Lesson 3: Protein Synthesis and Trafficking in Neurons

Why is the movement of proteins within a neuron essential?

It is essential for neuronal function and signaling.

What are the main functions of proteins in neurons?

They form cellular constituents, catalyze reactions, act as cell-surface markers and receptors, and serve as integral membrane channels and pumps.

What structure do amino acids have?

A central α-carbon, a primary amino group, and a carboxylic acid group.

What is the significance of the 'R' group in amino acids?

The 'R' group varies among different amino acids and determines their unique properties.

What happens to amino acids at physiological pH?

Both the carboxylic acid and amino groups are completely ionized.

What is an amphoteric amino acid?

An amino acid that can act as either an acid or a base.

How are amino acids linked to form proteins?

They are joined through the elimination of a water molecule, forming a peptide bond.

What is a polypeptide?

A protein composed of numerous amino acids and peptide bonds.

What are the four levels of protein structure?

Primary, Secondary, Tertiary, and Quaternary structure.

What defines the primary structure of a protein?

The amino acid sequence of the protein.

What characterizes the secondary structure of a protein?

The formation of alpha helices and beta-pleated sheets.

What occurs in the tertiary structure of a protein?

α-helical or β-pleated regions fold upon one another.

What is the quaternary structure of a protein?

Formed from two or more polypeptide chains that combine.

What is transcription in the context of protein synthesis?

The synthesis of RNA from the genetic code in DNA.

What is translation in protein synthesis?

The synthesis of proteins from amino acids.

What are codons in RNA?

Triplets of bases in mRNA that determine which amino acid is added to the growing peptide.

What role do enzymes play in neurons?

They catalyze various reactions, including the synthesis of neurotransmitters.

What is the sodium-potassium pump?

A protein that serves as an integral membrane channel for moving substances across the cell membrane.

How are excess dietary amino acids processed?

They are converted to common metabolic intermediates such as pyruvate and oxaloacetate.

What distinguishes essential amino acids from nonessential amino acids?

Essential amino acids must be obtained from the diet, while nonessential amino acids are synthesized from metabolic precursors.

What is the significance of the peptide bond?

It links amino acids together to form proteins.

What determines which amino acid is joined to the growing polypeptide chain?

The sequence of nucleotide bases in the mRNA codon.

How many possible combinations of codons exist?

64 possible combinations (4^3 = 64).

What does the codon AUG signify in protein synthesis?

It can code for methionine (Met) or function as a 'start' signal.

What are the three stop codons in mRNA?

UAA, UAG, UGA.

Where does protein synthesis occur?

On ribosomes.

What role does tRNA play in protein synthesis?

tRNA reads each codon and carries the corresponding amino acid.

What happens when a stop codon is encountered during translation?

Translation ends, and the polypeptide chain is released.

How is the nuclear membrane related to the endoplasmic reticulum (ER)?

The nuclear membrane is continuous with the ER membrane.

What is the fate of proteins synthesized in the cytosol?

Most remain in the cytosol for various cellular functions.

What is cotranslational modification?

Modification of proteins during their synthesis.

What is posttranslational modification?

Modification of proteins after their synthesis.

What is an example of a posttranslational modification?

Disulfide linkage (Cys-S-S-Cys) critical for protein structure.

What is the role of acylation in protein modification?

It helps anchor proteins into membranes.

What is the challenge of protein movement over long distances in cells?

Active transport is required due to distance from the soma.

What is the hydrophobic region of cell membranes?

The inner region that poses a challenge for protein insertion.

What is translational diffusion?

The random movement of molecules within cells.

What factors affect diffusion of proteins in cells?

Macromolecular crowding, viscosity, physical barriers, and binding.

How does protein size affect diffusion rate?

Larger proteins diffuse more slowly than smaller ones.

What directs proteins to their correct cellular location?

Signal peptides and signal patches.

What is a signal peptide?

A continuous stretch of amino acid sequence, typically 15 to 60 residues long, often removed from the finished protein after sorting.

What is a signal patch?

A specific three-dimensional arrangement of atoms on a protein's surface that can consist of amino acid residues distant from one another in the sequence.

What organelles do signal peptides direct proteins to?

Endoplasmic reticulum (ER), mitochondria, chloroplasts, peroxisomes, and nucleus.

What are the three mechanisms of protein transport between compartments?

Gated transport, trans-membrane transport, and vesicular transport.

What is gated transport?

Transport that occurs between compartments with continuity of their spaces, such as between the nucleus and cytosol.

What is trans-membrane transport?

Transport between two distinct compartments, where proteins are directed by a membrane-bound translocator and usually must be unfolded.

What is vesicular transport?

Transport where vesicles ferry proteins from one compartment to another, such as from the ER to the Golgi apparatus.

How are signal peptides and patches recognized?

By complementary receptor proteins in the target organelle.

What is the role of nuclear localization signals?

They direct nuclear proteins to the nucleus by binding with cytosolic proteins that help guide them to the nuclear pore complex.

What is required for the transport of large protein molecules into the nucleus?

Energy in the form of ATP hydrolysis.

What is the function of microtubules in protein transport?

They provide a track for the movement of organelles and vesicles by molecular motors.

What are the two types of axonal transport?

Fast axonal transport (anterograde and retrograde) and slow axonal transport.

What is the role of kinesin in axonal transport?

Kinesin aids in anterograde fast transport along microtubules, moving toward the plus end.

What is the role of dynein in axonal transport?

Dynein aids in fast retrograde transport along microtubules, moving toward the minus end.

What happens to transport function if microtubules are disrupted?

Transport function is lost.

What is exocytosis?

The process where proteins and phospholipids are transported to secretory vesicles that empty their contents by fusing with the cell membrane.

What is the purpose of endocytosis?

To take up the plasmalemmal membrane into the cell in the form of endocytic vesicles.

What is receptor-mediated endocytosis?

A process where specific uptake is mediated by clathrin-coated vesicles.

What happens to the clathrin coat after endocytosis?

The clathrin coat is shed once inside the cell.

How does vesicular traffic maintain the steady-state condition of the plasmalemma?

By balancing the transport of substances into and out of the cell membrane.