EXAM #1: BIOCHEMISTRY OF THE GI TRACT Flashcards
What enzymes are contained in the mouth that begin the breakdown of food?
1) Alpha-amylase
2) Lingual lipase
What is the function of alpha-amylase?
Breakdown of starch in the mouth
What is the function of lingual lipase?
Breakdown of TAGs in the mouth
What macronutrient is NOT brokendown in the mouth?
Protein
What are the components of gastric juice that breakdown food?
1) HCl
2) Pepsin
What is the inactive zymogen form of pepsin?
Pepsinogen
What converts pepsinogen into its active form?
Secretion of HCl that lowers the pH of the gastric juice
What are the three functions of HCl in the stomach?
1) Initial breakdown of protein
2) Activation of pepsinogen to pepsin
3) Kill microorganisms
What enzymes are produced by the pancreas?
Amylase
Lipase
Peptidase
In addition to enzymes, what else does the pancreas secrete?
NaCl
NaHCO3
What pancreatic enzyme is secreted in its active form?
Pancreatic alpha-amylase
What is the function of bile acids?
Emulsificaiton of aggregated lipids to make them more accessible for lipid digestion enzymes
What form must carbohydrates be absorbed in in the small intestine?
Monosaccharide
What are the common sources of glucose?
Honey
Fruit
Corn syrup
What are the common sources of fructose?
Honey
Fruit
What is the common source of sucrose?
Table sugar
What is amylose?
Plant carbohydrate/ starch with alpha 1,4 glycosidic bonds
What is amylopectin?
Plant carbohydrate/starch with alpha 1,4 glycosidic bonds AND alpha 1,6 branches
What is the specific molecular function of alpha-amylase?
Hydrolysis of non-terminal alpha-1,4 linkages
What is the function of pancreatic alpha-amylase?
Conversion of branched polysaccharides into oligosaccharide units in the small intestine
What are the end products of pancreatic alpha-amylase digestion?
1) Maltose
2) Maltotriose
3) alpha-limit dextrins
What is cellulose? Why can’t humans breakdown cellulose?
Polysaccharide plant cell wall component with BETA 1,4 GLYCOSIDIC BONDS
**Humans don’t have BETA 1,4 ENDOGLYOCSIDASES to breakdown cellulose
What is Chitin? Why can’t humans breakdown Chitin?
Polymer of N-acetyl-glucosamine with Beta 1,4 Glycosidic bonds that we do not have the enzymes to breakdown
Comes from fungi and arthropods*
How are the oligosaccharides made by alpha-amylase broken down into monosaccharides?
Membrane bound glycosidases on the luminal side of small intestine enterocytes
What is lactose hydrolyzed into via lactase?
1) Glucose
2) Galactose
What is maltose hydrolyzed into via maltase?
2x Glucose
What is sucrose hydrolyzed to via sucrase/isomaltase?
1) Glucose
2) Fructose
What transporter transports glucose across the membrane of the enterocyte for absorption?
Na+/Glucose cotransporter SGLT1
What transporter transports fructose across the enterocyte membrane for absorption?
Facilitated diffusion transporter, GLUT5
What is the role of Na+/K+ ATPase pumps in the absorption of monosaccharides?
- Cotransporters use Na+ as the driving force
- Concentration gradient is maintained by Na+/K+ ATPase
Which monosaccharide transporter requires ATP? Which does NOT?
SGLT1= ATP GLUT5= NO ATP
Once monosaccharides are in the enterocyte, how do they get into the circulation?
Facilitated diffusion transporter, GLUT2
Where does protein digestion begin? What enzyme?
Stomach via pepsin
What activates pepsinogen into pepsin?
Low pH from HCl secreted via parietal cells
What is the specific molecular function of pepsin?
Pepsin breaks proteins into large fragments
What happens to proteins in the small intestine?
Further breakdown into amino acids, dipeptides, and tripeptides
What pancreatic enzymes breakdown proteins in the small intestine?
Chymotrypsin
Trypsin
Carboxypeptidases
Elastase
What enzymes are produced by enterocytes of the small intestine to breakdown proteins?
