One of our principle sources of energy is sugar, or carbohydrates. Yes, carbohydrates and sugar are the same thing, so think about things bread and beer a bit differently. In this blog post, we’ll take a look at how we use them as an energy source!

Essentially, they all end up getting turned into glucose by one process or another, depending on the type of sugar it is. It’s going to help that glucose is a 6 carbon chain – just 6 carbons lined up in a row with hydrogens on one side and -OH (hydroxyl or an oxygen and a hydrogen) on the other. Oh, there is an aldehyde group at one end of the chain, too. That first carbon on our chain? There is an oxygen that has a double bond, or two sets of electrons, connecting it to the carbon, as well as a hydrogen attached to it.

Once those sugars have been turned into glucose, they go through a process called glyolysis. We’re going to focus on that today. Essentially, there are two basic phases of glycolysis – energy using and energy making. Gotta spend money to make money, right?

Fortunately, we only spend 2 ATP, which is our bodies primary energy form. Basically, everything we do, as far as breaking down food for energy and such, is about making ATP. It’s used for pretty much all of our bodies functions that require energy and can be used by virtually every cell. You can think of it as our bodies version of bitcoin, if bitcoin had actually worked out. Anyway, we spend 2 ATP, adenosine triphosphate, and make 4 of them instead. We also kick out a couple of NADH which are molecules that go on to make more energy and will probably be discussed in a future article.

So, we start with the energy spending phase and, of course, glucose is our starting molecule. An enzyme called hexokinase, which is used in most cells, or glucokinase, used in the liver, comes along to our glucose molecule, grabs an ATP, rips one a phosphate group off the ATP and slaps it onto the glucose. That leaves us with an ADP (Adenosine DiPhosphate) and Glucose 6-phosphate. So, the glucose and glucose 6-phosphate are almost the same thing. All we’ve done is added that phosphate group to the 6th carbon on the glucose chain. Okay, so not onto the carbon directly, but rather to the oxygen that is bound to that carbon. It’s important to know that this step is irreversible, meaning you can’t unmake glucose 6-phosphate. Glucose 6-phosphate also can’t move outside of the cell, so we’re essentially committed to this process already.

Step 2, we’re taking glucose 6-phosphate and making fructose 6-phosphate. All this means is that double bond oxygen at the beginning of our chain is moving from the first carbon to the second carbon. This makes them isomers (two compounds with the same formula but in a different order) of each other, so we’re going to use something called phosphoglucose isomerase. Still got that phosphate on our 6th carbon, just changing the structure from glucose (6-phosphate) to fructose (6-phosphate).

Next, we take our fructose 6-phosphate and adding a second phophate to it. This time, its on our first carbon, which is why we had to move that double bond out of the way. And yes, that phosphate group came from ATP once again, so the enzyme we’re using is phosphofructokinase 1, often abbreviated as PFK1, and it leaves an ADP in its path along with fructose 1,6-biphosphate. So, still fructose underneath it all, we’ve just added two phosphates – one on the 1st carbon and one on the 6th. Once again, this step is irreversible and it’s actually a really important step because it is a big control point for regulating glycolysis. We’ll get into that more later.

Up to this point, not too bad, right? Maybe a little hard to remember the names, but still pretty straight forward. Step 4 takes our fructose 1,6-biphosphate and reacts it with aldolase A. Remember the A because there is a aldolase B later which is really important, too. For our level, all it’s going to do is take our fructose 1,6-biphophate, which has 6 carbons and chop it in half. That gives us 2 chains of three carbons with one phosphate attached. The one that has the double bond on the second carbon, probably at the top if you visualize it the same way I do, is called dihydroxyacetone phosphate (DHAP). Don’t worry about it now, because that’s going on to gluconeogenesis and that’s DEFINITELY a different blog post.

Instead, we’re taking the bottom half, which in the process gained a double bond oxygen on its first carbon (phosphate still on the third which used to be the sixth), and we’re going to call it glyceraldehyde 3-P. A three carbon sugar with the bond on the first carbon is called a glyceraldehyde that phosphate is on the third carbon. Name makes sense then, right? That molecule is what is going to go on in glycolysis.

Quick side note, though, there is an enzyme called triose phosphate isomerase that allows DHAP and G3P to morph into each other. Remember, they’re almost the same except for the location of the double bonded oxygen. So, some texts will even have you assume that you’re going into the energy making phase with two glyceraldehyde 3-phosphates.

Okay, that takes us to the end of the energy spending phase. Let’s take a break there and next week, talk about the energy making phase!