Biochemistry Ch 9. Carbohydrate Metabolism Flashcards
Glucose transporters
GLUT 2 and GLUT 4
GLUT 2
Found in the liver (for glucose storage) and pancreatic beta cells (as part of the glucose sensor), has a high Km
GLUT 4
Found in adipose tissue and muscle and is stimulated by insulin, has a low Km
Glycolosis
Occurs in the cytoplasm of all cells and does not require oxygen, yields 2 ATP per molecule of glucose
Glucokinase
Converts glucose to glucose 6-phosphate, present in the pancreatic beta-islet cells as part of the glucose sensor and is responsive to insulin in the liver
Hexokinase
Converts glucose to glucose 6-phophase in peripheral tissues
Phosphofructokinase-1
PFK-1 - phosphorylates fructose 6-phosphate to fructose 1,6 bisphosphate in the rate limiting step of glycolysis, activated by AMP and fructose 2,6-bisphosphate, inhibited by ATP and citrate
Phosphofructokinase-2
PFK-2 - produces the F2,6-BP that activates PFK-1, activated by glucose and inhibited by glucagon
Glyceradehyde-3-phosphate dehydrogenase
Produces NADH which can feed into the electron transport chain
3 - phosphoglycerate kinase
Performs substrate level phosphorylation, placing an inorganic phosphate (Pi) onto ADP to form ATP
Pyruvate kinase
Performs substrate level phosphorylation, placing an inorganic phosphate (Pi) onto ADP to form ATP
Substrate-level phosphorylation
When an inorganic phosphate (Pi) in placed onto ADP to form ATP
NADH product of glycolysis
Oxidized by the mitochondrial electron transport chain when oxygen is present, if oxygen absent, the NADH is oxidized by cytoplasmic lactate dehydrogenase
Enzymes that catalyze irreversible reactions of glycolysis??
Glucokinase/hexokinase, PFK-1, and pyruvate kinase
Lactate dehydrogenase
Will oxidize NADH if oxygen is absent
Galactose
Comes from lactose in milk, trapped in the cell by galactokinase and converted to glucose 1-phosphate via galactose-1-phosphate uridyltransferase and an epimerase
Fructose
Comes from honey, fruit, and sucrose (common table sugar), it is trapped in the cell by fructokinase and cleaved by aldolase B to form glyceraldehyde and dHAP
Pyruvate dehydrogenase
Refers to a complex of enzymes that convert pyruvate to acetyl-CoA, stimulated by insulin and inhibited by acetyl-CaoA
Glycogenesis
Glycogen synthesis, the production of glycogen using two main enzymes: glycogen synthase and branching enzyme
Glycogen synthase
Creates alpha-1,4 glycosidic links between glucose molecules, activated by insulin in liver and muscle
Branching enzyme
Moves a block of oligoglucose from one chain and adds it to the growing glycogen as a new branch using an alpha-1,6 glycosidic link
Glycogenolysis
The breakdown of glycogen using two main enzymes: Glycogen phosphorylase and debranching enzyme
Glycogen phosphorylase
Removes single glucose 1-phosphate molecules by breaking alpha-1,4 glycosidic links, in the liver activated by glucagon to prevent low blood sugar, in exercising skeletal muscle, it is activated by epinephrine and AMP to provide glucose for the muscle itself
Debranching enzyme
Moves a block of oligoglucose from one branch and connects it to the chain using an alpha-1,4 glycosidic link, it also removes the branch point, which is connected via an alpha-1,6 glycosidic link, releasing a free glucose molecule
Gluconeogenesis
Occurs in both the cytoplasm and mitochondria predominantly in the liver, there is a small contribution from the kidneys, most of it is simply the reverse of glycolysis using the same enzymes, three irreversible steps bypassed by enzymes: pyruvate carboxylase, fructose-1,6-bisphosphatase, and glucose-6-phosphate
Pyruvate carboxylase
Converts pyruvate into oxaloacetate, which is converted to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase (PEPCK), these to enzymes bypass pyruvate kinase, activated by acetyl-CoA from beta-oxidation
Phosphoenolpyruvate carboxykinase
PEPCK - converts oxaloaetate to phosphoenolpyruvate, works with pyruvate cacrboxylase to bypass pyruvate kinase, activated by glucagon and cortisol
Fructose-1,6-bisphosphatase
Converts fructose 1,6-bisphosphate to fructose 6-phosphate, bypassing phosphofructokinase-1, rate limiting step of gluconeogenesis, activated by ATP directly and glucagon indirectly (via decreased levels of fructose 2,6-bisphosphate), inhibited by AMP directly and insulin indirectly (via increased levels of fructose 2,6-bisphosphate)
Glucose-6-phosphatase
Converts glucose 6-phosphate to free glucose, bypassing glucokinase, found only in the endoplasmic reticulum of the liver
Pentose phosphate pathway
PPP - aka hexose monophosphate shunt, occurs in the cytoplasm of most cells, generating NADPH and sugars for biosynthesis (derived from ribulose 5-phosphate), rate limiting enzyme is glucose-6-phosphate dehydrogenase
Glucose-6-phosphate dehydrogenase
Rate limiting enzyme in pentose phosphate pathway, activated by NADP+ and insulin and inhibited by NADPH
