Biochemistry-Digestion of Carbs, Proteins and Fats Flashcards
What is located in the brush border of absorptive cells in the GI tract?
Transporters for nutrients and hydrolases are bound to the plasma membrane for breakdown of oligosaccharides and disaccharides.
Once nutrients are absorbed by the enterocytes, how do they get into the blood?
Portal vein -> Liver -> Hepatic vein
What enzymes begin the process of carbohydrate breakdown?
Alpha-amylases secreted by the salivary glands and pancreas
How is the structure shown below linked together?
This is amylose. This is made up of straight chain glucose molecules linked together by alpha-1,4 glycosidic bonds.
How is the structure shown below linked together?
This is amylopectin. This is made up of glucose molecules linked together by alpha-1,4 and alpha-1,6 glycosidic bonds.
You go the cafeteria during lunch and eat a sandwich. You chew it up and swallow it. How are the carbohydrates in the bread separated by enzymes in your saliva?
Salivary and pancreatic alpha-amylases are endo-glucosidases. They split the carbohydrates into maltose, isomaltose, trisaccharites and alpha-dextrins (oligosaccharides with alpha-1,6 branching)
What end of a starch is the reducing end?
The end with the anomeric carbon not involved in a glycosidic bond.
How does the low pH of the stomach help us out?
Kills bacteria and denatures proteins
What are some of the components of fiber?
Indigestible carbohydrates like cellulose, alpha-L-arabinose, beta-D-xylose and components of pectin.
Why do beans give you gas? What other compounds can give you gas for this reason?
Raffinose has an alpha-1-6 galactose-sucrose linkage that we cannot degrade. This leaves it for degradation by bacteria which produces H2, CO2 and CH4. Short chain fatty acids and lactic acid can also do this?
Dietary fiber
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A mom brings her daughter in to see you complaining of distention, gas and watery diarrhea whenever she eats dairy products. What causes the watery diarrhea?
She is lactose intolerant, characterized by beta-galactosidase deficiency. Inability to separate lactose into glucose and galactose makes it so lactose cannot cross the gut wall. It gets fermented by bacteria to lactic acid, which is osmotically active. This pulls water into the gut lumen and causes watery diarrhea.
What is a zymogen?
Inactive form of a digestive enzyme (trypsinogen, chyotrypsinogen, proelastase, procarboxypeptidases A and B.
Where does protein digestion first take place?
The stomach. Acid denatures it and pepsin chops it up into oligopeptides.
How does ingestion of protein trigger activation of the different pancreatic proteases in the small bowel?
Intestinal endocrine cells secrete hormones when contacted by oligopeptides -> pancreatic acinar cell secretes trypsinogen -> intestinal mucosal epithelial cell secretes enteropeptidase -> enteropeptidase converts trypsinogen to trypsin -> trypsin converts more tripsinogen to trypsin -> trypsin converts chymotrypsinogen to chymotrypsin, proelastase to elastase and procarboxypeptidase to carboxypeptidase.
What enzymes are responsible for the final conversion of oligopeptides to free amino acids?
Aminopeptidase. This enzyme is found in the brush border of enterocytes.
By what method are amino acids and glucose transported into the cell with Na+?
Facilitate transport with Na+. If Na/K ATPase no longer functioned, the Na+ gradient would dissipate and you could no longer transport amino acids or glucose from the intestinal lumen.
SGLT
Sodium-Glucose linked transporter, active transport of glucose by using Na+ gradient.
How is glucose transported into the cell without Na+?
GLUT transporters. These are passive transporters that allow glucose into the enterocytes.
What does pancreatic lipase do to triacylglycerols?
It pops of fatty acids so you go from a triacylglycerol, to a diacylglyerol, to a 2-monoacylglycerol.
What circuit might be messed up if your poo is always green?
Entero-hepatic circulation. Normally bile is secreted by the gall bladder through the common bile duct and circulates back to the liver, which sends it back to the gallbladder.
How does bile help us better digest fat?
The bile salts form micelles that act as detergents. Hydrophobic tails in the center of the micelle encircle the triacylglycerols of the fat droplet and pinch it off into smaller droplets.
How is the structure of a bile salt prime for the task of emulsification?
The salt has a hydrophilic head with a steroid ring tale. The tale has a polar face and a nonpolar face that sticks to the fat droplet.
Why is phosphatidylcholine a good molecule for emulsification of fat?
Note that it is also called lecithin, its polar surface keeps small oil droplets from coalescing.
Where does cholesterol fit in as it is being transported by a low density lipoprotein?
Fatty acids form a “microemulsion” of cholesterol esters. Cholesterol has a single polar end because of an attached hydroxyl group. The polar end is in the shell of the LDL with the phospholipid heads. The tail cholesteryl ester is in the center with the phospholipid tails. Note that the lipoprotein B-100 is what gives LDL its structure.
What happens to the emulsified 2-monoacylglycerol once it enters an enterocyte in the bowel?
A triglyceride is reformed in the smooth ER, it is packed with phospholipids and apoB in the golgi to form a chylomicron. The chylomicron leaves the enterocyte via lactiles and enters the lymphatics. From the lymphatics it enters the left subclavian vein.
How does the ingested triacylglycerol get digested by lipases when it is protected by the bile salt micelle?
A colipase binds with lipase to break open the micelle. Then the enzyme goes to work making fatty acids and monoglycerides.
How does our body break down the content in the center of an LDL particle?
At the center of an LDL particle, you find cholesterol ester. Cholesterol esterae pops off the ester to form cholesterol
