Physiology of Digestion and Absorption 2 Flashcards
What glucose polymers are present in starch?
Amylose and amylopectin.
What are the links between glucose subunits in amylose and amylopectin?
Amylose - alpha-1,4 linkage.
Amylopectin - alpha-1,4 linkage and alpha-1,6 linkage (branches).
Is glycogen or amylopectin more branched?
Glycogen.
What are the linkages in glycogen molecules?
Alpha-1,4 and alpha-1,6 (branches).
What are oligosaccharides?
Sugar dimers.
What makes up sucrose and what are the bonds between the monomers?
Glucose and fructose. Alpha-1,2 linkages.
What makes up lactose and what are the bonds between the monomers?
Glucose and galactose. Beta-1,4 linkages.
What breaks down starch into oligosaccharides and where does this occur?
Alpha-amylase (salivary and pancreatic). In the lumen (intra-luminal hydrolysis).
What breaks down oligosaccharides into monosaccharides and where does this occur?
Oligosaccharidases (lactase, maltase, sucrase-isomaltase).
Is alpha-amylase and exoenzyme or an endoenzyme?
Endoenzyme (not sure about this).
What bonds can amylase break and not break?
Can: internal alpha-1,4 linkages. Can’t: terminal alpha-1,4 linkages, alpha1-6 linkages or alpha-1,4 linkages adjacent to branch points.
What is produced from amylase digestion of starch?
Maltotriose, maltose and alpha-limit dextrins.
What is the substrate of lactase?
Lactose (the only one).
What are the functions of the other oligosaccharidases?
They cleave the terminal alpha-1,4 linkages.
What are the specific functions of maltase, sucrase and isomaltase?
Maltase - can degrade alpha-1,4 linkages in straight chain monomers up to 9 in length.
Sucrase - hydrolyses sucrose.
Isomaltase - only one that can split branching alpha-1,6 linkages of alpha-limit dextrins.
What is the only oligosaccharidase where the hydrolysis by the enzyme is the rate limiting step in assimmilation?
Lactase.
What gene affects lactase persistence (LP)?
MCM6 gene (regulates expression of the lactase gene).
Describe primary, secondary and congenital lactase deficiency.
Primary - lack of lactase persistence allele (commonest).
Secondary - damage to/infection of the proximal small intestine.
Congenital - rare autosomal recessive disease, no ability to digest lactose from birth.
What is the name for lactase insufficiency?
Hypolactasia.
In hypolactasia, what do the colonic microflora produce when lactose is delivered to the colon?
Short chain fatty acids (absorbed), hydrogen (can be detected in breath following lactose challenge), carbon dioxide, methane.
What symptoms do the substances produced by the colonic microflora cause?
Bloating, abdominal pain, flatulence.
What does undigested lactose cause?
Acidification of the colon, an increased osmotic load (loose stools and diarrhoea).
How are glucose and galactose absorbed from the lumen?
Secondary active transport mediated by SGLT1 (sodium symporter).
How is fructose absorbed from the lumen?
Facilitated diffusion mediated by GLUT5.
How do monosaccharides get transported across the basolateral membrane of the enterocyte?
All by facilitated diffusion mediated by GLUT2.
What are the 2 conditions for a substrate to go through the SGLT1 transporter?
Must be a hexose in the D-confomation and one that can form a pyranose ring.
What maintains a low sodium concentration in the enterocytes?
Na+/K+ATPase.
What binds first to the SGLT1 transporter?
Sodium.
What 3 types of enzymes can break down peptides?
Luminal enzymes, brush border enzymes and intracellular enzymes.
How much of daily energy intake does protein assimilation account for in developed countries?
10-15%.
What is the difference between an endopeptidase and an exopeptidase?
Endopeptidase - attacks bonds in middle of chain.
Exopeptidase - attacks bonds at end of chain.
What pancreatic enzymes are endopeptidases and exopeptidases?
Endopeptidases - trypsin, chymotrypsin, elastase.
Exopeptidases - procarboxypeptidase A+B.
What are aminopeptidases and carboxypeptidases?
Aminopeptidases - cleave off amino acid at N-terminal.
Carboxypeptidases - cleave off amino acid at C-terminal.
Why are there numerous brush border peptidases?
Each enzyme attacks a limited number of peptide bonds and the oligopeptides to be digested are extremely varied in their structure.
What types of peptides do brush border peptidases have a higher affinity for?
Larger oligopeptides (3-8 amino acids).
At the brush border how are amino acids absorbed?
5 Na+-dependent co-transporters (secondary active transport), 2 Na+ independent transporters (facilitated diffusion of cationic amino acids).
What diseases does dysfunction of the Na+ dependent and Na+independent transporters cause?
Na+ dependent: Hartnup disease.
Na+ independent: cystinuria.
How are amino acids transported over the basolateral membrane?
3 mechanisms mediate efflux and are sodium independent, 2 mediate influx and are sodium dependent.
What is the name of mechanism of transport of oligopeptides into the enterocyte?
H+-dependent mechanism called PepT1.
