Module 3 - Gluconeogenesis Flashcards
What is the function of gluconeogenesis?
the primary function of gluconeogenesis is to assist in maintaining adequate glucose levels in the blood
particularly important during periods of fasting (e.g., when you are sleeping) or starvation
Where does gluconeogenesis take place in the animal?
occurs only in liver and kidney, with liver being the largest contributor
Gluconeogenesis
The synthesis of glucose from non-carbohydrate precursors
Bifunctional Enzyme
A single protein that possesses two catalytic activities
Cori Cycle
A metabolic pathway in which lactate produced by glycolysis in muscle is transported via the bloodstream to the liver, where it is used for gluconeogenesis. The glucose that is generated is returned to muscle
Obligate Allosteric Activator
An allosteric activator which is required to be bound to an enzyme for it to have catalytic activity
Gluconeogenic precursor: Lactate
one of the fates of pyruvate from glycolysis is that it can be reduced to lactate
This reaction, catalyzed by lactate dehydrogenase, is a reversible reaction
Most of the lactate formed in our bodies is produced in active muscle during anaerobic metabolism, which is released into the blood and eventually taken up by liver
it is converted back to pyruvate by lactate dehydrogenase and thereby enters the gluconeogenesis pathway
Gluconeogenic precursor: Amino Acids
Some amino acids, that are derived from the diet or from protein degradation, can be metabolized to intermediates in the gluconeogenic pathway
Gluconeogenic precursor: Glycerol
Triacylglycerols, the form of fat that we store in our bodies, can be broken down into fatty acids and glycerol
While fatty acids are used by various organs in our body for fuel, the glycerol is released from the adipose tissue into the blood and is taken up by the liver
There, glycerol is converted to dihydroxyacetone phosphate in a two-step process
How does pyruvate convert into Phosphoenolpyruvate?
In glycolysis, the conversion of phosphoenolpyruvate to pyruvate is catalyzed by pyruvate kinase and generates ATP as a second product
It is a strongly exergonic reaction, and thus not reversible
for gluconeogenesis to proceed, another mechanism involving different enzyme(s) has to be found in order to convert pyruvate to phosphoenolpyruvate
The process starts in the mitochondria with the carboxylation of pyruvate to form a four-carbon molecule called oxaloacetate
the reaction involves the hydrolysis of ATP which provides the energy to drive this reaction forward
Pyruvate carboxylase
requires a covalently bound prosthetic group called biotin (derivative of vitamin B7) that carries the CO2 in a manner that facilitates its reactivity with pyruvate
also requires that acetyl CoA is bound to the enzyme in order for it to catalyze carboxylation
In the absence of bound acetyl CoA, carboxylation does not occur
How does oxaloacetate move into the cytoplasm?
Oxaloacetate is converted to malate (by malate dehydrogenase), which is able to leave the mitochondria.
Once in the cytosol, malate is converted back to oxaloacetate by the same enzyme
How does oxaloacetate convert to phosphoenolpyruvate?
Happens in the cytosol
uses the enzyme phosphoenolpyruvate carboxykinase
Note that the hydrolysis of GTP rather than ATP is used to drive this reaction forward, at least in animals. Some bacterial forms of the enzyme use ATP.
How does phosphoenolpyruvate convert to fructose 1,6-bisP?
Once phosphoenolpyruvate is formed, it is metabolized by the enzymes of glycolysis to fructose 1,6-bisP.
This is possible because the reactions are all close to equilibrium at intracellular conditions, and therefore are reversible.
Conversion of Fructose 1,6-Bisphosphate to Fructose 6-P
The enzyme used is fructose 1,6-bisphosphatase, a hydrolase which cleaves off the phosphate group using water from carbon 1 on fructose.
This step is an important regulatory point of gluconeogenesis and the enzyme involved is regulated allosterically
Where does the conversion of glucose 6-P to glucose take place?
The enzyme glucose 6-phosphatase resides in the lumen (i.e., inside) of the endoplasmic reticulum, thus requiring that the glucose 6-P produced in the cytosol be shuttled into the ER.
