Chapter 9 and 10: Carbohydrate Metabolism Flashcards
GLUT 2
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; GLUT 2 captures the excess glucose primarily for storage
What happens when the glucose concentration drops below the Km for GLUT 2?
Much of the remainder leaves the liver and enters the peripheral circulation
What is the Km for GLUT 2?
Quite high (~15 mM)
What does the liver do?
The liver will pick up excess glucose and store it only after a meal, when blood glucose levels are high; in the β-islet cells of the pancreas, GLUT 2, along with the glycolytic enzyme glucokinase, serves as the glucose sensor for insulin release
GLUT 4
Adipose tissue and muscle; responds to the glucose concentration in the peripheral blood
What does insulin do?
The rate of glucose transport is increased in GLUT 4; insulin stimulates the movement of additional GLUT 4 transporters to the membrane by a mechanism involving exocytosis
What is the Km of GLUT 4?
5 mM (normal glucose concentration in the blood is 5.6 mM or between 4-6 mM); the transporters become saturated when blood glucose levels are just a bit higher than normal
What does muscle store excess glucose as? Adipose tissue?
Glycogen; dihydroxyacetone phosphate (DHAP) —> glycerol phosphate to store incoming fatty acids as triacylglycerols
Glycolysis
Cytoplasmic pathway that converts glucose into two pyruvates, releasing a modest amount of energy captured in two SLP and one oxidation reaction; if the cell has mitochondria and oxygen, the energy-carriers produced in glycolysis (NADH) can feed into the aerobic respiration pathway to generate energy for the cell; also provides intermediates for other pathways
What are GLUT transporters specific for?
Glucose (not phosphorylated glucose AKA glucose-6-phosphate); this traps the glucose inside the cell once it has entered via facilitated diffusion or active transport
Hexokinase
Widely distributed in tissues and is inhibited by its product glucose-6-phosphate
Glucokinase
Found only in liver cells and pancreatic β-islet cells; in the liver, glucokinase is induced by insulin
Where is hexokinase found?
Present in most tissues
Where is glucokinase found?
Present in hepatocytes and pancreatic β-islet cells (along with GLUT 2, acts as the glucose sensor)
What is the Km of hexokinase? Glucokinase?
Low Km (reaches maximum velocity at a low [glucose]); high Km (acts on glucose proportionally to its concentration)
What is hexokinase inhibited by?
Glucose-6-phosphate
What is glucokinase induced by?
Insulin in hepatocytes
Phosphofructokinase 1 (PFK-1)
PFK-1 is the rate-limiting enzyme and main control point in glycolysis; phosphorylates fructose 6-phosphate to form fructose 1,2-bisphosphate using ATP
What inhibits PFK-1?
ATP and citrate; activated by AMP (when the cell has high ATP)
Phosphofructokinase 2
PFK-2 converts a tiny amount of fructose 6-phosphate to fructose 2,6-bisphosphate (F2,6-BP); F2,6-BP activates PFK-1; insulin stimulates PFK-1 and glucagon inhibits PFK-1 via an indirect mechanism involving PFK-2 and fructose 2,6-bisphosphate
What does activation of PFK-2 do?
PFK-2 is found mostly in the liver; by activating PFK-1, it allows these cells to override the inhibition caused by ATP so that glycolysis can continue, even when the cell is energetically satisfied; the metabolites of glycolysis can thus be fed into the production of glycogen, fatty acids, and other storage molecules rather than just being burned to produce ATP
Glyceraldehyde-3-phosphate dehydrogenase
Catalyzes an oxidation and addition of inorganic phosphate (Pi) to its substrate, glyceraldehyde 3-phosphate; results in the production of a high-energy intermediate 1,3-bisphosphoglycerate and the reduction of NAD+ to NADH
3-Phosphoglycerate Kinase
Transfers the high-energy phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate; substrate-level phosphorylation
Pyruvate kinase
The last enzyme in aerobic glycolysis, it catalyzes a substrate-level phosphorylation of ADP using the high-energy phosphoenolpyruvate (PEP); activated by fructose 1,6-bisphosphate from the PFK-1 reaction; referred to as feed-forward activation, meaning that the product of an earlier reaction of glycolysis stimulates or prepares a later reaction in glycolysis
What is the key fermentation enzyme in mammalian cells?
In the absence of oxygen, fermentation will occur; lactate dehydrogenase which oxidizes NADH to NAD+, replenishing the oxidized coenzyme for glyceraldehyde-3-phosphate dehydrogenase
What is fermentation in yeast cells?
