Core Concepts
In this tutorial, you will learn about an important biochemical pathway: Glycolysis.
A pathway in biochemistry is a sequence of reactions that perform a function such as breaking down or synthesizing a molecule. This particular pathway, glycolysis, is among the most well-studied pathways because it is essential for the survival of most life!
What is Glycolysis?
Glycolysis is a biochemical pathway that starts with the sugar glucose (C6H12O6), breaks it down into smaller molecules, and generates useful energy for a living system. Depending on how you view it, glycolysis can be described in different ways. Here are a few important perspectives.
In Life
At a glance, without getting into the details, we can say that glycolysis is a way that the majority of organisms on Earth break down or “metabolize” sugar to obtain energy. Organisms rely on the energy generated this way to live, move, and grow. Plants use photosynthesis to create sugar and then use glycolysis to break those sugars down into energy. Animals, like penguins, use the same pathway to process the sugars in the food that they eat. Only a few organisms, such as some bacteria, do not make use of glycolysis. They instead use other pathways to metabolize their source of energy.
Glycolysis in Biochemistry
To biochemists, glycolysis is just one important pathway and a part of a greater system of chemical pathways that occur in life. In the image below, the red line represents glycolysis, while other known pathways remain in black and white. You can see that glycolysis is just one piece of the puzzle.
Biochemists have an interest in the details of these pathways: how chemicals change, and how energy moves throughout the process. When you approach it methodically and keep track of what comes in and out of each process, it starts to make some more sense. By doing some of our own “bookkeeping” we can begin to understand glycolysis like a biochemist.
Glycolysis at the Molecular Level
From a more detailed chemical perspective, glycolysis is the breaking down of sugar by various reactions. This is where its name comes from: the Greek roots “glukús” meaning sweet and “lysis” meaning to cut or release. One molecule of glucose, a sugar, transforms chemically in ten distinct reactions or steps. These steps produce energy (ATP) and create important molecules. One of these important molecules is pyruvate, the end product of glycolysis. Pyruvate is special because it is a starting molecule for other energy-producing processes, like the citric acid cycle.
1: Glucose phosphorylation
In the first reaction of the pathway, an enzyme called hexokinase takes a phosphate group from ATP and adds it to carbon 6 of the glucose molecule. The new molecule is glucose-6-phosphate. A good way to remember this is from the name of the enzyme. “Kinase” enzymes all add a phosphate to (or remove one from) something, and this one has “hex” in its name, which means six, so hexokinase adds a phosphate to carbon 6!
2: Isomerization
Here, the enzyme phosphoglucose isomerase changes the structure of the sugar from a 6-atom ring (glucose-6-phosphate) to a 5-atom ring (fructose-6-phosphate). It doesn’t add or take away anything, it just rearranges the bonds, an action called isomerization. Again, the clue is in the name of the enzyme: phosphoglucose gets isomerized!
3: Fructose phosphate phosphorylation
We’re back at it with phosphorylation! Another kinase, phosphofructokinase, takes phosphofructose and steals another phosphate from ATP to add to it (this time to carbon 1), making fructose-1,6-bisphosphate (bis- just means two, so there are two phosphates on a fructose molecule here). If you feel like the name of the enzyme is familiar from step 1, you’re right on: -kinase means it adds a phosphate, and phosphofructo- indicates that it does this to fructose-6-phosphate.
4: Aldolase C-C bond breaking
This step makes a big change: the single 6-carbon molecule breaks apart using the enzyme aldolase and forms two 3-carbon compounds called dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP or G3P).
5: Isomerization
Triosephosphate isomerase has a small but important role to play, changing the DHAP into GAP so we just have one kind of 3-carbon sugar (triose is a word for such a sugar, since tri- means 3). If not for this step, half of every glucose molecule would be a by-product! Every following step can be thought of as happening twice for every initial molecule put in, since we’ve made two GAP molecules out of a single glucose.
6: Dehydrogenation and phosphorylation
Glyceraldehyde phosphate dehydrogenase (GAPDH) enzyme removes a hydrogen from the aldehyde group on GAP, then adds a free phosphate (instead of one from ATP) to make it into 1,3-bisphosphoglycerate (again, bis- means two).