The products of the reaction, Pi and glucose, are subsequently transported back out to the cytosol which allows glucose to be transported out of the cell into the blood
What is the overall reaction of gluconeogenesis?
the synthesis of glucose from pyruvate is energetically unfavourable except for the fact that it is coupled with reactions that release energy, such as the hydrolysis of ATP and GTP
What is the major point of regulation in gluconeogenesis?
the step where fructose 1,6-bisP is converted to fructose 6-P catalyzed by fructose 1,6-bisphosphatase.
What is the major point of regulation in glycolysis?
the conversion of fructose 6-P to fructose 1,6-bisP catalyzed by phosphofructokinase
note the reciprocal effects AMP on the two key enzymes
AMP, which reflects a low energy charge, activates phosphofructokinase and thus stimulates glycolysis and produces ATP, while at the same time it inhibits fructose 1,6-bisphosphatase, thereby slowing down gluconeogenesis.
Thus, under a low energy state of the cell, the net flux through this step would highly favour glycolysis.
note the reciprocal effects of fructose-2,6-bisP on the two key enzymes
Fructose 2,6-bisP has a similar effect, and interestingly the concentration of this biomolecule increases in the fed state, when one would want to metabolize the glucose that is available.
note the reciprocal effects of citrate on the two key enzymes
Citrate is an intermediate of the citric acid cycle which is the major pathway for oxidizing fuels when oxygen is available.
Thus, citrate reflects a high-energy state in the cell.
Not surprisingly, citrate inhibits phosphofructokinase but stimulates fructose 1,6-bisphosphatase, and thus signals for a net flux in the direction of gluconeogenesis.
The interconversion of pyruvate and phosphoenolpyruvate is another point of reciprocal regulation
Pyruvate kinase is inhibited by biomolecules that reflect a high-energy charge, such as ATP and alanine.
Conversely, high levels of ADP which occur in a low energy state of the cell, inhibit the conversion of pyruvate to phosphoenolpyruvate, which leads to a net flux favouring glycolysis and ATP production.
The Balance Between Glycolysis and Gluconeogenesis in Liver Is Sensitive to the Blood Glucose Concentration
One of the liver’s major responsibilities is to maintain blood sugar levels, which it does by adjusting the rates of glycolysis and gluconeogenesis.
It follows then that the blood sugar levels somehow influence the rates of these pathways.
It turns out that it achieves this through the allosteric modifier fructose 2,6-bisP.
Fructose 2,6-bisP activates phosphofructokinase, thus activating glycolysis; but it also inhibits fructose 1,6-bisphosphatase, thus slowing up gluconeogenesis
how do blood sugar levels influence the concentration of fructose 2,6-bisP?
The relative activities of PFK-2 and FBPase-2 determine the concentration of fructose 2,6-bisP in the cell and thus the activities of glycolysis and gluconeogenesis.
As scientists began researching these two enzymes in order to understand how they were regulated, they found something very surprising: both were present in the same protein
Further research found that there was a single serine residue in the regulatory domain that became phosphorylated when glucose was scarce, such as when we are asleep and not eating
This occurred as a result of a rise in the hormone glucagon which is secreted by the pancreas as blood sugar levels fall.
Glucagon stimulates a cAMP signal cascade that leads to the phosphorylation of the single serine residue
what does the phosphorylation of the serine residue result in?
it activates the phosphatase activity domain but at the same inhibits the kinase domain activity.
Together, this results in the lowering of fructose 2,6-bisP in the cell.
The Cori Cycle
Recall that during strenuous activity when oxygen may become limiting, muscle produces a significant amount of lactate from pyruvate which is released into the blood.
Much of this lactate is taken up by the liver, converted back to pyruvate by the enzyme lactate dehydrogenase, and then converted to glucose by the gluconeogenesis pathway.
In this way, the liver helps to replenish the blood glucose levels in the blood, which provides the contracting muscle with the glucose it needs to generate ATP via glycolysis.
These combined reactions in the muscle and liver constitute the Cori Cycle
