Digestion and Absorption

Flashcards covering the digestion and absorption of dietary proteins and amino acids, including enzymes, transporters, and related conditions.

What is the primary nutritional purpose of dietary protein?

To provide amino acids and nitrogen for the synthesis of body proteins and other nitrogen-containing molecules.

What are the two main categories of protein sources for the body?

Exogenous (dietary) and endogenous proteins.

In protein digestion, what is the role of HCl in the stomach?

HCl denatures proteins but does not hydrolyze peptide bonds (primary structure).

How is pepsinogen converted to its active form, pepsin, in the stomach?

Pepsinogen is converted to pepsin by HCl and by pepsin itself.

What is the main function of pepsin in protein digestion?

Pepsin hydrolyzes interior peptide bonds (acting as an endopeptidase) at low pH, producing large polypeptides, oligopeptides, and some free amino acids.

Which two hormones are released from the duodenum to stimulate pancreatic enzyme and bicarbonate secretion into the small intestine?

Cholecystokinin (CCK) and secretin.

What is the inactive form of the major pancreatic protease, and what activates it?

Trypsinogen is the inactive form, converted to active trypsin by enteropeptidase in the small intestine lumen.

Describe the action and source of enteropeptidase in the small intestine.

Enteropeptidase, secreted from brush border enterocytes, activates trypsinogen to trypsin, which then activates other pancreatic zymogens.

Which pancreatic protease, an endopeptidase, specifically targets peptide bonds adjacent to basic amino acids?

Trypsin.

Which pancreatic protease, an endopeptidase, specifically targets peptide bonds adjacent to aromatic amino acids, methionine, asparagine, and histidine?

Chymotrypsin.

What is the function of carboxypeptidases (A and B) during protein digestion?

Carboxypeptidases are exopeptidases that cleave C-terminal amino acids from peptides, releasing free amino acids.

Aminopeptidases

Exopeptidase that cleaves N terminal end

What happens to di- and tri-peptides at the brush border of intestinal cells?

They are either hydrolyzed by brush border peptidases into free amino acids or absorbed intact into the enterocytes.

How are amino acids primarily absorbed into the intestinal cells (enterocytes)?

Via carrier-mediated transport systems, mostly sodium-dependent.

Which types of amino acids are generally absorbed most rapidly into enterocytes?

Essential amino acids, particularly methionine, leucine, isoleucine, and valine, due to properties like hydrocarbon mass.

How do di- and tri-peptides enter the intestinal cells, and is this process faster or slower than free amino acid transport?

They are co-transported with H+ into the intestinal cells, and this process is generally faster than free amino acid transport.

What is Hartnup disease?

An autosomal recessive disorder affecting the absorption of tryptophan and other neutral amino acids into intestinal and kidney cells, leading to a risk of niacin deficiency.

Why might amino acid supplements not always be more beneficial or easier to absorb than intact dietary protein?

Amino acid supplements can lead to competition for shared transporters, potentially impairing the absorption of other amino acids, and peptides are often absorbed more rapidly than free amino acids.