Amino Acid Metabolism Flashcards
What are the primary ways the body acquires amino acids?
Dietary intake, intracellular protein turnover, and protein synthesis.
How does the body use amino acids from dietary proteins?
Amino acids can be used to build proteins and other molecules or oxidized to yield energy.
Where does the digestion of dietary proteins begin, and what processes are involved?
In the stomach:
Dietary protein stimulates secretion of gastrin (from G cells).
Gastrin stimulates secretion of HCl and pepsinogen.
Pepsinogen is activated to pepsin at low pH.
Pepsin cleaves polypeptides into smaller peptides.
What happens to proteins in the small intestine during digestion?
Acidic stomach contents trigger secretion of secretin and cholecystokinin:
Secretin stimulates bicarbonate secretion (neutralizes HCl).
Cholecystokinin stimulates the release of pancreatic proteases (trypsinogen, chymotrypsinogen, procarboxypeptidases A and B).
Active proteases cleave peptides into free amino acids.
How are free amino acids absorbed into the bloodstream?
Free amino acids are transported into intestinal epithelial cells and then into the blood.
Why are digestive proteases synthesized as inactive precursors (zymogens)?
To prevent proteases from degrading organ linings.
What is the function of specific digestive proteases, and how are they activated?
Proteases cleave specific peptide bonds:
Examples: Chymotrypsinogen is activated to chymotrypsin; trypsinogen is activated to trypsin.
What is protein turnover, and why is it important?
Protein turnover refers to the degradation and resynthesis of proteins, maintaining proteostasis (a balanced functional proteome).
What role does ubiquitin play in protein turnover?
Ubiquitin is a 76-residue protein that tags proteins for destruction via a covalent bond with Lys residues on target proteins.
How is ubiquitin attached to target proteins?
Activation: Ubiquitin’s C-terminal Gly forms a thioester linkage with an E1 activating enzyme (requires 2 ATP equivalents).
Conjugation: Ubiquitin is transferred to an E2 conjugating enzyme.
Ligation: An E3 ligase transfers ubiquitin from E2 to the target protein.
What is tetraubiquitin, and why is it significant?
Tetraubiquitin consists of 4 ubiquitin monomers linked by isopeptide bonds (via Lys48).
It signals protein degradation.
How does the N-terminal residue affect protein turnover?
The half-life of cytoplasmic proteins is correlated with their N-terminal residue:
Met at the N-terminus: Half-life >20 hours.
Arg at the N-terminus: Half-life 2 minutes.
Posttranslational modifications can alter N-terminal residues.
What is the proteasome, and how does it degrade proteins?
The 26S proteasome degrades ubiquitinated proteins:
Consists of two 19S regulatory particles and one 20S core particle.
Proteins are recognized, unfolded, deubiquitinated, and translocated into the core for degradation
What are the structural components of the 20S core particle?
Barrel-like structure with four rings:
Outer rings: 7 α subunits.
Inner rings: 7 β subunits (3 with protease activity).
What is the function of the 19S regulatory particle?
Recognizes ubiquitinated proteins, removes ubiquitin, unfolds proteins, and translocates them into the 20S core.