7: Dephosphorylation
Before, we saw kinases adding phosphates to our molecules, taking them from ATP. Now, phosphoglycerate kinase removes the brand new phosphate and adds it to ADP to make energy-storing ATP. This leaves us with 3-phosphoglycerate.
8: “Mutation” of the phosphate
Here comes another isomerization! Only this time it doesn’t come from isomerase or aldolase, it’s an enzyme called phosphoglyceromutase, which just moves the phosphate group over from carbon 3 to carbon 2, making it 2-phosphoglycerate instead of 3-phosphoglycerate.
9: Dehydration
Even though step 8 seems small, it was necessary to let the enzyme enolase dehydrate (remove a water molecule from) the phosphoglycerate, leaving us with phosphoenolpyruvate (try saying that 5 times fast!).
10: Dephosphorylation
A familiar type of enzyme shows up to finish the pathway: a kinase. Pyruvate kinase removes our last phosphate, changing another ADP to ATP in the process. Now we have our final product, pyruvate!
We did it! That’s the entire glycolysis pathway, from familiar glucose all the way to pyruvate. The enzyme names might seem confusing, but if you think about what they mean, they can actually be a memory aid, to help you keep track of what chemical change is occurring in each part of the series! If you need to have this pathway at your fingertips for an exam, try rewriting the steps, reactants, products, and enzymes used in each step. For even better results, be sure to draw out the structures of the molecules as you go. Below is a schematic to help you out:
Adding it all up
From start to finish, a lot changes through the process of glycolysis. It can be a little overwhelming seeing everything at once, so we will break down some of the main points and show how they fit together below.
Counting Carbons
Carbon is an essential element in glucose, pyruvate, and all of the intermediate molecules created throughout the process. One molecule of glucose has six carbons. When broken down, it forms the intermediate glyceraldehyde 3-phosphate and final product pyruvate, each of which has 3 carbons. Since one glucose molecule creates two molecules of pyruvate, the number of carbon atoms stays constant throughout the process (3 carbons from pyruvate x 2 pyruvate molecules formed = 6 carbons).
Looking at Energy
Energetically speaking, glycolysis has two phases: one in which energy is invested (Phase I) and one which generates usable energy in the form of ATP or NADH (Phase II).
It is common for chemical pathways to involve energy-storing molecules like ATP or NADH. As the “currency” of biochemical reactions, they change forms when a process uses up or releases energy. Paying attention to the occurrences of these molecules in biochemical pathways can help you identify steps that involve energy generation or consumption. First, let’s take a look at ATP (adenosine triphosphate). In Phase I of glycolysis, two ATP molecules are consumed and converted into ADP (adenosine diphosphate). In Phase II, 4 ADP react with free phosphate (called inorganic phosphate, or Pi) to create ATP.
If you were keeping track, that means during glycolysis we lose 2 ATP but gain 4 ATP. It produces a net total of 2 ATP.
Another energy-storing molecule, NADH, also finds a place in glycolysis. In Phase II, 2 molecules of NAD+ change into 2 molecules of NADH. This is important because NADH stores an even greater amount of energy than ATP. Cells can access its stored energy using something called the electron transport chain.
Is that it?
As we saw earlier, glycolysis is a complex process that impacts almost all life on Earth. If you spend some time looking, you will find much more detailed information about the individual steps, the molecules involved, and the other biological processes that rely on glycolysis.
The basics of the process, however, are just what we have discussed in this article. Six-carbon glucose breaks down into three-carbon pyruvate, we generate ATP and NADH, and there is a “preparatory phase” (Phase I) and a “payoff phase” (Phase II).
Gluconeogenesis: glycolysis backward!
Gluconeogenesis just means the new (“neo”) creation (“genesis”) of sugar (“gluco”), and it refers to the exact reverse process of glycolysis. This pathway starts with pyruvate and results in the synthesis of glucose. Many of the same enzymes are involved in this process, and the steps are closely related. If you try to make connections between the forward and backward directions, both of them will be easier to remember.