Carbohydrate metabolism I Flashcards
This glucose transporter:
-Found in hepatic and pancreatic cells
-Has a low affinity for glucose
-Appears when glucose levels are high (Km~15 mM)
-Acts as sensor for insulin release by interacting with pancreatic B-cells
-Has a low affinity for glucose
GLUT 2
This glucose transporter:
-Found in adipose tissue and skeletal muscles
-Responds to glucose levels in peripheral blood
-Is saturated when glucose levels are only slightly elevated (Km~5 mM)
-Is modulated by insulin, where insulin increases the amount of transporters brought to the cell membrane
GLUT 4
The end products of glycolysis are: __ ATP, __ pyruvate and __ NADH
2, 2, 2
This glycolysis enzyme:
-Prevents glucose from leaving the cell by phosphorylating it
-Is irreversible
-Is widely distributed in tissue and has a high affinity for glucose
-Is inhibited by it’s product glucose-6-phosphate
Hexokinase
This glycolysis enzyme:
-Prevents glucose from leaving the cell by phosphorylating it
-Is irreversible
-Is found only in liver and pancreatic B-cells, and has a low affinity for glucose
-Is induced by insulin (in the liver)
Glucokinase
Compare and contrast hexokinase and glucokinase?
Both are involved in the phosphorylation of glucose so that it cannot leave once it enters the cell
Hexokinase is found in every cell because it helps with normal glycolysis and is inhibited by glucose-6-phosphate, its product. Associate it with GLUT 4, in that it’s saturated at normal glucose levels and has a high affinity for glucose
Glucokinase is found only in liver and pancreatic B-cells. It only helps with glycolysis in the liver and is stimulated by the presence of insulin (which indicates the levels of glucose are increased). Think of it like GLUT 2, where it isn’t saturated at normal glucose levels and has a low affinity for glucose
This phosphofructokinase:
-Is a rate limiting enzyme
-Is irreversible
-Turns fructose-6-phosphate into fructose-1,6-biphosphate
-Is inhibited by ATP and citrate
-Is activated by AMP
PFK 1
Essentially determines whether or not glycolysis needs to proceed or if the cell has enough energy for the time being
Why is PFK-1 activated by AMP but inhibited by citrate and ATP?
PFK-1 is involved in an energy making process, so if the cell already has enough energy (indicated by levels of ATP and citrate) there’s no need for glycolysis to continue. Similarly, if the cell only has AMP, it’s lacking in energy and glycolysis is necessary
This phosphofrutokinase:
-Activates PFK-1
-Allows PFK-1 to override the presence of ATP and citrate and keep glycolysis active
PFK-2
This dehydrogenase:
-Adds phosphate to glyceraldehyde-3-phosphate to form 1,3-biphosphoglycerate
-Forms NADH from NAD+, which is then brought into the electron transport chain
(Has a name based on the molecule it’s adding to)
Glyceraldehyde-3-phosphate dehydrogenase
This kinase:
-Forms ATP from ADP and 1,3-biphosphoglycerate
-Is the only means of ATP generation in anaerobic tissue
-Is irreversible
3-Phosphoglycerate kinase
This kinase:
-Creates ATP from ADP and phosphoenolpyruvate
-Is activated by PFK-1
-Is irreversible
Pyruvate kinase
_____ occurs when ADP is directly phosphorylated to ATP via a high energy intermediate
Substrate-level phosphorylation
What are the four irreversible enzymes of glycolysis?
