USMLE® Step 1 style questions USMLE
Biochemistry researchers are studying the pathway of glycolysis with students in the laboratory. Which of the following is the rate-limiting step of glycolysis?
Content Reviewers:Rishi Desai, MD, MPH
Let’s say that you just ate a big slice of pizza with onions, mushrooms, bell peppers, and jalapenos. To pull energy out of the glucose in that pizza or really any food, requires glycolysis.
Glycolysis is a series of enzymatic reactions in which glucose, a 6 carbon sugar molecule, is broken down into two 3 carbon pyruvate molecules.
And as glucose gets processed, energy is produced in the form of adenosine triphosphate, or ATP.
Now, glycolysis happens in the cytoplasm of cells, and no special organelles or even oxygen are needed to turn glucose into ATP.
Therefore, all cells can use glucose to make energy; and it’s possible to do glycolysis even when oxygen levels are low.
It’s like a business investment - the cell needs to spend some energy before it can start making energy, and like any good investment the cell gets more energy back than it puts in.
The energy-consuming phase requires ATP, and the energy-producing phase generates ATP, as well as other molecules like reduced nicotinamide adenine dinucleotide, or NADH, which can be used to make ATP.
We can keep track of all of this using an energy counter.
Going back to that delicious pizza, first, glucose from those ingredients has to first get from the small intestine into the bloodstream.
In response to high blood glucose, the pancreatic beta-cells secrete insulin.
Now, to get inside the cells, glucose utilizes glucose transporters, or GLUT, which are on the cell membrane.
In fact, some GLUTs like GLUT2 in the liver and pancreatic beta-cells are particularly responsive to glucose in the presence of insulin.
Once glucose gets inside the cell, it’s prevented from diffusing across the cell membrane back into the circulation by enzymes called kinases which phosphorylate the glucose.
Adding a phosphate group changes the shape of the glucose molecule, which means it can’t easily diffuse out of the cell, a bit like a criminal that’s handcuffed to the table in an interrogation room.
The phosphate comes from the breakdown of ATP into ADP and phosphate - so this initial phosphorylation step drops us to -1 on that energy counter.
Specifically, there are two enzymes called hexokinase and glucokinase, and they both add a phosphate group to the 6th carbon in the glucose molecule, turning it into glucose-6-phosphate.
Both enzymes pretty much do the same thing, but hexokinase is found in all cells, whereas glucokinase, like GLUT2, is induced by the presence of insulin, and is found in the liver cells and the beta-cells of the pancreas.
This first step is irreversible, meaning that the reaction can only go in the glucose to glucose-6-phosphate direction, and not vice versa.
Glucose-6-phosphate is converted to its isomer, fructose-6-phosphate by an enzyme called phosphoglucoisomerase.
So at this point, it’s still a 6 carbon molecule.
Fructose-6-phosphate is then phosphorylated by the enzyme phosphofructokinase-1, or PFK1, which adds a phosphate group to the 1st carbon on the fructose molecule, making fructose-1,6-bisphosphate.
This is the second irreversible reaction in glycolysis and it also uses ATP as a phosphate source - so we’re at -2 on that counter now.
This reaction is considered the rate-limiting step of glycolysis - meaning that how fast PFK1 converts fructose-6-phosphate to fructose-1,6-bisphosphate determines the speed at which all of glycolysis happens.
In other words, it’s the rate limiting step of glycolysis. It’s a bit like an assembly line in a factory, if the slowest step is putting tires on a car, then that’s the step that determines how many cars get built in a day.
Because of this, cells closely regulate PFK1 activity by using another enzyme, called phosphofructokinase 2 - or PFK2.
You see - PFK2 can also phosphorylate fructose-6-phosphate - but it adds phosphate to the 2nd carbon instead, making fructose 2,6-bisphosphate. PFK2 activity varies depending on the level of glucose in the blood.
When the body is well-fed, like right after eating that slice of pizza, blood glucose levels go up, and the pancreas secretes insulin, which activates PFK2 - resulting in more fructose-2,6 bisphosphate.
Now, here’s the key - increased levels of fructose-2,6 bisphosphate activates PFK1, which means it increases the rate of available PFK1 enzymes.
So more PFK1 means that the slowest step in glycolysis speeds up, and more glucose is turned into energy. More tires, more cars.
PFK1 is also inhibited in other ways. For example, when cells are in high energy states, there is a lot of ATP floating around as well as citrate, because that’s a by product of fatty acid synthesis.