Carbohydrate Metabolism: Gluconeogenesis Flashcards
What is glucongeogensis?
Synthesis of glucose
Where is gluconeogenesis most active in animals?
Most active when glycogen stores in the liver and muscle has been depleted
Which organ is most responsible for gluconeogenesis?
It is mostly the liver’s job as it is unselfish and gives it to those in need
The muscle is selfish as it stores its own glucose
What are the 4 major carbon sources for glucose synthesis?
Glycerol
Amino acids
Lactate
CO2 fixation (in plants)
How is glycerol a major carbon source for glucose synthesis?
Triglycerides (fats) are converted into glycerol, which can be converted to DHAP (Dihydroxyacetone phosphate)
How are amino acids a major carbon source for glucose synthesis?
Nutrient limitation increases the conversion of amino acids into pyruvate or citrate cycle metabolites
How is lactate a major carbon source for glucose synthesis?
Anaerobic respiration increases the pool of lactate, which can be converted into pyruvate for entry into the gluconeogenic pathway
How is CO2 fixation (in plants) a major carbon source for glucose synthesis?
Plants use gluconeogenesis to synthesize glucose from triose phosphates generated from CO2 fixation, which is used to produce sucrose and starch for energy storage
Compare and contrast between glycolysis and gluconeogenesis.
Glycolysis is the breakdown of glucose while gluconeogenesis is the synthesis of glucose
They are opposing pathways - end product of one is the starting point of the other
They share 7 enzymes (those that catalyze reversible reactions)
- The direction of the reversible reactions is simply a response to the relative concentrations of metabolites
Main determinants of the flux through either pathway are the IRREVERSIBLE reactions
Name 4 gluconeogenic enzymes that bypass 3 exergonic reactions in glycolysis.
Pyruvate carboxylase and Phosphoenolpyruvate (PEP) carboxykinase in gluconeogenesis bypass Pyruvate kinase in glycolysis
Fructose-1,6-bisphosphate-1 in gluconeogenesis bypass Phosphofructokinase-1 in glycolysis
Glucose-6-phosphate in gluconeogenesis bypass Hexokinase in glycolysis
Describe the reaction of Pyruvate to Phosphoenolpyruvate.
Requires two energy-consuming steps
1st step: pyruvate carboxylase converts pyruvate to OAA (oxaloacetate)
- Carboxylation using a biotin cofactor
- Requires transport of pyruvate into the mitochondria
2nd step: PEP carboxykinase converts OAA to PEP
- Phosphorylation and decarboxylation
- Occurs in mitochondria or cytosol depending on aerobic vs. anaerobic conditions
Describe the synthesis of oxaloacetate.
Biotin = an important cofactor for pyruvate carboxylase
Covalently linked to the e amino acid group on Lys in the active site
Biotinylated Lys functions as a swinging arm in the reaction to carry a carboxyl group from one region of the active site to another
What is Biotin?
CO2 Carrier; long biotinyl-Lys tether moves CO2 from site 1 to site 2
A vitamin required in the human diet; abundant in many foods; also synthesized by intestinal bacteria
Is biotin deficiency rare?
Yes its rare, but can be caused by a diet rich in raw eggs
What is the relation between eggs and biotin?
Eggs have a protein called avidin, which binds biotin and prevents its absorption (which will also affect the synthesis of oxaloacetate)
In cooked eggs, avidin is denatured and inactivated
