AA 1

• What are dietary proteins a source of?

  Amino acids
• Why must proteins be broken down for absorption?

  They must be converted to amino acids
• What forms can digestive products of protein be absorbed as?

  Amino acids, dipeptides, and tripeptides
• Where does the digestion of proteins begin?

  In the stomach
• What completes the digestion of proteins?

  The intestine
• What are proteolytic enzymes produced as?

  Zymogens
• How are zymogens activated?

  By cleavage after entering the gastrointestinal lumen
• What is the major proteolytic enzyme in the stomach?

  Pepsin
• How is pepsin produced?

  From inactive zymogen pepsinogen
• What causes pepsinogen to cleave itself?

  HCl causes a conformational change
• What type of peptide bonds does pepsin cleave?

  Bonds with aromatic amino acids or leucine
• What neutralizes stomach acid in the intestine?

  Pancreatic bicarbonate
• What do pancreatic endopeptidases do?

  Cleavage of peptide bonds within protein chains
• What do pancreatic exopeptidases cleave?

  One amino acid at a time from C-terminal
• What completes the conversion of dietary proteins to amino acids?

  Proteases produced by intestinal epithelial cells
• What do aminopeptidases cleave?

  One amino acid at a time from N-terminal
• What do dipeptidases and tripeptidases produce?

  Amino acids from dipeptides and tripeptides
• What happens to digestive enzymes in the intestine?

  They digest themselves and dietary protein
• How are most free amino acids taken into enterocytes?

  Via sodium-dependent secondary active transport
• What drives the transport of amino acids from the intestinal lumen to the blood?

  Hydrolysis of ATP
• What transporter takes up di- and tripeptides?

  Proton-linked peptide transporter (PepT1)
• What happens to peptides in enterocytes?

  They are hydrolyzed to free amino acids
• What type of transporters release free amino acids into the portal system?

  Sodium-independent transporters
• What happens to free amino acids after a meal?

  They are metabolized by the liver or released
• Can adults increase muscle by eating excess protein?

  No, they cannot
• What happens to excess dietary protein?

  Converted to glycogen and triacylglycerols
• Are amino acids stored by the body?

  No, they are not stored
• How must amino acids be obtained?

  From diet, synthesized, or degraded proteins
• What is the input to the amino acid pool?

  Degradation of proteins, dietary protein, and synthesis
• What is the output from the amino acid pool?

  Synthesis of proteins and conversion to other molecules
• What is protein turnover?

  Constant synthesis and degradation of proteins
• How much body protein is turned over daily?

  300–400 g
• What varies widely among individual proteins?

  The rate of protein turnover
• What characterizes short-lived proteins?

  Rapidly degraded with short half-lives
• What is nitrogen balance?

  Match of nitrogen consumed and excreted
• What indicates a negative nitrogen balance?

  More nitrogen excreted than consumed
• What indicates a positive nitrogen balance?

  More protein synthesis than breakdown
• What is the role of lysosomes in protein degradation?

  Participate in autophagy and degrade proteins
• What is the process of ubiquitination?

  Tagging proteins for degradation with ubiquitin
• What happens to ubiquitinated proteins?

  They are recognized and degraded by proteasomes