Aminopeptidase
Dipeptidase
Endopeptidase
Which enzymes are zymogens, pancreatic proteases or small intestine proteases?
Pancreatic
What is the difference between an Endoprotease and an Exoprotease? Which pancreatic proteases are Endoproteases and which are Exoproteases?
Endoprotease= hydrolyze internal peptide bonds
- Trypsin
- Chymotrypsin
- Elastae
Exoprotease= hydrolyze only terminal peptide bonds
- CARBOXYPEPTIDASE
What enzyme in the small intestine enterocyte beings conversion of pancreatic zymogens to active proteases?
Enteropeptidase
What is the function of enteropeptiase?
Conversion of Trypsinogen to Trypsin
What is the general role of the pancreatic proteases? What is the follow-up role of the proteases produced by the small intestine enterocytes?
Pancreatic= large fragments into oligopeptides
Small intestine= oligopeptides into di/tripeptides and amino acids
Where does the further digestion of di/tripeptides occur?
Intracellularly i.e. in the enterocyte
What prevents Trypsin activation in the pancreas?
Trypsin inhibitor in the pancreas
What happens when pancreatic ducts are blocked i.e. how does this lead to acute pancreatitis?
Blocked ducts lead to an accumulation of Trypsinogen that overwhelms inactivating enzymes; activation occurs in the pancreas and leads to tissue damage
What disease of the pancreas is associated with CF?
Pancreatic insufficiency
How does CF lead to pancreatic insufficiency?
1) CFTR mutation= decreased Sodium Bicarboante
2) Dehydrates pancreatic juice/ thickens it
3) Thickened pancreatic juice reduces production of pancreatic digestive enzymes
What is the manifestation of pancreatic insufficiency?
Insufficient digestion of:
1) Lipids
2) Proteins
3) Carbohydrates
Plus symptoms excess FAT in the large intestine
Describe the etiology of Celiac Disease.
- Exact cause is unknown, but there is a peptidase defect
- Small peptides are present in the intestine that are toxic to mucosal cells
- Ingestion of gluten causes an inflammatory response
What transporter is present on the luminal membrane of the intestine and facilitate the uptake of amino acid, di and tripeptides?
SLC type transporters
What disease does a defect in SLC3A1/7A9 result in?
Cystinuria
Describe the etiology of Cystinuria.
- Defect in the amino acid transporter that catalyzes the reuptake of cystine in the kidney
- Cystine accumulates in the urine
- Cytine is insoluble and forms kidney stones
What do TAGs need to be digested into for uptake?
2x fatty acids
1x glycerol
What three enzymes facilitate TAG digestion? Which does the bulk of the work?
1) Lingual lipase
2) Gastric lipase
3) Pancreatic lipase*
*Pancreatic lipase does most of the digestion of lipids in the small intestine
What happens to lipids in the small intestine?
Emulsifiaction by bile acids to prevent the formation of large lipid droplets
What prevents pancreatic lipase from being denatured by bile acids?
Binding to colipase in the small intestine
What reaction is catalyzed by pancreatic lipase?
Hydrolysis of 2 fatty acids from TAG to form:
- Monoglycerol
- 2x fatty acids
Where are bile acids formed, stored, and secreted?
- Formed= liver from cholesterol
- Stored= gallbladder
- Secreted= duodenum
Where are bile acids reabsorbed for intrahepatic circulation/ recycling?
Ileum
What happens to fatty acids once they are absorbed into enterocytes?
Resynthesis to TAG in the endoplasmic reticulum
What happens to TAGs in the enterocyte, once they have been re-synthesized?
Packaging into Chylomicrons
What apolipoprotein facilitates the packacking of TAGs into chylomicrons?
Apo B-48
Where are chylomicrons secreted once formed in the enterocytes?
Lymphatic system
What is Steatorrhea?
Lipid malabsorption resulting in increased lipids in the feces
What are the causes Steatorrhea?
1) Disruption in delivery of bile acids to the duodenum
2) Compromised pancreas function e.g. CF
3) Defective intestinal mucosa
4) Products such a Olestra, a fat substitute that cannot be digested
