gluconeogenesis Flashcards
what is gluconeogenesis?
when glucose is formed from non carbohydrate substances such as lactate/glycerol/amino acids
when do we use gluconeogenesis?
when we are fasting and the body has low blood sugar and the liver has already dispensed its glycogen
where does gluconeogenesis occur?
in the liver mostly and a little in the kidneys, cytosol
what is the starting substance of gluconeogenesis?
oxaloacetate
name the enzymes that change pyruvate into PEP
pyruvate carboxylase and PEP carboxykinase
what does pyruvate carboxylase do?
turns pyruvate and HCO-3 to oxaloacetate (needs ATP)
what does PEP carboxykinase do?
converts oxaloacetate into PEP (using GTP)
describe pyruvate carboxylase
is a tetramer and each subunit has a biotin molecule
describe biotin
it acts as a CO2 carrier, by acquiring a carboxyl substitute as its ureido group.
how is biocytin formed?
when a carboxyl group is bound to a ureido group by an amide link between its valeryl carboxyl group and the E-amino group of a lys side chain
describe the 2 steps involved in the binding of a carboxyl group to biotin
1- biotin is carboxylated by a bicarbonate ion to dehydrate the bicarbonate. This releases CO2 which attaches to biotin.
2- the active carboxyl group is transferred from carboxybiotin to pyruvate to oxaloacetate
can PEP leave the mitochondria?
yes through specific transfer membrane proteins
how does oxaloacetate leave the mitochondria?
it must be converted to malate or aspartate
how is oxaloacetate converted to malate?
by the enzyme Malate Dehydrogenase (uses NADH)
how is oxaloacetate converted to aspartate?
by Aspartate Aminotransferase, (doesn’t use NADH) note that Aspartate can be converted to fumarate, which is hydrated to malate and then dehydrogenated to oxaloacetate
what are the 3 irreversible reactions of glycolysis that gluconeogenesis reverses?
turning pyruvate back to PEP (using pyruvate kinase), turning fructose-1,6-biphosphate back to fructose-6-phosphate (using PFK) , turning glucose-6-phosphate back to glucose (using glucokinase)
where is glucose-6-phosphatase present?
only in the liver and kidney
what is the purpose of Fructose-2,6-biphosphate?
it is an allosteric effector, it activates PFK, and deactivates FBPase, it is synthesized by PFK-2
glycogen breakdown is stimulated by?
phosphorylation
when gluconeogenesis is activated what happens to glycolysis?
glycolysis is inhibited at the pyruvate kinase level by alanine and phosphorylation
describe the cori cycle?
slow twitch muscle fibers produce lactate when ATP demand is unachievable. Lactate goes through blood stream to liver where it is turned into pyruvate by the enzyme lactate dehydrogenase. Then pyruvate turns into glucose using gluconeogenesis