The glucokinases (hexo and gluco)
PFK-1
3-phosphoglycerate kinase
Pyruvate kinase
This pathway occurs anaerobically to replenish NAD+ that can then be used to continue glycolysis
Fermentation
This glycolysis intermediate:
-Is nicknamed DHAP
-Is found in hepatic and adipose tissue
-Forms the backbone of triacylglycerol
Dihydroxyacetone
These two glycolysis intermediates:
-Form ATP from substrate level phosphorylation
-Are the only anaerobic source of ATP
1,3-biphosphoglycerate (1,3-BPG) and phosphoenolpyruvate
In the galactose metabolism, galactose is transformed into ____
Glucose-1-phosphate
In the fructose metabolism, fructose is transformed into ____
DHAP and glyceraldehyde
This complex of enzymes:
-Can enter the citric acid cycle as either acetyl-CoA if ATP is needed, or become fatty acids if there’s already enough ATP
-Is activated by insulin (when it’s in the liver)
-Is inhibited by acetyl CoA
Pyruvate dehydrogenase
Glycogen is synthesized and degraded in ____ and ___ muscle, and is stored in the ____ as granules
Liver and skeletal muscle
Cytoplasm
This pathway creates new glycogen
Glycogenesis
This glycogenesis enzyme:
-Forms the linear alpha 1,4 bond in glycogen
-Is the rate limiting enzyme
-Is stimulated by insulin and glucose-6-phosphate
-Is inhibited by ephinephrine and glucagon
Glycogen synthase
This glycogenesis enzyme:
-Creates the alpha 1,6 branches in glycogen by hydrolyzing areas of the linear 1,4 chain and moving them into the branched position at carbon 6
Branching enzyme
This pathway breaks down glycogen
Glycogenolysis
This glycogenolysis enzyme:
-Breaks alpha 1,4 bonds but cannot break 1,6 branches
-Is the rate limiting step
-Is activated by glucagon in the liver and by AMP and epinephrine in the skeletal muscle
-Is inhibited by ATP
Glycogen phosphorylase
This glycogenolysis enzyme breaks the 1,6 branching in two steps: first by breaking the 1,4 bond closest to the branch and placing it back on the linear chain and second by hydrolyzing the 1,6 bond to release the single free glucose left from the branch
Debranching enzyme
This pathway:
-Forms new glucose
-Is inhibited by insulin
-Is activated by glucagon and epinephrine
-Activates or increases during the the first 12 hours of fasting
Gluconeogenesis
____ amino acids can turn into intermediates for gluconeogenesis, except for lysine and leucine
Glucogenic AA’s
____ amino acids can turn into ketone bodies and serve as alternative fuel during starvation
Ketogenic AA’s
This important gluconeogenesis enzyme:
-Is mitochondrial
-Forms Oxaloacetate as a product
-Is activated by the beta oxidation of acetyl-CoA
Pyruvate carboxylase
This important gluconeogenesis enzyme:
-Is in the cytoplasm
-Is induced by glucagon and cortisol
-Circumvents pyruvate kinase by converting pyruvate into phosphoenolpyruvate
Phosphoenolpyruvate carboxykinase
This important gluconeogenesis enzyme:
-Is the rate limiting step
-Circumvents PFK-1 by creating fructose-6-phosphate
Fructose-1,6-biphosphatase
This important gluconeogenesis enzyme:
-Is only found in the lumen of the rough ER
-Circumvents hexo and glucokinase by tranposting glucose-6-phosphate to the ER and releasing it as glucose
Glucose-6-phosphatase
This pathway:
-Occurs in the cytoplasm of all cells
-Has two major functions: to act as a source of NADPH and ribose-5-phosphate for nucleotide synthesis
Pentose phosphate pathway
(Also called the hexose monophosphate shunt)
This enzyme of the pentose phosphate pathway:
-Is the rate limiting enzyme
-Is induced by insulin
-Is activated by NADP+
-Is inhibited by NADPH
Glucose-6-phosphate dehydrogenase
What is the difference between NADH and NADPH?
NADH indirectly produces ATP by feeding into the electron transport chain. As NAD+, it’s a powerful oxidizing agent and electron acceptor
NADPH acts an electron donor and reducing agent, and is required for fatty acid and cholesterol synthesis, bleach production in some white blood cells, and maintaining glutathione to prevent free radicals
