Glucose Transport Flashcards
(32 cards)
GLUT 2
A low-affinity transporter in hepatocytes and pancreatic cells. After a meal, blood traveling through the hepatic portal vein from the intestine is rich in glucose – captures the excess glucose primarily for storage.
GLUT 4
In adipose tissue and muscle and responds to the glucose concentration in peripheral blood.
The transporter is saturated when blood glucose levels are just a bit higher than normal.
Glycolysis
A cytoplasmic pathway that converts glucose into 2 pyruvate molecules, releasing a modest amount of energy captured in 2 substrate-level phosphorylations and one oxidation reaction.
Hexokinase
- Present in most tissues
- Low Km (reaches max velocity at low [glucose])
- Inhibited by glucose-6-phosphate
Glucokinase
- Present in hepatocytes and pancreatic Beta-islet cells
- High Km
- Induced by insulin in hepatocytes
Phosphofructokinase-1 (PFK-1)
The rate-limiting enzyme and main control point in glycolysis: inhibited by ATP and citrate, and activated by AMP.
Insulin stimulates and glucagon inhibits PFK-1 in hepatocytes by an indirect mechanism involving PFK-2 and fructose 2,6-biphosphate.
Why is activation of PFK-1 by insulin important
It allows those cells to override the inhibition caused by ATP so that glycolysis can continue, even when the cell is energetically satisfied.
Glyceraldehyde-3-phosphate dehydrogenase
Catalyzes an oxidation and addition of inorganic phosphate (Pi) to its substrate, glyceraldehyde 3-phosphate. Results in high-energy intermediate 1,3-bisphosphoglycerate and the reduction of NAD+ to NADH.
3-phosphoglycerate kinase
Transfers the high-energy phosphate from 1,3-bisphoglycerate to ADP, forming ATP and 3-phosphoglycerate –> this step is substrate-level phosphorylation
Pyruvate kinase
Catalyzes substrate-level phosphorylation of ADP using high-energy substrate phosphoenolpyruvate (PEP).–> activated by fructose 1,6-biphosphate from the PFK-1 reaction
Why is activation of pyruvate kinase feed-forward activation
Product of an earlier reaction of glycolysis stimulates, or prepares, a later reaction in glycolysis.
Lactate dehydrogenase
Key fermentation enzyme in mammalian cells: reduces pyruvate to lactate and oxidizes NADH to NAD+, replenishing NAD+ supply needed for glycolysis.
Dihydroxyacetone phosphate (DHAP)
Used in hepatic and adipose tissue for triacylglycerol synthesis.
Formed from fructose 1,6-bisphosphate –> can be isomerized to glycerol 3-phosphate –> converted to glycerol –> backbone of triacylglycerols
What are the irreversible enzymes of glycolysis
How Glycolysis Pushes Forward the Process: Kinases Hexokinase Glucokinase PFK-1 Pyruvate Kinase
What is the only pathway for ATP production in erythrocytes
Anaerobic glycolysis, yielding a net 2 ATP per glucose.
Effect of 2,3 - BPG on red blood cells
Binds allosterically to the beta-chains of hemoglobin A (HbA) and decreases its affinity for oxygen.
Adaptation to high altitudes involve:
- Increased respiration
- Increased oxygen affinity for hemoglobin (initial)
- Increased rate of glycolysis
- Increased [2,3-BPG] in RBC (over a 12-24 hour period)
- Normalized oxygen affinity for hemoglobin restored by the increased level of 2,3-BPG
- Increased hemoglobin
What 2 monosaccharides make up lactose
Glucose and galactose
What 2 monosaccharides make up sucrose
Glucose and fructose
Glycogenesis
The synthesis of glycogen granules.
Glycogen synthase
The rate-limiting enzyme of glycogen synthesis and forms the alpha-1,4 glycosidic bond found in the linear glucose chains of the granule.
Branching enzyme
Responsible for introducing the alpha-1,6-linked branches into the granule as shown.
Glycogenolysis
The process of breaking down glycogen
Glycogen phosphorylase
Rate-limiting enzyme of glycogenolysis. Breaks alpha-1,4, glycosidic bonds, releasing glucose 1-phosphate from the periphery of the granule. Activated by glucagon in the liver
