Biochemistry Lecture 14 - Carbohydrate Absorption Flashcards
Do people that have undergone trauma (e.g. burn victims, post-surgery, etc.) have higher calorie requirements?
Oh for sho.
How many calories per gram are protein, carbs, and fat, respectively?
4, 4, and 9 (fat)
Where does digestion start? What enzyme is present there?
Mouth - alpha amylase.
What links can amylase break? What can it NOT break?
Alpha 1,4 links. Cannot break alpha 1,6 linkages.
What are the three products of alpha amylase digestion?
Maltose, maltotriose, and limit dextrins.
Does carbohydrate digestion occur in the stomach? Why or why not?
No, the acidity of the stomach inactivates amylase.
Aside from salivary glands, where is amylase produced? Where does amylase from this organ function?
The pancreas. Amylase is spat out from the pancreas to the small intestine to continue carb digestion.
Name four enzymes that digest carbs which are found in the intestinal mucosa of the brush border.
Maltase, isomaltase, sucrase, lactase.
Which enzyme breaks the limit dextrins? Which links does it cleave?
Isomaltase cleaves glucose-glucose alpha 1,6 linkages.
What linkages does sucrase cleave?
glucose-alpha (1,2) beta fructose linkages in sucrose.
What linkages does lactase cleave?
galactose-beta (1,4) glucose linkages in lactose.
What plant carb is not digestible by humans? Why?
Cellulose has beta (1,4) linkages and we do not have an enzyme to cleave those
Why does bacterial metabolism in the lower gut make us feel like shit and give us diarrhea?
Bacteria metabolize undigested stuff such as lactose (in a lactase deficient person), giving off lactic and fatty acids as well as H2 and CO2 gases. The osmolites draw H2O via osmosis into the intestinal lumen. So you get watery diarrhea and gas.
What are the beneifts of undigested polysaccharides?
This is dietary fiber. It affects gastric emptying, intestinal mobility/transit time, contributes to stool bulk, and may adsorb carcinogens and cholesterol.
Does lactose intolerance go away as one ages?
No, the opposite. It gets worse.
What ethnic groups commonly have lactose intolerance?
Africans and Asians
Can lactose intolerance be temporary?
Yes, it can sometimes be caused by illnesses that injure the intestinal mucosa.
Describe the process of glucose absorption in the gut.
The Na+/Gluc transporter co-transports both glucose and Na+ into the cell from the intestinal lumen. Glucose then goes through a Gluc transporter (facilitated diffusion) into the blood, and the Na+ is pumped out of the cell using the Na+/K+ pump, which uses ATP.
Which GLUT transporter is insulin-responsive? Is this transporter found in the liver?
The GLUT 4 transporter is insulin responsive and it is NOT found in the liver! GLUT2 is in the liver (and in pancreatic beta cells, too)
Where are GLUT4 transporters found?
Muscle, fat, and white blood cells!
How does insulin affect GLUT4 transporters?
When insulin goes into a muscle, fat, or WB cell, the GLUT4 transporters are recruited to the surface to take glucose in.
Where are the only two places where active transport is used to transport glucose?
Intestine and kidney.
Where do carbs go first after being absorbed?
The liver via portal circulation.
What factors affect the glycemic index?
- Sugar content (eg glucose vs. fructose)
- Type of starch (eg amylose vs amylopectin)
- Physical barriers (eg bran)
- Vistocity of soluble fiber
- Fat and protein content of accompanying foods (affects gastric transport)
- Acid content (affects gastric transport)
- Food processing (eg rolled vs. quick oats)
How do cells keep glucose in? What enzymes do this?
Cells phosphorylate glucose to keep them in. The enzymes that do this are hexokinase and glucokinase.
Where is glucokinase found? Is it product inhibited?
It is found in the liver and pancreatic beta cells. It is NOT product-inhibited.
Where are hexokinases found? Is it product inhibited?
It is ubiquitous (found in all cells), and it IS product inhibited.
Of the isoenzymes glucokinase and hexokinase, which enzyme has a lower Km and Vmax? What is the benefit of this?
Hexokinase has a low Km and Vmax. Its Km is much lower than normal [glucose] in cells so it is constantly going full-speed. This makes it less sensitive to fluctuations in glucose concentration.
What are the benefits of glucokinase’s kinetics?
Glucokinase works at high glucose concentrations in the liver and can handle high glucose concentrations to convert to g-6-P for the cell.
What is the defect in MODY patients?
They have a mutation in the TFs that regulate glucokinase or a mutation in the glucokinase gene itself (more common).
How does G-6-P raise insulin levels?
G-6-P is used in glycolysis to generate ATP. ATP regulates the ATP-sensitive K+ channel, which causes membrane depolarization. Ca2+ then flows into the cell, which causes secretion of insulin-containing secretory vesicles.
What is Tauri’s disease?
Defect in the PFK that is used in muscle. Fructose makes these patients feel better because it bypasses this step.