Conversion of pyruvate to ethanol and CO2; result is the same as mammalian cells: replenishing NAD+
What are the important intermediates of 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, which can then be converted to glycerol 1,3-bisphosphoglycerate (1,3-BPG) and phosphoenolpyruvate (PEP) are high-energy intermediates used to generate ATP by SLP (the only ATP gained in anaerobic respiration)
Irreversible enzymes
Keeps the pathway moving in one direction; glucokinase/hexokinase; PFK-1; pyruvate kinase
What enzymes do RBCs have in particular?
Bisphophoglycerate mutase - produces 2,3-BPG from 1,3-BPG in glycolysis; (mutases are enzymes that move a functional group from one place in a molecule to another); 2,3-BPG binds allosterically to the β-chains of HbA and decreases its affinity for oxygen
Irreversible steps of glycolysis
How Glycolysis Pushes Forward the Process: Kinases Hexokinase Glucokinase PFK-1 Pyruvate kinase
What are the important enzymes in galactose metabolism?
Galactokinase - phosphorylates galactose to trap it in the cell
Galactose-1-phosphate uridyltransferase - converts galactose 1-phosphate to glucose 1-phosphate
Epimerase - enzyme that catalyzes the conversion of one sugar epimer to another
Fructokinase
Phosphorylates fructose to trap it in the cell
Aldolase B
Cleaves fructose 1-phosphate into glyceraldehyde and DHAP
Uses of acetyl-CoA
Entry into the citric acid cycle if ATP is needed or for fatty acid synthesis if sufficient ATP is present
Pyruvate dehydrogenase complex (PDH)
Irreversible; activated by insulin in the liver; complex of enzymes carrying out multiple reactions in succession
What are the three possible fates of pyruvate?
Conversion to acetyl-CoA by PDH, conversion to lactate by lactate dehydrogenase, or conversion to oxaloacetate by pyruvate carboxylase
What cofactors and coenzymes are required by PDH?
Thiamine pyrophosphate, lipoic acid, CoA, FAD, and NAD+
What is PDH inhibited by?
Acetyl-CoA; build up of acetyl-CoA shifts pyruvate conversion into oxaloacetate
Glycogen
Branched polymer of glucose, represents a storage form of glucose; stored in the cytoplasm as granules - central protein cores with polyglucose chains radiating outward to form a sphere; glycogen granules composed entirely of linear chains have the highest density of glucose near the core; if the chains are branched, the glucose density if highest at the periphery of the granule, allowing more rapid release of glucose on demand
Purpose of glycogen in the liver? Muscle?
Source of glucose that is mobilized between meals to prevent low blood sugar; stored as an energy reserve for muscle contraction
Glycogenesis
Synthesis of glycogen granules; begins with a core protein called glycogenin; glucose addition to a granule begins with glucose 6-phosphate which is converted to glucose 1-phosphate which is activated by coupling to a molecule of uridine diphosphate (UDP) which permits integration into the glycogen chain by glycogen synthase; activation occurs when glucose 1-phosphate interacts with uridine triphosphate (UTP), forming UDP-glucose and pyrophosphate
Glycogen synthase
Rate-limiting enzyme of glycogen synthesis and forms the α-1,4 glycosidic bond found in the linear glucose chains of the granule; stimulated by glucose 6-phosphate and insulin; inhibited by epinephrine and glucagon through a protein kinase cascade that phosphorylates and inactivates the enzyme
Branching enzyme (glycosyl α-1,4:α-1,6 transferase)
Hydrolyzes one of the α-1,4 bonds to release a block of oligoglucose (a few glucose molecules bound together in a chain) which is then moved and added in a slightly different location; forms an α-1,6 bond to create a branch
Glycogen phosphorylase
Breaks α-1,4 glycosidic bonds, releasing glucose 1-phosphate from the periphery of the granule; cannot break α-1,6 bonds and therefore stops when it nears the outermost branch points; activated by glucagon in the liver and AMP and epinephrine in skeletal muscle; inhibited by ATP
Debranching enzyme (glucosyl α-1,4:α-1,4 transferase and α-1,6 glucosidases)
Breaks an α-1,4 bond adjacent to the branch point and moves the small oligoglucose chain that is released to the exposed end of the other chain; forms a new α-1,4 bond; hydrolyzes the α-1,6 bond, releasing a single free glucose (only free glucose released in glycogenolysis)
Isoforms
Slightly different versions of the same protein
Gluconeogenesis
Performed by the liver and kidneys (but mainly the liver); pathways are promoted by glucagon and epinephrine which act to raise blood sugar levels and are inhibited by insulin which acts to lower blood sugar levels; after 24 hours of starvation, it becomes the sole source of glucose