Ribosome Nature: Ribosomes are massive ribonucleoprotein macromolecular complexes that act as the structural and catalytic machines of protein synthesis.

• Subunit Organization: Each functional ribosome contains one small and one large ribosomal subunit, named according to sedimentation rates in ultracentrifugation, measured in Svedberg units ($\text{S}$

• Comparative Ribosomal Composition:

  • Prokaryotic Ribosome ($70\text{S}$):

  • Large Subunit ($50\text{S}$): Composed of two $\text{rRNA}$ species ($23\text{S}\;\text{rRNA}$ and $5\text{S}\;\text{rRNA}$) plus $31$ ribosomal proteins.

  • Small Subunit ($30\text{S}$): Composed of one $\text{rRNA}$ species ($16\text{S}\;\text{rRNA}$) plus $21$ ribosomal proteins.

  • Eukaryotic Ribosome ($80\text{S}$):

  • Large Subunit ($60\text{S}$): Composed of three $\text{rRNA}$ species ($28\text{S}\;\text{rRNA}$, $5.8\text{S}\;\text{rRNA}$, and $5\text{S}\;\text{rRNA}$) plus $49$ ribosomal proteins.

  • Small Subunit ($40\text{S}$): Composed of one $\text{rRNA}$ species ($18\text{S}\;\text{rRNA}$) plus $33$ ribosomal proteins.

• Structural and Catalytic Folding: Ribosomal RNA molecules fold into complex, stable three-dimensional architectures stabilized by extensive intramolecular base pairing. The $\text{rRNA}$ molecules provide the primary catalytic activities of the ribosome.

• Specific Binding Sites for $\text{RNA}$:

  • The $\text{mRNA}$ transcript binds exclusively across the small ribosomal subunit.

  • Three functional $\text{tRNA}$ binding sites bridge the interface between the small and large ribosomal subunits:

  • $\text{A}$ (Aminoacyl) Site: Accurately accommodates an incoming aminoacyl-tRNA whose anticodon complements the codon positioned in the small subunit.

  • $\text{P}$ (Peptidyl) Site: Holds the peptidyl-tRNA attached to the growing nascent polypeptide chain.

  • $\text{E}$ (Exit) Site: Transiently accommodates the deacylated (uncharged) $\text{tRNA}$ immediately prior to its dissociation from the ribosome.

• Specialized Catalytic Centers:

  • Decoding Center: Located within the small ribosomal subunit; monitors base-pairing geometry between codon and anticodon, ensuring that only appropriately paired tRNAs are accepted into the $\text{A}$ site.

  • Peptidyltransferase Center: Located deep within the large ribosomal subunit; coordinates and accelerates peptide bond synthesis.

  • Catalytic Nature: The 2009 Nobel Prize in Chemistry was awarded for structural work demonstrating that both the decoding center and peptidyltransferase center are composed entirely of $\text{rRNA}$ folds that act as ribozymes, proving that RNA, not ribosomal protein, executes the catalytic steps of translation.

The Three Steps of Translation

• Protein translation proceeds through three discrete, sequential stages: initiation, elongation, and termination, executed by ribosomes, $\text{mRNA}$, $\text{tRNA}$, and protein factors.

• Initiation in Prokaryotes:

  • Primary Objective: Place the specialized initiator aminoacyl-tRNA directly into the ribosomal $\text{P}$ site over the start codon.

  • Initiator Amino Acid: The universal start codon is $\text{AUG}$, encoding methionine. In prokaryotes, translation initiates with a chemically modified form, $N$-formylmethionine ($\text{fMet}$

  • The $5'\text{-UTR}$ and Shine-Dalgarno Sequence: The $\text{mRNA}$ contains an untranslated leader sequence ($5'\text{-UTR}$) upstream of the protein-coding sequence. Within this region lies the Shine-Dalgarno sequence ($5'\text{-AGGAGGU-}3'$

  • The Shine-Dalgarno sequence base-pairs directly with a complementary pyrimidine-rich sequence at the $3'$ end of the $16\text{S}\;\text{rRNA}$ ($3'\text{-UCCUCCA-}5'$) of the $30\text{S}$ subunit.

  • This base pairing aligns the $\text{AUG}$ initiation codon directly in the future $\text{P}$ site of the ribosome.

  • Role of Initiation Factors:

  • $\text{IF3}$: Binds the free $30\text{S}$ subunit to prevent premature reassociation with the $50\text{S}$ subunit.

  • $\text{IF1}$ and $\text{IF2}$: Form a complex with $\text{GTP}$ and coordinate the entry of initiator $\text{fMet-tRNA}_{\text{f}}$ exclusively into the $\text{P}$ site, simultaneously blocking the $\text{A}$ site.

