Gluconeogenesis

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Carbohydrates are made of sugar molecules, and the most important one is the 6-carbon sugar, glucose. It’s important to keep a steady amount of glucose in the blood, because cells use it to produce energy - in the form of adenosine triphosphate, or ATP.
One of the ways you can do this is by eating carbohydrate-rich foods like pasta. But in between meals, when fasting, the body maintains glucose levels using gluconeogenesis.Gluconeogenesis is a metabolic pathway that uses enzymatic reactions to make glucose from other molecules, like amino acids, lactate, and glycerol.
Gluconeogenesis primarily takes place in liver cells, but it can also happen in the epithelial cells of the kidney and the intestines.
Specifically, it takes place in the cytoplasm, mitochondria, and endoplasmic reticulum of cells found in these tissues. Okay, so let’s say you’re going hiking in the woods, and you fuel up on some pasta before you leave.
Now, during the hike, you get lost and end up stranded with no food. Initially, the glucose in your pasta is broken down by a series of enzymatic reactions to make pyruvate, producing ATP in the process.
This is called glycolysis, and it keeps you going for a couple of hours. Some of the extra glucose is stored in the liver cells in the form of glycogen, which is a bunch of glucose molecules stringed together.
When you’re fasting, you still need glucose, in particular for your red blood cells and your brain. And you might need it to find your way out of the woods.
So, it’s up to your liver to maintain adequate blood glucose levels while fasting. There are two pathways that can contribute glucose: glycogenolysis and gluconeogenesis.
So with glycogenolysis, the liver breaks down glycogen into individual glucose molecules, but that only helps for 12 to 24 hours of fasting, because glycogen stores are finite.
In contrast, gluconeogenesis makes glucose from scratch, so it can keep on going in the event that you fast for more than a day.
Actually, by 12 hours of fasting, gluconeogenesis is the main provider of glucose in the bloodstream.The process of gluconeogenesis is almost reverse glycolysis.
In glycolysis, you’re using 10 enzymatic reactions to convert glucose to pyruvate in order to make ATP. And in gluconeogenesis, you’re working backwards and using ATP to convert pyruvate to glucose.
7 of the reactions in glycolysis and gluconeogenesis are reversible, meaning they can go in both directions using the same enzymes.
However, the other 3 are irreversible, meaning that the enzyme mediating the reaction can only go in the direction of glucose to pyruvate, but not the opposite.
So if you think of the path between glucose and pyruvate as a two-way road within the cell. When driving from glucose to pyruvate, it’s a smooth straightforward ride.
On the other hand, when driving from pyruvate to glucose, most of the ride is straightforward, but there are 3 roadblocks which represent irreversible reactions.
And for those, you have to take an alternate route to bypass them, which represent unique reactions that use different enzymes only found in gluconeogenesis.
Alright, so the ingredients we need to cook up some glucose from scratch in our liver are a source of pyruvate and ATP. In gluconeogenesis, the two main sources of pyruvate are lactate and amino acids like alanine.
Lactate is produced as a byproduct of anaerobic respiration in red blood cells and exercising skeletal muscle cells. An enzyme called lactate dehydrogenase removes a hydrogen from lactate, turning it into pyruvate.
In that reaction, the hydrogen goes to a nearby nicotinamide adenine dinucleotide, or NAD+, which turns into NADH. The second source of pyruvate is amino acids, which are the building blocks of proteins.
We have 20 different amino acids, and 18 of them, with the exception of leucine and lysine, are glucogenic amino acids, meaning they can be used to make glucose.
You can remember the exceptions - leucine and lysine - as the “Lazy Ls” of amino acids. The body breaks down protein in skeletal muscle cells into individual amino acids, with the main amino acid being alanine.
The enzyme alanine transaminase, or ALT, removes an amino group from alanine, and turns it into pyruvate. The amino group attaches to an acid called alpha ketoglutarate, which then turns into glutamate.
And in general, transaminase enzymes require pyridoxine, or vitamin B6 as a cofactor. Now, to get ATP when you’re stranded in the woods, your body starts breaking down fats which come in the form of triacylglycerides.
During fasting, the pancreatic alpha cells sense blood glucose levels decreasing, and release glucagon. In the meantime, the low level of insulin, the presence of epinephrine, as well as ACTH are causing adipocytes or fat cells to stimulate hormone-sensitive lipase or HSL, an enzyme that breaks triacylglycerides down into free fatty acids and glycerol.
The fatty acids enter the bloodstream and go to various tissues to be used for energy. In the hepatocyte mitochondria, free fatty acids are broken down into acetyl CoA and ATP by a metabolic process called beta oxidation.
The glycerol, on the other hand, is used to make glucose in gluconeogenesis. So always remember, lactate, glycerol and all amino acids except leucine and lysine will be used to build glucose.
Now let’s cook up some glucose! The first step is converting pyruvate to phosphoenolpyruvate, or PEP - but this is also our first roadblock.
In glycolysis, the enzyme pyruvate kinase catalyzes PEP turning into pyruvate, but not vice versa. The alternate route involves 3 enzymes; pyruvate carboxylase, malate dehydrogenase, and phosphoenolpyruvate carboxykinase, or PEPCK.So the alternate route starts when pyruvate enters the mitochondria, and pyruvate carboxylase adds a carbon to it, making a 4-carbon molecule, oxaloacetate.
