LESSON 3: THE KREBS CYCLE
After the pyruvate from glycolysis has been prepped in pyruvate oxidation and has formed the 2-C Acetyl CoA, it enters the Krebs Cycle. This cycle, as complicated at it may seem, is simply a series of repeated reactions that serve to produce key molecules in the process of cellular respiration.
First, 2-C Acetyl CoA combines with a 4-C oxaloacetate to form a 6-C citrate molecule. This is simple addition. Next, with the help of an enzyme of course, citrate rearranges itself into isocitrate. Isocitrate has all the same components, or elements, as citrate does however simply differs in its arrangement of those elements. This makes isocitrate an isomer of citrate.
First, 2-C Acetyl CoA combines with a 4-C oxaloacetate to form a 6-C citrate molecule. This is simple addition. Next, with the help of an enzyme of course, citrate rearranges itself into isocitrate. Isocitrate has all the same components, or elements, as citrate does however simply differs in its arrangement of those elements. This makes isocitrate an isomer of citrate.
Isomerisation is much like when you change up your appearance when it gets hot by maybe zipping down your jacket, putting your tuque in your pocket, and stashing your gloves in your purse. You are still the same person with all the same clothes and accessories but simply have arranged yourself a little differently and may appear slightly different to another person.
In the next step of the Krebs cycle, isocitrate "de-weeds" itself and "pays the bank". If you'll remember, de-weeding would be akin to removing a carbon from the isocitrate through a decarboxylation reaction and releasing it as CO2. In "paying the bank", isocitrate gives up 1 H to NAD+ to form NADH through the reaction called oxidation. As a result, the 5-C molecule of α-ketoglutarate is formed.
That same "de-weeding" and "payment to the bank" is performed on and by α-ketoglutarate and it produces CO2 and NADH as products. The carrier CoA attaches to the molecule and forms succinyl-CoA. Seeing as a carbon was "de-weeded" as CO2, succinyl co-A is a 4-C molecule. The CoA on succinyl-coA is promptly replaced with a phosphate group, which can be seen as a sort of bully since it pushes the CoA out and takes it spot on the Krebs Cycle. This sort of attitude is not appreciated in the Krebs Cycle and therefore the Pi is given away to a guanosine diphosphate (GDP) molecule to create guanosine triphosphate. However, an ADP molecules comes along and steals the Pi from the GTP to make profit and produce 1 ATP molecule. This is called substrate level phosphorylation. After all these reactions, succinyl-CoA becomes a 4-C succinate.
In the next step of the Krebs cycle, isocitrate "de-weeds" itself and "pays the bank". If you'll remember, de-weeding would be akin to removing a carbon from the isocitrate through a decarboxylation reaction and releasing it as CO2. In "paying the bank", isocitrate gives up 1 H to NAD+ to form NADH through the reaction called oxidation. As a result, the 5-C molecule of α-ketoglutarate is formed.
That same "de-weeding" and "payment to the bank" is performed on and by α-ketoglutarate and it produces CO2 and NADH as products. The carrier CoA attaches to the molecule and forms succinyl-CoA. Seeing as a carbon was "de-weeded" as CO2, succinyl co-A is a 4-C molecule. The CoA on succinyl-coA is promptly replaced with a phosphate group, which can be seen as a sort of bully since it pushes the CoA out and takes it spot on the Krebs Cycle. This sort of attitude is not appreciated in the Krebs Cycle and therefore the Pi is given away to a guanosine diphosphate (GDP) molecule to create guanosine triphosphate. However, an ADP molecules comes along and steals the Pi from the GTP to make profit and produce 1 ATP molecule. This is called substrate level phosphorylation. After all these reactions, succinyl-CoA becomes a 4-C succinate.
Next, this 4-C succinate goes through an oxidation reaction. Much like many times before, hydrogens are released from 4-C succinate but this time 2 are released and attached to FAD to create FADH2. This can be seen as another bank payment, almost like a tax that has to be paid 4 times on the Krebs Cycle. After becoming oxidized, succinate becomes 4-C fumarate. 1 H2O is added onto the fumarate molecule to create the 4-C molecule of malate. Since water is being added to the molecule it can be seen as the opposite of sweating, which would be "fueling up". Just remember that fumarate adds fuel to become malute and it should help you remember the addition of water in this step. In the last step of the Krebs Cycle, malate makes one last tax payment (oxidation reaction) to the bank of 1 H and allows NAD+ to become NADH. The final product of the Krebs Cycle is 4-C oxaloacetate. Oxaloacetate is continuously recycled and used again as the second pyruvate from glycolysis goes through pyruvate oxidation, becomes Acetyl-CoA, and enters the cycle.
OVERVIEW
To simplify the entire process of the Krebs Cycle, think of it as a water wheel. The initial supply of water is continuously added to and taken from before it comes back to the top. 1 cycle of the Krebs Cycle is very similar in that it starts off with Acetyl-CoA and produces of 2 CO2, 3 NADH, 1 FADH2, and 1 ATP as molecules like Hydrogen, Carbon and Phosphates are taken from the cycle. It also has the addition of H2O as well before oxaloacetate is recycled over and over. The following are the products of the Krebs Cycle after 2 cycles. Remember, per 1 glucose molecule the Krebs Cycle happens twice since the 2 pyruvates produced in glycolysis each go through pyruvate oxidation and the Krebs Cycle one by one.