  • Following $\text{fMet-tRNA}_{\text{f}}$ binding, $\text{IF3}$ dissociates, enabling the $50\text{S}$ subunit to dock. $\text{GTP}$ hydrolysis triggers the release of $\text{IF1}$ and $\text{IF2}$, leaving an active $70\text{S}$ initiation complex.

  • Coupled Transcription and Translation: Because prokaryotic cells lack a membrane-bound nucleus, translation initiates directly on nascent $\text{mRNA}$ transcripts while transcription is still actively occurring.

• Initiation in Eukaryotes:

  • Spatial and Temporal Separation: Transcription and $\text{mRNA}$ processing occur inside the nucleus, whereas translation is restricted to the cytoplasm.

  • $\text{mRNA}$ Post-Transcriptional Processing: Before export, eukaryotic transcripts undergo $5'$ capping with a 7-methylguanosine cap and $3'$ polyadenylation.

  • Removal of Secondary Structures: Eukaryotic mRNAs often form local secondary structures (hairpins) in the $5'\text{-UTR}$ that must be unwound prior to translation. Initiation factors $\text{eIF4A}$, $\text{eIF4B}$, and $\text{eIF4G}$ bind to the $5'$ cap and unwind these duplexes.

  • Scanning Mechanism:

  • The small $40\text{S}$ subunit forms a complex with eukaryotic initiation factors ($\text{eIFs}$

  • The complex binds the $5'$ cap and scans along the $\text{mRNA}$ in a $5' \rightarrow 3'$ direction until locating the first $\text{AUG}$ start codon.

  • Once the codon is recognized, the $60\text{S}$ subunit joins, initiation factors dissociate, and an active $80\text{S}$ initiation complex forms.

• Elongation Phase:

  • Delivery of Aminoacyl-tRNA: Elongation factor Tu ($\text{EF-Tu}$) forms a ternary complex with $\text{GTP}$ and a charged aminoacyl-tRNA ($\text{EF-Tu}\cdot\text{GTP}\cdot\text{aa-tRNA}$

  • Codon Recognition: The ternary complex enters the empty $\text{A}$ site. Upon correct codon-anticodon pairing, $\text{GTP}$ is hydrolyzed to $\text{GDP}$, and $\text{EF-Tu}\cdot\text{GDP}$ is released from the ribosome.

  • Peptide Bond Formation: The peptidyltransferase center catalyzes nucleophilic attack by the amino group of the amino acid in the $\text{A}$ site on the ester bond linking the growing polypeptide to the $\text{P}$-site $\text{tRNA}$.

  • Translocation:

  • Elongation factor G ($\text{EF-G}$), complexed with $\text{GTP}$, binds the $\text{A}$ site.

  • Hydrolysis of $\text{GTP}$ drives a conformational rearrangement that shifts the ribosome by exactly three nucleotides ($1$ codon) toward the $3'$ end of the $\text{mRNA}$.

  • The deacylated $\text{tRNA}$ moves from the $\text{P}$ site into the $\text{E}$ site and exits the ribosome.

  • The peptidyl-tRNA moves from the $\text{A}$ site into the $\text{P}$ site, opening the $\text{A}$ site to accept the next cognate ternary complex.

• Termination Phase:

  • Elongation cycles repeat until a stop codon ($\text{UAA}$, $\text{UAG}$, or $\text{UGA}$) enters the $\text{A}$ site.

  • No standard aminoacyl-tRNA possesses an anticodon matching a stop sequence.

  • Specialized protein release factors recognize stop codons directly through protein-RNA interactions:

  • In bacteria, $\text{RF1}$ recognizes $\text{UAA}$ and $\text{UAG}$; $\text{RF2}$ recognizes $\text{UAA}$ and $\text{UGA}$.

  • $\text{RF3}$ facilitates release factor binding and dissociation via $\text{GTP}$ hydrolysis.

  • Hydrolytic Cleavage: Binding of a release factor alters peptidyltransferase activity, causing it to transfer a water molecule to the peptidyl-tRNA, cleaving the ester linkage and releasing the completed polypeptide chain.

  • Ribosomal Recycling: Ribosome recycling factors cause dissociation of the small and large ribosomal subunits, liberating the uncharged $\text{tRNA}$ and $\text{mRNA}$.

The Proteome, Alternative Splicing, and Isoform Diversity

• Definitions:

  • Genome: The complete, invariant set of genetic material ($\text{DNA}$) present within an organism; represents what could happen.

  • Transcriptome: The full complement of $\text{mRNA}$ transcripts synthesized by an organism, organ, or cell type under specific conditions; represents what might be happening.

  • Proteome: The complete catalog of functional proteins expressed and active within an organism, tissue, or cell at a given time; represents what is happening.

