Ch 9: Carbohydrate Metabolism 1 Flashcards
GLUT 2
low affinity transporter in hepatocytes and pancreatic cells; found in the liver (for glucose storage) and pancreatic B-islet cells (as part of the glucose sensor); has a high Km
GLUT 4
glucose transport; found in adipose tissue and muscle and is stimulated by insulin; has a low Km
Glycolysis
occurs in cytoplasm of all cells, and does not require oxygen; it yields 2 ATP per molecule of glucose
Glucokinase
important glycolysis enzyme which converts glucose to glucose 6-phosphate. present in the pancreatic B-islet cells as part of the glucose sensor and is responsive to insulin in the liver
Hexokinase
important glycolysis enzyme converts glucose to glucose 6-phosphate in peripheral tissues
Phosphofructokinase-1 (PFK-1)
important glycolysis enzyme that phosphorylates fructose 6-phosphate to fructose 1,6-biphosphate in the rate-limiting step of glycolysis. PFK-1 is activated by AMP and fructose 2,6-biphosphate (F2,6-BP) and is inhibited by ATP and citrate
Phosphofructokinase-2 (PFK-2)
important glycolysis enzyme produces the F2,6-BP that activates PFK-1. It is activated by insulin and inhibited by glucagon
Glyceraldehyde-3-phosphate dehydrogenase
important glycolysis enzyme produces NADH, which can feed into the electron transport chain
3-phosphoglycerate kinase & pyruvate kinase perform
important glycolysis enzyme that performs substrate-level phosphorylation
substrate-level phosphorylation
placing an inorganic phosphate (Pi) onto ADP to form ATP
4 enzymes that catalyze irreversible reactions in glycolysis are
glucokinase
hexokinase
PFK-1
pyruvate kinase
when oxygen/mitochondia are present, NADH produced in glycolysis is
oxidized by mitochondrial electron transport chain
when oxygen/mitochondia are NOT present, NADH produced in glycolysis is
oxidized by cytoplasmic lactate dehydrogenase
examples: red blood cells, skeletal muscle (during short, intense bursts of exercise), and any cell deprived oxygen
Galactose
comes from lactose in milk; trapped in cell by galactokinase and converted to glucose 1-phosphate via galactose-1-phosphate uridyltransferase and epimerase
galactose-1-phosphate uridyltransferase
converts galactose to glucose 1-phosphate
Fructose
comes from honey, fruit, and sucrose (table sugar); trapped in cell by fructokinase, and cleaved by aldolase B to form glyceraldehyde and DHAP
Pyruvate dehydrogenase
refers to a complex of enzymes that converts pyruvate to acetyl-CoA; stimulated by insulin and inhibited by acetyl-CoA
Glycogenesis (glycogen synthesis)
production of gylcogen using two main enzymes: glycogen synthase and branching enzyme
Glycogen synthase
enzyme in glycogenesis that creates alpha-1,4 glycosidic linkages between glucose molecules. it is 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
breakdown of glycogen using two main enzymes: glycogen phosphorylase and debranching enzyme
Glycogen phosphorylase
enzyme in glycogenolysis which removes single glucose 1-phosphate molecules by breaking alpha-1,4 glycosidic links. in the liver, it is 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 branchpoint, which is connected via an alpha-1,6 glycosidic link, releasing a free glucose molecule.
Gluconeogenesis
liver maintains glucose levels in blood during fasting through this (or glycogenolysis); occurs in both the cytoplasm and mitochondria, predominantly in the liver; small contribution from the kidneys
Gluconeogenesis is the reverse of
glycolysis
Gluconeogenesis must surpass the 3 irreversible steps of glycolysis with these different enzymes:
pyruvate carboxylase & phosphoenolpyruvate carboxykinase (PEPCK)
fructose-1,6-bisphosphatase
glucose-6-phosphatase
Pyruvate carboxylase
& PEPCK
converts pyruvate into oxaloacetate, which is converted to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase (PEPCK); together these 2 enzymes bypass pyruvate kinase
Pyruvate carboxylase is activated by acetyl-CoA from B-oxidation
PEPCK activated by glucagon and cortisol
Fructose-1,6-bisphosphatase
converts fructose 1,6-bisphosphate to fructose 6-phosphate, bypassing phosphofructokinase-1. this is the rate limiting step of gluconeogenesis; is activated by ATP directly and glucagon indirectly (via decreased levels of fructose 2,6-bisphosphate) and 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. it is found only in the endoplasmic reticulum of the liver
Pentose Phosphate Pathway (PPP)
also known as the hexose monophosphate (HMP) shunt
occurs in the cytoplasm of most cells,
two main functions: generating NA-DPH and sugars for biosynthesis (derived from ribulose 5-phosphate- PPP is the source of this)
Glucose-6-phosphate dehydrogenase
rate-limiting enzyme which is activated by NADP+ and inhibited by NADPH and insulin in the PPP
Rate-limiting enzyme of Glycolysis
phosphofructokinase-1
Rate-limiting enzyme of Fermentation
lactate dehydrogenase
Rate-limiting enzyme of Glycogenesis
glycogen synthase
Rate-limiting enzyme of Glycogenolysis
glycogen phosphorylase
Rate-limiting enzyme of Gluconeogenesis
fructose-1,6-bisphosphatase
Rate-limiting enzyme of Pentose Phosphate Pathway
glucose-6-phosphate dehydrogenase
Glycogenesis
synthesis of glycogen granules, beginning with core protein called glycogenin
NADPH
primary functions are lipid biosynthesis, bacterial bleach formation in white blood cells, and maintenance of glutathione stores to protect against reactive oxygen species
How Glycolysis Pushes Forward the Process: Kinases
is a mnemonic for
Irreversible steps of glycolysis
Hexokinase
Glucokinase
PFK-1
Pyruvate Kinase
