Carbohydrates - Digestion, Absorption, & Glycolysis Flashcards
Where is the reducing end of a sugar located? What is the function of the reducing end?
The reducing end is the end of a sugar with a free aldehyde/ketone group. The reducing end is the active end of the sugar, which donates electrons in the formation of a bond.
What enantiomer form of sugars is more abundant in humans?
D enantiomer
Why do cancer cells produce more lactate from pyruvate even under aerobic conditions? What is this phenomenon called?
Warburg Effect - Provides more intermediary molecules for anabolism and cell growth. Increased acidity also helps cancer cells evade the immune response.
Hexokinase and glucokinase catalyze the same reaction, glucose to glucose-6-phosphate. What is the difference between the enzymes?
Hexokinase is expressed in most tissues and inhibited by glucose-6-phosophate.
Glucokinase is expressed in liver and pancreatic beta cells and is inhibited by fructose-6-phosphate.
What are the effects of insulin and glucagon on pyruvate kinase activity in the liver?
Insulin activates a protein phosphatase, which dephosphorylates and activates the pyruvate kinase for glycolysis to proceed. Glucagon activates protein kinase A, which phosphorylates and deactivates the pyruvate kinase.
Why do starch and glycogen have multiple branches and non-reducing ends?
Branches with non-reducing ends provide multiple locations for glucose to be released from the starch/glycogen as opposed to amylose which is linear in form.
What is the result of Aldolase B deficiency?
Buildup of fructose-1-phosphate that leads to trapped phosphate, reduced ATP/ADP ratio, and liver damage
What differentiates alpha and beta sugars?
In alpha sugars, the hydroxyl group at the 1C is pointed down. In beta sugars, the hydroxyl group at 1C is pointed up.
What glucose transporter is constitutively expressed in most cells? What is the general Km of this transporter?
GLUT1 - low Km indicating high glucose sensitivity
At what steps is glycolysis regulated?
Hexokinase, Phosphofructokinase, Pyruvate kinase
Why do citrate and ATP serve as inhibitors of PFK1?
Citrate is the first molecule of TCA. High levels of citrate and ATP indicate that the cell has adequate energy and intermediates are beginning to build up.
What transporter transports fructose? Where is this transporter located?
GLUT5 - small intestine
Under anaerobic conditions, pyruvate cannot undergo oxidative phosphorylation. What happens to the molecule instead?
Pyruvate is converted to lactate. This allows for the generation of NAD+ which can be used for other glycolytic processes.
What symptoms are associated with pyruvate kinase deficiency?
Hemolytic anemia, pale skin, fatigue, lethargy - RBCs are most affected
How is glucokinase activity regulated?
When glucose levels are high, glucose is transported into liver and pancreatic beta cells via the GLUT2 transporter. The presence of glucose enables glucokinase to dissociate from its regulatory protein in the nucleus and translocate to the cytoplasm. When fructose-6-phosphate levels become very high, this indicates the cell no longer needs glucose for the PPP or glycogen synthesis. Fructose-6-phosphate then deactivates the glucokinase.
What is the function of the SGLT1?
The SLGT1 is a glucose cotransporter that facilitates glucose absorption from the lumen. Glucose transport is coupled with the movement of Na+ down its concentration gradient. The concentration gradient is maintained by the Na+/K+ pump.
Fructose-2,6-bisphosphate is not a glycolytic intermediate. What is its role in glycolysis?
F-2,6-bisP is an activator of PFK1.
F-6-P is converted to F-2,6-bisP by PFK2. High levels of F-2,6-bisP activates PFK1 to convert fructose-6-P to F-2,6-bisP.
Describe the process of carbohydrate digestion beginning at ingestion.
Food is mechanically broken down by the teeth. In addition, salivary amylase breaks a1-4 bonds of starches. Sucrose and lactose remain intact.
In the stomach, starches exist as 6-10 sugars and are called alpha-dextrins. Sucrose and lactose remain intact.
As the carbohydrates enter the intestine, the pancreas releases bicarbonate to raise pH and reactivate enzymes. Pancreatic amylase is also released.
In the small intestine, alpha-dextrins are further broken down to maltose and isomaltose. Maltase and isomaltase convert maltose to glucose.
Also in the small intestine, sucrase breaks down sucrose to glucose and fructose. Lactase breaks down lactose to glucose and galactose.
F-2,6-bisP levels are managed by PFK2 and the Fructose-2,6-bisphosphotase. How is enzymatic function regulated?
When insulin levels are high, PFK2 is dephosphorylated and activated while the F-2,6-bisPase is dephosphorylated and inactivated. This results in production of F-2,6-bisP.
When insulin levels fall and glucagon levels increase, PFK2 is phosphorylated and inactivated while F-2,6-bisPase is phosphorylated and activated. This results in the conversation of F-2,6-bisP to F-6-P.
