Protein Structure, Translation, and the Genetic Code

Protein Structure

Amino Acids

Amino acids are the building blocks of proteins, each consisting of a central alpha carbon bonded to an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R-group). The side chain determines the chemical properties and classification of each amino acid.

• Structure: All amino acids share a common backbone but differ in their R-groups.

• 20 different side chains: These confer distinct chemical properties.

• Polarity: Amino acids have polarity, with a free amino (N) terminus and a carboxyl (C) terminus.

Example: Glycine has a hydrogen as its side chain, while phenylalanine has a benzyl group.

Classes of Amino Acid Side Chains

Amino acids are classified based on the properties of their side chains, which affect protein folding and function.

Class	Examples	Properties
Nonpolar	Alanine, Methionine, Cysteine, Phenylalanine, Glycine, Proline, Isoleucine, Tryptophan, Leucine, Valine	No charge or electronegative atoms at pH 7.0; hydrophobic
Polar	Asparagine, Threonine, Glutamine, Tyrosine, Serine	Partial charges at pH 7.0; can form hydrogen bonds
Basic	Arginine, Histidine, Lysine	Positively charged at pH 7.0; can form ionic bonds
Acidic	Aspartate, Glutamate	Negatively charged at pH 7.0; can form ionic bonds

Additional info: The table above summarizes the main classes and their representative amino acids.

The Peptide Bond

A peptide bond is a covalent linkage formed between the carboxyl group of one amino acid and the amino group of another, resulting in a polypeptide chain. This bond is formed via a condensation (dehydration) reaction.

• Polypeptide backbone: Repeating sequence of N-C-C atoms.

• Polarity: Polypeptides have an N-terminus (amino end) and a C-terminus (carboxyl end).

Equation:

$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

Levels of Protein Structure

Proteins exhibit hierarchical structural organization, which determines their function.

• Primary structure: Linear sequence of amino acids in a polypeptide.

• Secondary structure: Local folding patterns stabilized by hydrogen bonds, including alpha-helices and beta-sheets.

• Tertiary structure: Overall 3D shape of a single polypeptide, formed by interactions among side chains (hydrophobic, ionic, hydrogen bonds, disulfide bridges).

• Quaternary structure: Assembly of multiple polypeptide subunits into a functional protein complex.

Example: Hemoglobin is a quaternary protein composed of four polypeptide chains.

Translation (Protein Synthesis)

Adaptor Function of tRNAs

Transfer RNAs (tRNAs) serve as adaptors that translate the nucleotide sequence of mRNA into the amino acid sequence of proteins. Each tRNA has an anticodon that pairs with a specific mRNA codon and an acceptor arm that binds the corresponding amino acid.

• tRNA charging: Attachment of an amino acid to tRNA by aminoacyl tRNA synthetase enzymes.

• Specificity: Each synthetase recognizes one amino acid and its corresponding tRNAs.

Ribosomes

Ribosomes are large ribonucleoprotein complexes that catalyze protein synthesis. They consist of a small and a large subunit, each made of rRNA and proteins.

• Binding sites: Ribosomes have three tRNA binding sites: A (aminoacyl), P (peptidyl), and E (exit).

• Function: Facilitate codon-anticodon pairing, peptide bond formation, and translocation along mRNA.

tRNAs and tRNA Charging

tRNA charging is the process by which an amino acid is covalently attached to its corresponding tRNA by an aminoacyl tRNA synthetase. This step is essential for accurate translation.

• Accuracy: The ribosome does not check the amino acid; accuracy depends on correct charging.

• Enzymes: 20 different synthetases, one for each amino acid.

Initiation of Translation

Prokaryotic Initiation

In prokaryotes, translation initiation requires the Shine-Dalgarno sequence on mRNA, which base-pairs with a complementary sequence on the small ribosomal subunit rRNA. Initiation factors (IF1, IF2, IF3) and a specialized initiator tRNA (charged with N-formylmethionine) are involved.

• Start codon: Usually AUG, positioned in the P site.

• Assembly: Small subunit, mRNA, initiator tRNA, and initiation factors assemble; large subunit joins after GTP hydrolysis.

Eukaryotic Initiation

Eukaryotic translation initiation requires a 5' methyl-guanosine cap, polyA tail, and the Kozak consensus sequence surrounding the start codon. Initiation factors bind the cap and polyA tail, and the small ribosomal subunit scans the mRNA for the start codon.

• Scanning: Initiation complex scans 5' to 3' until it finds the Kozak sequence.

• Assembly: Large subunit joins, and translation begins at the start codon.

Elongation (Prokaryotic)

During elongation, aminoacyl-tRNAs are delivered to the A site by elongation factor EF-Tu. The ribosome catalyzes peptide bond formation, transferring the growing peptide chain to the tRNA in the A site. Translocation moves the ribosome along the mRNA, shifting tRNAs from A to P to E sites, facilitated by EF-G and GTP hydrolysis.

• Peptidyl transferase: Catalyzes peptide bond formation.

• Translocation: Moves ribosome and tRNAs to next codon.

Termination (Prokaryotic)

Translation terminates when a stop codon (UAG, UGA, UAA) enters the A site. No tRNAs recognize stop codons; instead, release factors bind and promote hydrolysis of the bond between the polypeptide and tRNA, releasing the completed protein. GTP hydrolysis is required for dissociation.

The Genetic Code

Properties of the Genetic Code

The genetic code is the set of rules by which nucleotide triplets (codons) in mRNA specify amino acids in proteins.

• Triplet code: Each amino acid is specified by a codon of three nucleotides.

• Non-overlapping: Codons are read sequentially, without overlap.

• Degeneracy: Most amino acids are encoded by more than one codon.

• Ordered: Codons for similar amino acids often have similar sequences, minimizing mutation effects.

• Universality: The code is nearly universal across organisms, with few exceptions.

Example: There are 20 amino acids, 61 codons that encode amino acids, and 3 stop codons. Organisms typically have 40-60 tRNAs, so some tRNAs must recognize multiple codons.

Wobble Base Pairing

Wobble pairing refers to relaxed base-pairing rules at the third position of the codon (5' end of the anticodon), allowing some tRNAs to recognize more than one codon. This contributes to the degeneracy of the genetic code.

Summary Table: Amino Acid Classes

Class	Representative Amino Acids	Key Properties
Nonpolar	Ala, Val, Leu, Ile, Met, Phe, Trp, Pro, Gly, Cys	Hydrophobic, no charge at pH 7.0
Polar	Ser, Thr, Tyr, Asn, Gln	Hydrophilic, partial charges, can form hydrogen bonds
Basic	Arg, Lys, His	Positively charged at physiological pH
Acidic	Asp, Glu	Negatively charged at physiological pH

Additional info: Antibiotics can target prokaryotic translation by interfering with ribosomal function, exploiting structural differences between prokaryotic and eukaryotic ribosomes.