STEP 8: enzyme: phosphoglycerate mutase; reaction: 3-phosphoglycerate ⇆ 2-phosphoglycerate - this is another isomerase, but I guess they thought “mutase” sounded cooler or something - but “all” you’re doing here is shifting the phosphate to the middle C STEP 9: enzyme: enolase; reaction: 2-phosphoglycerate ⇆ phosphoenolpyruvate (PEP) + H₂O - making PEP is a kind of “prep” - when you kick out that water you make a double bond between 2 of the carbons - making it a really awkward situation for that middle carbon - this molecule is really unstable - so the phosphate is now attached to a carbon that doesn’t really want it and can better survive without it - so that phosphate can more easily get removed in…
STEP 10: enzyme: pyruvate kinase; reaction: PEP + pyruvate kinase + ADP → pyruvate - the last step & final payout of glycolysis - involves another kinase named for the reverse reaction - because PEP is so unstable, it’ll easily give up that phosphate to an ADP that pyruvate kinase helps it meet - and because of how unstable PEP is, although you’re not spending energy money, the reaction’s really unlikely to go backwards. So you do all that for each copy of G-3-P you got from the investment phase. So, you end up with 2 X NADH (from step 6), 2 X ATP from step 7 and 2 x ATP from step 8. And you used 2 ATP in the investment phase. So, on net you have: 2 NADH + [(-2) + 2 + 2 = 4] ATP. And you also have those 2 pyruvates which can get further processed for more energy.
Gluconeogenesis is the “reverse” of glycolysis but it’s not a direct reverse because it has to “reroute” around the irreversible steps using different enzymes. It reverses step 10 (the de-pep-ing) in 2 or 3 steps - it’s a bit “steppy” because that pyruvate that got made in glycolysis gets shipped into the mitochondria. In the mitochondria, carboxylase converts pyruvate to oxaloacetate (at the cost of 1 ATP) & then phosphoenol-pyruvate carboxykinase converts that oxaloacetate. That can’t go through the mitochondrial membranes, so it first gets made into malate by malate dehydrogenase, then that malate goes into the cytoplasm where another malate dehydrogenase turns it back to oxaloacetate which can then be turned into to phosphoenolpyruvate (PEP) (at the cost of 1 GTP) by PEP carboxykinase in the cytoplasm. Alternatively, the PEP can be made in the mitochondria in some animals and transported out like that.
It uses fructose 1,6-bisphosphatase to reverse step 3 (remove the second phosphate that was added to go from fructose 1,6-bisphosphate to fructose 6-phosphate.
And to reverse the 1st step of glycolysis (initial phosphate-adding), glucose-6-phosphatase removes the phosphate to form glucose. This happens in the lumen of the endoplasmic reticulum (another membrane-bound compartment that’s often used for protein-modifying) and shipped out into the cytoplasm by glucose transporters.
I learned some cool things about glycolysis in the book “For the Love of Enzymes” by Arthur Kornberg that president-elect of the IUBMB, Dr. Alexandra Newton, gave me. For example, the discovery of glycolysis came with the birth of “biochemistry” - and it came by accident - in a story not so well-known as the Pasteur and the Petri dish one about the discovery of the antibiotic penicillin being made by mold on a contaminated bacteria culture plate.
Basically, a couple of guys in the late 1900s (1897 to be exact) - Hans & Eduard Buchner - thought there could be medicinal value to the stuff inside of yeast cells (cell-free yeast extracts). So they ground up & squished a bunch of yeast cells, filtered it and, to preserve it, added some sugar (hey - it works for jams right?!) Well, they found that the yeast extract started getting all bubbly - it was fermenting (fermentation is a process you can learn more about in yesterday’s post whereby NAD+ can be regenerated from NADH without going through oxidative phosphorylation)
This was really surprising because, before this, it was thought that complex processes like this could only take place INSIDE OF LIVING CELLS - but this was just the cell extract! Further excitement came when studies with muscle extracts showed that lactic acid fermentation used a lot of the same reactions as the alcoholic fermentation that the yeast were doing. In fact, glycolysis is a nearly universal process in all sorts of organisms. Lots of pioneering biochemists joined in on the fun and, by 1940, the glycolytic pathway was figured out! Nowadays, we study reactions with purified proteins outside of cells all the time because it gives us better control more on all sorts of metabolic stuff: bit.ly/bbmetabolism & ruclips.net/p/PLUWsCDtjESrHXBgulruKEOrNXQ21_0gyc
more about all sorts of things: #365DaysOfScience All (with topics listed) 👉 bit.ly/2OllAB0 or search blog: thebumblingbiochemist.com
STEP 8: enzyme: phosphoglycerate mutase; reaction: 3-phosphoglycerate ⇆ 2-phosphoglycerate
- this is another isomerase, but I guess they thought “mutase” sounded cooler or something - but “all” you’re doing here is shifting the phosphate to the middle C
STEP 9: enzyme: enolase; reaction: 2-phosphoglycerate ⇆ phosphoenolpyruvate (PEP) + H₂O
- making PEP is a kind of “prep” - when you kick out that water you make a double bond between 2 of the carbons - making it a really awkward situation for that middle carbon - this molecule is really unstable
- so the phosphate is now attached to a carbon that doesn’t really want it and can better survive without it - so that phosphate can more easily get removed in…
STEP 10: enzyme: pyruvate kinase; reaction: PEP + pyruvate kinase + ADP → pyruvate
- the last step & final payout of glycolysis
- involves another kinase named for the reverse reaction
- because PEP is so unstable, it’ll easily give up that phosphate to an ADP that pyruvate kinase helps it meet
- and because of how unstable PEP is, although you’re not spending energy money, the reaction’s really unlikely to go backwards.
