Biochemistry Chapter 9: Carbohydrate Metabolism I Flashcards
GLUT 2
found in the liver (for glucose storage) and pancreatic beta-islet 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
Where does glycolysis take place?
in the cytoplasm
Does glycolysis require oxygen?
No
What does glycolysis yield?
2 ATP
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-phosphate in peripheral tissues.
Phosphofructokinase-1
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-bisphosphate and is inhibited by ATP and citrate
Phosphofructokinase-2
produces the F2,6 - BP that activates PFK-1. It is activated by insulin and inhibited by glucagon
Glyceraldehyde-3-phosphate dehydrogenase
produces NADH, which can feed into the electron transport chain.
3-phosphoglycerate kinase / pyruvate kinase
substrate-level phosphorylation - placing an inorganic phosphate onto ADP to form ATP.
Which enzyme perform irreversible reactions?
glucokinase/hexokinase, PFK-1, pyruvate kinase
Where is the NADH from gylcolysis oxidized?
The mitochondrial electron transport chain
If oxygen or mitochondria are absent, then the NADH produced in glycolysis…
is oxidized by cytoplasmic lactate dehydrogenase. ex. red blood cells, skeletal muscle, and any cell deprived of oxygen.
Galactose
Comes from lactose in milk - trapped by galactokinase and converted to glucose 1-phosphate via galactose-1-phosphate uridyl-transferase and an epimerase
fructose
comes from honey, fruit, sucrose. It is trapped in the cell by fructokinase and then cleaved by aldolase B to form glyceraldehyde and DHAP.
Pyruvate dehydrogenase
a complex of enzymes that convert pyruvate to acetyl-CoA. It is stimulated by insulin and inhibited by acetyl-CoA
Glycogenesis
production of glycogen using two main enzymes: glycogen synthase and branching enzyme
glycogen synthase
creates a alpha-1,4 glycosidic link between glucose molecules. Activated by insulin in the 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
a breakdown of glycogen using two main enzymes: glycogen phosphorylase and debranching enzyme
Glycogen phosphorylase
removes single glucose 1-phosphate molevules 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 occurs in..
occurs in both the cytoplasm and mitochondria, predominantly in the liver. There is a small contribution from the kidneys.
What is gluconeogenesis?
the reverse of glycolysis.
What are the three irreversible steps of gluconeogenesis?
Pyruvate carboxylase, fructose 1, 6 BP, glucose-6-phosphatase
Pyruvate carboxylase
- converts pyruvate into oxaloacetate which is converted to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase.
- activated by acetyl-CoA from beta-oxidation
- PEPCK is activated by glucagon and cortisol
Fructose-1,6-bisphosphatase
- converts fructose 1,6 bisphosphate to fructose 6-phosphate
- It is activated by ATP directly and glucagon indirectly
- inhibited by AMP directly and insulin indirectly
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)
occurs in the cytoplasm of most cells, generating NADPH and sugars for biosynthesis
Rate limiting enzyme for PPP
glucose-6-phosphate dehydrogenase (activated by NADP+ and inhibited by NADPH and insulin
