143. Absorption of Carbs/Proteins

Question 1

How does the mouth digest carbs?

Answer 1

Salivary a-amylase: breaks interior a1,4 bonds
pH optimum 7.0 - inactivated by gastric acid
Products: maltose (2), maltriose (3), a-dextrin (5-9 with branch)

Question 2

How does small intestine digest carbs?

Answer 2

Pancreatic a-amylase: same specificity but greater activity than salivary enzyme
Brush border enzyme: on epithelium of small intestine, highest activity in JEJUNUM
Lactase: lactose = glu + gal
Sucrase: sucrose = fru + glu
Maltase: maltose = glu + glu
Glucoamylase: maltooligos = many glu
a-dextrinase: breaks a1,6 branches

Question 3

How does small intestine absorb carbs? Where?

Answer 3

In duodenum and upper jejunum
Only Glu, Fru, Gal absorbed

Apical:
SGLT1: Na-dependent indirectly active; Glu/Gal with 2 Na cotransport
GLUT5: Passive Fru Transport

Basolateral:
GLUT2: passive glu, gal, fru transport
Na/K ATPase: drives SGLT1 activity

Question 4

How are proteins digested in stomach?

Answer 4

Acidic pH: denatured protein, activates pepsin

Pepsin: endopeptidase (interior polypeptide chain cleaves into large protein fragments), hydrolyzes 15% dietary protein

Question 5

How are proteins digested in small intestine?

Answer 5

Pancreatic pro-enzyme secretion by CCK/ACh
Enteropeptidase: secreted by INTESTINAL mucosal epithelial cells into lumen by CCK stim; activates trypsin (pancreatic enzyme)
Trypsin: pancreatic enzyme activated by enteropeptidase, converts other pancreatic pro-enzymes to active form, endopeptidase (interior bond cleavage)

Chymotrypsin, Elastase (pancreatic endopeptidases)
Carboxypeptidase A/B (pancreatic exopeptidase)

All active at neutral pH - need bicarb from pancreas to give good pH

50% proteins digested in duodenum

Question 6

How are proteins absorbed in small intestine?

Answer 6

Brush border aminopeptidases: breakdown peptides into free AA and di/tri peptides

Apical:
Na/AA cotransporter: Na dependent indirectly active powered by Na/K ATPase
di/tri peptide cotransporter with H+: breakdown to free AA within cell

Basal:
many passive carrier types for specific AAs
Na/K ATPase - driver for Na/AA cotransporter

Phagocytosis: 10% direct (absorb intact protein), 90% degradative (absorb and process protein to free AAs)

90% proteins absorbed as free AAs