So you do all that for each copy of G-3-P you got from the investment phase. So, you end up with 2 X NADH (from step 6), 2 X ATP from step 7 and 2 x ATP from step 8. And you used 2 ATP in the investment phase. So, on net you have: 2 NADH + [(-2) + 2 + 2 = 4] ATP. And you also have those 2 pyruvates which can get further processed for more energy.
Gluconeogenesis is the “reverse” of glycolysis but it’s not a direct reverse because it has to “reroute” around the irreversible steps using different enzymes. It reverses step 10 (the de-pep-ing) in 2 or 3 steps - it’s a bit “steppy” because that pyruvate that got made in glycolysis gets shipped into the mitochondria. In the mitochondria, carboxylase converts pyruvate to oxaloacetate (at the cost of 1 ATP) & then phosphoenol-pyruvate carboxykinase converts that oxaloacetate. That can’t go through the mitochondrial membranes, so it first gets made into malate by malate dehydrogenase, then that malate goes into the cytoplasm where another malate dehydrogenase turns it back to oxaloacetate which can then be turned into to phosphoenolpyruvate (PEP) (at the cost of 1 GTP) by PEP carboxykinase in the cytoplasm. Alternatively, the PEP can be made in the mitochondria in some animals and transported out like that.
It uses fructose 1,6-bisphosphatase to reverse step 3 (remove the second phosphate that was added to go from fructose 1,6-bisphosphate to fructose 6-phosphate.
And to reverse the 1st step of glycolysis (initial phosphate-adding), glucose-6-phosphatase removes the phosphate to form glucose. This happens in the lumen of the endoplasmic reticulum (another membrane-bound compartment that’s often used for protein-modifying) and shipped out into the cytoplasm by glucose transporters.
I learned some cool things about glycolysis in the book “For the Love of Enzymes” by Arthur Kornberg that president-elect of the IUBMB, Dr. Alexandra Newton, gave me. For example, the discovery of glycolysis came with the birth of “biochemistry” - and it came by accident - in a story not so well-known as the Pasteur and the Petri dish one about the discovery of the antibiotic penicillin being made by mold on a contaminated bacteria culture plate.
Basically, a couple of guys in the late 1900s (1897 to be exact) - Hans & Eduard Buchner - thought there could be medicinal value to the stuff inside of yeast cells (cell-free yeast extracts). So they ground up & squished a bunch of yeast cells, filtered it and, to preserve it, added some sugar (hey - it works for jams right?!) Well, they found that the yeast extract started getting all bubbly - it was fermenting (fermentation is a process you can learn more about in yesterday’s post whereby NAD+ can be regenerated from NADH without going through oxidative phosphorylation)
This was really surprising because, before this, it was thought that complex processes like this could only take place INSIDE OF LIVING CELLS - but this was just the cell extract! Further excitement came when studies with muscle extracts showed that lactic acid fermentation used a lot of the same reactions as the alcoholic fermentation that the yeast were doing. In fact, glycolysis is a nearly universal process in all sorts of organisms. Lots of pioneering biochemists joined in on the fun and, by 1940, the glycolytic pathway was figured out! Nowadays, we study reactions with purified proteins outside of cells all the time because it gives us better control
more on all sorts of metabolic stuff: bit.ly/bbmetabolism & ruclips.net/p/PLUWsCDtjESrHXBgulruKEOrNXQ21_0gyc
more about all sorts of things: #365DaysOfScience All (with topics listed) 👉 bit.ly/2OllAB0 or search blog: thebumblingbiochemist.com
Thanks so much for these excellent videos!
Thank you! Glad you find them helpful!