• Enrichment of the Proteome: The total number of unique proteins in the proteome substantially exceeds the number of individual protein-coding genes in the genome due to two major cellular mechanisms:

  • Alternative splicing of precursor $\text{mRNA}$ ($\text{pre-mRNA}$

  • Post-translational modifications of polypeptides.

• Alternative Splicing and Protein Isoforms:

  • Alternative splicing enables a single primary pre-mRNA transcript to generate multiple distinct mature mRNA variants, allowing one gene to code for multiple related proteins termed isoforms.

  • Functional Domains: Discrete sequences within a protein that fold independently and carry out defined biological functions (e.g., substrate binding, catalytic activity, subcellular targeting).

  • Because individual functional domains are typically encoded by distinct exons, alternative splicing selectively includes or excludes specific exons, creating protein isoforms with different combinations of functional domains.

• Case Study: Fibroblast Growth Factor Receptor 2 ($\text{FGFR2}$) Gene:

  • The $\text{FGFR2}$ gene contains multiple exons that undergo alternative splicing to generate distinct receptor isoforms.

  • Selective exon inclusion alters the extracellular ligand-binding domain:

  • First Isoform: Includes a specific exon sequence that configures the binding pocket to recognize fibroblast growth factors $\text{FGF10}$ and $\text{FGF7}$.

  • Second Isoform: Selects an alternative exon sequence that restructures the binding pocket to interact with an entirely different set of ligands: $\text{FGF2}$, $\text{FGF9}$, $\text{FGF4}$, $\text{FGF8}$, and $\text{FGF6}$.

Post-Translational Events: Folding and Molecular Chaperones

• Nascent Polypeptides: Newly synthesized proteins emerging from the ribosome exit tunnel are generally non-functional and must fold into specific three-dimensional conformations to acquire biological activity.

• The Cellular Folding Problem: The crowded aqueous interior of the cell can cause non-productive hydrophobic interactions and protein aggregation during the folding process.

• Molecular Chaperones:

  • A conserved class of proteins present in all living organisms that assist the non-covalent folding, unfolding, assembly, or disassembly of macromolecular protein structures.

  • Distinguishing Characteristic: Chaperones do not form part of the final macromolecular assembly when it performs its normal biological function once folding is complete.

  • Chaperone complexes (such as GroEL-GroES) provide isolated hydrophilic microenvironments that allow hydrophobic regions of nascent proteins to fold properly without aggregating.

Post-Translational Chemical Modifications of Proteins

• More than $300$ distinct post-translational chemical modifications of amino acid side chains have been characterized in living systems. Two of the most common and regulatory modifications are phosphorylation and ubiquitinylation.

• Phosphorylation and Dephosphorylation:

  • Kinase Activity: Protein kinases catalyze the transfer of a negatively charged phosphate group ($\text{PO}_4^{3-}$) from $\text{ATP}$ to the free hydroxyl ($-\text{OH}$) groups of serine, threonine, or tyrosine side chains.

  • Phosphatase Activity: Protein phosphatases reverse this process by hydrolytically removing phosphate groups as inorganic phosphate ($\text{P}_i$

  • Conformational Switching: Because phosphate groups introduce a bulky negative charge, their addition or removal alters the local electrostatic environment, triggering reversible conformational changes.

  • Regulatory Utility: Acts as a reversible molecular on/off switch regulating catalytic activity, subcellular localization, and macromolecular interactions.

  • Frequency: Kinase-mediated regulation is widespread; for example, the model plant Arabidopsis possesses more than $1000$ kinase-encoding genes.

  • The Interactome: The dynamic physical interaction network among all proteins within an organism or cell, heavily governed by phosphorylation cascades.

• Ubiquitinylation (Ubiquitination):

  • Ubiquitin: A small ($76$-amino acid), highly conserved regulatory protein expressed in nearly all eukaryotic tissues.

  • Chemical Mechanism: Enzymatic modification where the carboxylic acid group of the C-terminal glycine of an activated ubiquitin forms a covalent isopeptide (amide) bond with the epsilon-amino group ($\epsilon\text{-NH}_2$) of a specific lysine residue on the target protein.

  • Targeted Protein Degradation: Covalent polyubiquitination targets damaged or unneeded proteins to the $26\text{S}$ proteasome.

  • $26\text{S}$ Proteasome Architecture and Function:

  • A massive, barrel-shaped proteolytic complex that recognizes polyubiquitinated substrate proteins.

  • Deubiquitinating enzymes remove the ubiquitin chain so ubiquitin monomers can be recycled.

  • The target protein is unfolded and threaded into the proteasome core, where internal proteases degrade it into short oligopeptides.

  • Scientific Recognition: The discovery of ubiquitin-mediated protein degradation was recognized with the 2004 Nobel Prize in Chemistry.

  • Non-Degradative Roles: Ubiquitin tags also act as non-destructive regulatory signals directing protein trafficking and governing cellular signaling pathways.