Translation is the process by which messenger RNA (mRNA) is translated into a polypeptide chain (protein).
Occurs in the cytosol where mRNA base pair sequences are translated into amino acid sequences.
Nucleotides are read in groups of three known as codons.
There are 64 codons made from 4 different bases (A, U, C, G) with each codon specifying either a start, stop signal, or an amino acid.
The relationship between a codon and its corresponding amino acid is established by the genetic code.
tRNA (transfer RNA) molecules serve as the essential link between codons on mRNA and their corresponding amino acids.
Each tRNA is approximately 80 nucleotides in size, featuring an anti-codon region that binds to its complementary codon on mRNA.
tRNA molecules are synthesized by RNA polymerase III (POL III) as large transcripts that are subsequently trimmed at both ends and potentially modified in the middle.
These modifications create non-traditional bases (Ψ, Im, Gm, I, T), which contribute to the unique final conformation of the tRNA.
Aminoacyl Synthetase is an enzyme that catalyzes the linking of an amino acid to its specific tRNA, forming a high-energy bond.
The amino acid-activated tRNA is crucial for peptide bond formation during translation.
The reaction consumes energy from ATP, producing AMP and two inorganic phosphates (Pi).
Prior to catalyzing peptide bond formation, tRNA synthetases can proofread and edit tRNAs, removing incorrect amino acids to maintain fidelity in protein synthesis.
The ribosome is a large ribonucleoprotein complex composed of two subunits (large and small) made up of several rRNAs and over 50 proteins.
Eukaryotic ribosomes assemble in the nucleolus and are exported into cytosol for protein synthesis. Millions of ribosomes exist per eukaryotic cell.
The small subunit contains an mRNA binding site and multiple binding sites for tRNAs, while the large subunit catalyzes peptide bond formation.
The initiation process starts with the initiator aminoacyl-tRNA binding to the small ribosomal subunit.
Additional initiation factors (eIFs) bind to guide this small ribosomal subunit to the 5’ cap of mRNA.
The small subunit scans the mRNA for the AUG start codon, driving movement powered by ATP hydrolysis.
When AUG is encountered, initiation factors dissociate, and the large ribosomal subunit binds, starting translation.
Sometimes, the first one or two AUGs can be ignored in favor of a later AUG, a phenomenon called leaky scanning, which allows for the synthesis of functionally identical proteins with different localization signals.
Bacterial mRNAs lack a 5’ cap and instead utilize consensus sequences for small subunit ribosome attachment, which leads to polycistronic mRNAs.
This allows for the translation of multiple proteins from a single mRNA strand.
Elongation continues with incoming aminoacyl-tRNAs bringing new amino acids to the A-site of the ribosome.
Energy from the breaking of high-energy bonds assists in peptide bond formation, while the large ribosomal subunit catalyzes the reaction, shifting the ribosome down the mRNA template without moving the tRNAs.
Stop codons (UAA, UAG, UGA) signal termination of translation, binding release factors (RFs) that mimic tRNAs.
These RFs introduce a water molecule instead of an amino acid, triggering the release of the newly synthesized polypeptide and terminating translation.
Polyribosomes are formations of multiple ribosomes translating the same mRNA simultaneously, enhancing the rate of protein synthesis in both prokaryotes and eukaryotes.
In eukaryotes, polyribosomes can be formed after transcription is completed, but bacteria begin translation even before transcription ends.
Protein folding begins during synthesis, with the assistance of molecular chaperones (hsp60 and hsp70) that help proteins reach their correct conformations.
Proteins that cannot be properly folded are marked for degradation by proteasomes and ubiquitin chains, ensuring cellular quality control by removing defective proteins.
Plasma membrane (in prokaryotes and eukaryotes)
Defines cellular space and boundaries.
Maintains biochemical and electrical differences.
Acts as a conduit for extracellular communication.
Organelle membranes (found in eukaryotes only):
Define organelle lumens and boundaries.
Membrane composition:
Lipid bilayer
Many inserted and surface-associated proteins
A few other types of molecules
Membrane lipids are amphipathic molecules:
Contain both polar and non-polar bonds.
Membrane lipids have a unique shape:
Hydrophilic “head” region (polar) and
Hydrophobic “tail” region (non-polar).
Membrane lipids spontaneously form bilayers in aqueous solution.
Energetically Unfavorable: Lipid micelle (edges exposed to water).
Energetically Favorable: Sealed compartment formed by phospholipid bilayer.
Most abundant lipid in cell membranes.
Many types of phospholipids:
Four major types found in cell membranes.
Phosphatidyl-ethanolamine
Phosphatidyl-serine
Phosphatidyl-choline
Sphingomyelin
Individual lipids diffuse laterally.
Rotate rapidly about their axis.
Nonpolar tails are extremely flexible.
Through:
Cis-double bonds in fatty acid tails.
Addition of other types of molecules.
Allows cells to maintain constant fluidity independent of temperature fluctuations.
Affected by the addition of certain non-phospholipid amphipathic molecules, e.g., cholesterol:
Interacts with phospholipids to increase rigidity between tails near head region.
Lower ends of tails remain flexible and disrupt attractive interactions between phospholipids.
This prevents crystallization (freezing).
Weak associations between lipids, often include cholesterol and proteins.
Lipid rafts drift as a group.
Membrane in raft region is often thicker, may serve as a mechanism for holding membrane proteins.
Lipid molecules + attached sugar molecules.
Found only on non-cytoplasmic surface (extracellular and intra-organelle).
Protection.
Charged glycolipids concentrate ions at cell surface.
Cell recognition.
Unintended function: may allow bacterial toxins to enter cells.
Define much of membrane function.
Amount and type of protein in any region of the membrane is highly variable.
Membrane proteins often have oligosaccharide chains attached to their non-cytosolic domains (similar to glycolipids).
Transmembrane proteins:
Always amphipathic; may be single or multi-pass.
Spanning segment often α-helix or β-barrel.
Surface proteins:
Usually located in cytosolic region; sometimes extracellular.
Variety of attachment mechanisms include:
α-helix associated with lipid bilayer.
Covalent attachment via oligosaccharide chain.
Noncovalent attachment to transmembrane protein.
Peripheral proteins: Weak noncovalent surface attachment.
Integral proteins:
Transmembrane or covalently attached to the surface.
All peripheral proteins are surface proteins, but not all surface proteins are peripheral.
All transmembrane proteins are integral but not all integral proteins are transmembrane.
Spectrin:
Cytosolic and noncovalently associated (peripheral) protein found in red blood cells.
Associates with cytoskeleton and other proteins to form a flexible mesh, maintaining cell shape (concave red blood cell).
Glycophorin:
Transmembrane red blood cell protein with a membrane-spanning α helix domain.
Extensive extracellular domain composed of oligosaccharide chains, providing adaptability for motility.
Bacteriorhodopsin:
Multi-pass transmembrane protein functioning as a proton pump in archaea.
Contains retinal, responding to photons and enabling mechanical work to pump protons out of the cell.
Include several protein subunits such as cytochrome, forming complex cellular structures that participate in various membrane functions.
Extracellular surface covered with sugars.
Covalently attached to:
Membrane proteins (glycoproteins)
Lipids (glycolipids)
Functions:
Provides protection against mechanical damage
Facilitates cell recognition (as further discussed in Chapter 19).
A type of glycoprotein in which one or more attached sugars are amino sugars (e.g., glycosaminoglycan).