A general rule of thumb is that any carboxylase enzyme requires 3 cofactors, which you can remember with an ABC mnemonic, “a” for ATP, which comes from fatty acid oxidation, “b” for biotin, or Vitamin B7, and “c” for carbon dioxide.
Also, the acetyl-CoA that’s made during fatty acid oxidation enhances pyruvate carboxylase activity. So now we have oxaloacetate, and it needs to get out of the mitochondria and into the cytoplasm to continue gluconeogenesis.
Unfortunately, oxaloacetate can’t go through the mitochondrial membrane - so malate dehydrogenase in the mitochondria comes to the rescue and adds a hydrogen to oxaloacetate, converting it to malate.
Malate can exit the mitochondria and enter the cytoplasm, where malate dehydrogenase in the cytoplasm converts it back to oxaloacetate.
This process of conversion and reconversion is called the malate shuttle. Okay, now that oxaloacetate is in the cytoplasm, PEPCK removes a carbon group, and adds a phosphate group, turning oxaloacetate into PEP.
This reaction requires an energy source, but instead of ATP, the energy comes in the form of guanosine triphosphate, or GTP.
Don’t worry, it doesn’t make a difference since the energy is contained in the phosphate group. So-called “stress” hormones, like glucagon, epinephrine, and cortisol enhance the activity of PEPCK by induction- and being stranded in the woods generates a lot of these hormones, speeding up gluconeogenesis.
Now that we have PEP, it goes through a series of reversible reactions, which are common to glycolysis, until it’s converted to dihydroxyacetone phosphate - or DHAP.
Alternatively, glycerol from hormone-sensitive lipase activity can also be used to make DHAP. In that process, the enzyme glycerol kinase adds a phosphate group from ATP to the 3rd carbon of glycerol, making glycerol-3-phosphate, or G3P.
Then, another enzyme called glycerol-3-phosphate dehydrogenase converts G3P to DHAP. Either way, once DHAP is formed, it’s converted to fructose 1,6 bisphosphate by a reversible reaction involving the enzyme aldolase.
Now, here’s our second roadblock. In glycolysis, the enzyme phosphofructokinase-1, or PFK1 adds a phosphate group to fructose-6-phosphate, making fructose-1,6-bisphosphate.
In gluconeogenesis, there’s a different enzyme called fructose-1,6-bisphosphatase, that removes a phosphate from the 1st carbon of fructose-1,6-bisphosphate, making fructose-6-phosphate.
This reaction is the rate limiting step of gluconeogenesis, meaning that how fast fructose 1,6 bisphosphatase catalyzes this reaction determines the speed at which all of gluconeogenesis happens.
As a result, this step is really tightly controlled to make sure we make glucose when we need to, and stop when we don’t.
For example, one trigger that speeds up gluconeogenesis is when the liver is burning up a lot of fatty acids, yielding a lot of ATP.
This ATP enhances the activity fructose-1,6 bisphosphatase. Remember, the liver needs energy to carry out gluconeogenesis!
It also relies on the presence of stress hormones to enhance the activity of PEPCK. Glucagon activates whereas insulin inhibits fructose-1,6 bisphosphatase.
So next, fructose-6-phosphate is converted to its isomer, glucose-6-phosphate by the enzyme isomerase, which changes the molecular shape, but not its chemical structure.
That’s why the phosphate on the 6th carbon is still there. But glucose can’t go into the bloodstream with a phosphate clinging to it, so cutting it off is the final roadblock.
In glycolysis, when glucose enters a cell, the enzyme hexokinase adds a phosphate to glucose to keep glucose inside the cell.
Removing the phosphate off of the 6th carbon of glucose, requires an enzyme located in the endoplasmic reticulum called glucose-6-phosphatase.
Once the phosphate is removed, glucose can enter the bloodstream. Now - one final thought on gluconeogenesis relates to chronic alcoholism.
Alcohol, or ethanol is also processed in liver cells, and over time it results in an accumulation of NADH and ATP in the cytoplasm.
Increased NADH levels signal the lactic, malate, and glycerol-3-phosphate dehydrogenase enzymes in gluconeogenesis to go the other way around: from pyruvate to lactate, from oxaloacetate to malate and from DHAP to G3P, respectively.
On the other hand, high ATP levels in the liver slow down fatty acid beta oxidation and cause abnormal accumulation of fat in the liver.
Both of these effects impair gluconeogenesis, resulting in chronic low blood sugar when fasting - not a good way to get stranded in the woods.
Alright, a quick recap. Gluconeogenesis involves the production of glucose from pyruvate to help maintain steady blood glucose levels during prolonged fasts.
In essence, it is reverse glycolysis, but there are 3 irreversible reactions that are bypassed using enzymes that are unique to gluconeogenesis.
Pyruvate kinase is bypassed by pyruvate carboxylase, malate dehydrogenase and PEPCK. PFK1 is bypassed by fructose-1,6-bisphosphatase, and hexokinase is bypassed by glucose-6-phosphatase.
way to get stranded in the woods. Alright, does a quick recap gluconeogenesis, involves the production of glucose from pyruvate to help maintain steady blood glucose levels during prolonged fast, since it is reverse glycolysis, but there are three reversible reactions that are bypassed using enzymes that are unique to gluconeogenesis.
Pyruvate. Kinase is bypassed by pyruvate carboxylase malate, dehydrogenase.
And Pepsi k p. F k.
1 is bypassed by fructose 16. Bisphosphatase and hexo.
Kindness is bypass my glucose 6 phosphatase.