Ch. 14: Carbohydrate Metabolism Flashcards
NADPH
A strong reductant, available in biosynthetic and detoxification pathways, used to reduce reactive oxygen species
What is the main product of the pentose phosphate pathway (PPP)?
NADPH
What are the main functions of the pentose phosphate pathway?
- Provide NADPH
- Maintain homeostasis by providing carbons for nucleotide and amino acid biosynthesis
What happens with the PPP if ATP is needed?
Glucose-6-P is reserved for glycolysis
What molecule does the PPP start with?
Glucose-6-P
What the intermediates of the oxidative phase of PPP?
- 6-Phosphogluconolacetone
- 6-phosphogluconate
- Ribulose-5-P
What is the rate limiting step of the oxidative phase of the PPP?
Glucose-6-P to 6-phosphogluconolacetone
What are the 3 enzymes involved in the oxidative phase of the PPP?
- Glucose-6-phosphate dehydrogenase
- Lactonase
- 6-phosphogluconate dehydrogenase
How is the buildup of harmful oxygen radicals in cells prevented?
By the detoxification reactions that require reduced glutathione and the enzyme glutathione peroxidase
Why is NADPH required as a coenzyme?
It is required as a coenzyme in the glutathione reductase reaction to convert one molecule of oxidized glutathione (GSSG) to two molecules of reduced glutathione (GHS), which is a highly reactive electron donor in coupled redox reactions.
Carbon shuffle
When transketolase or transaldolase is put into another molecule, both are molecules that partake in the PPP
What do transketolase and transaldolase do in the PPP?
Catalyze the reactions of the non-oxidative phase of PPP, both link glycolysis to the PPP
Allosteric Regulation of NADPH
In high levels of NADP+, it will act as an activator for the PPP and the formation of NADPH; in high levels of NADPH, it will act as an inhibitor and stop PPP from occurring, and favor glycolysis and ATP formation instead
What are the four main carbon sources for glucose synthesis?
- Lactate
- Amino acids
- Glycerol
- CO2 Fixation (in plants)
Glyconeogenesis
Production of glucose from noncarbohydrate compounds; occurs mostly in the cytoplasm
When does glyconeogensis occur?
It is most active in animals when glycogen stores in the liver and muscles have been depleted
Gluconeogenic Enzymes
- Pyruvate carboxylase; uses the cofactor biotin
- Phosphoenolpyruvate carbokinase
Biotinylated lysine group
This group in pyruvate carboxylase functions as a swinging arm in the reaction to carry a carbonyl group from the biotin carboxylation domain to the caryboxyltransferase domain
Anaerobic Metabolism
During vigorous exercise, lactate produced in muscle cells is transported to the liver, where it is converted to pyruvate by the enzyme lactate dehydrogenase
The Cori Cycle
Link between lactate and glucose metabolism between the muscle and the liver; Under extreme conditions where glucose levels are depleted
Why can the Cori cycle be problematic?
It uses 4 ATP
What is glycogen degradation and synthesis regulated by?
- Glycogen phosphorylase
- Glycogen synthase
- Glycogen debranching and branching enzymes
Glycogen degradation
Releasing units of glucose into the bloodstream when glucose levels are low
Activity of the glycogen phosphorylase dimer
Stimulated by phosphorylation, which shifts equilibrium from the inactive T-state conformation to the active R-state conformation
Glycogen debranching enzyme
Transfers 3 glucose units to the nearest nonreducing end; cleaves alpha (1-6) glycosidic bond to release free glucose; no debranching unless the chain is long enough
Glycogen synthesis
UMP combines with GIP to form UDP glucose which is needed for the growing glycogen chain
What is required for glycogen synthesis?
The synthesis of uridine diphosphate glucose (UDP glucose)
Glycogen synthase
ATP is hydrolyzed to reform UTP which is needed to make UDP glucose that is in turn required for adding glucose molecules onto non-reducing ends of sugars
Glycogen branching enzyme
Transfers 7 glucose residues from one end of the glycogen chain to a nearby chain
Glycogenin
Anchor protein for the glycogen core particle; catalyzes the glycosyltransferase and synthesis reactions needed to generate glycogen; extends chain to 7 glucose residues
Hormonal Regulation of Glycogen Metabolism
Phosphylation can inactivate glycogen synthase, thereby degrading glycogen and transporting glucose out of the cell
When does hormonal regulation of glycogen metabolism occur?
Under low blood glucose conditions (e.g. fasting)
What is used when blood glucose levels are high?
Insulin is used to synthesize glycogen
What happens when blood glucose levels are low?
Glycogen is released from the liver and brought to the bloodstream, where it is broken down into glucose and used to bring the glucose levels back to normal.
How many NADPH molecules are generated from the oxidative step of the PPP?
2 NADPH molecules
If 6 molecules of glucose-6-P were used for the PPP, how many NADPH molecules would be produced?
12 NADPH molecules
