Basic Biochemistry. Energy Production - Second Step: Krebs Cycle (Citric Acid Cycle)

Navigating through the fascinating world of biochemistry, we continue with the flow of cellular respiration! We've learned how glycolysis, the first step of cell respiration, can provide us with 2 ATP, pyruvate, and NADH molecules. Remember that in the complete cycle of cellular respiration, 38 ATP molecules are generated. That leaves us with a deficit of 36! But where do these other molecules come from?

Well, pyruvate is a highly energetic 3-carbon molecule, but it needs to undergo a series of cyclic chemical reactions to transform into molecules that provide energy, such as NADH (remember the AAA battery analogy?). However, since cells have a high demand for ATP, all this NADH will be used as a substrate. Think of it as an AAA battery providing energy to form AA batteries – quite remarkable, isn't it?

Got it, so a series of complex chemical reactions will occur in a cycle to generate ATP molecules at the end? Exactly. These cyclic reactions were observed by Sir Hans Adolf Krebs in 1937, which he called the "Citric Acid Cycle," also known as the Krebs Cycle (now referred to as the tricarboxylic acid cycle). These reactions take place in a highly critical location, the mitochondria.

Mitochondria are elongated spherical organelles ranging in size from 1 µm to 10 µm in length and 0.5 µm to 1.0 µm in width. These organelles have been present in every cell for thousands of years. The quantity of mitochondria in each cell type varies depending on the metabolic needs of the tissue. Interestingly, mitochondria have a double membrane structure, consisting of the outer mitochondrial membrane (OMM) housing pores for metabolite passage and transporters, and the inner mitochondrial membrane (IMM) with numerous invaginations called mitochondrial cristae. The space enclosed by the IMM is known as the mitochondrial matrix, where mitochondrial DNA and enzymes involved in cellular respiration reside, as well as other proteins, genetic material (DNA and RNA), and ribosomes. Between the OMM and IMM, there is an "intermembrane space" (IMS). We will delve deeper into mitochondria when we reach the final stage of cellular respiration, oxidative phosphorylation. For now, let's focus on the fact that the Krebs cycle occurs within the mitochondrial matrix.

So, you're saying that the metabolites required to drive this cycle must pass through these membranes to reach the matrix? Exactly.

As mentioned, the OMM contains metabolite transporters, and pyruvate is one of them. There is a protein on the OMM known as the Mitochondrial Pyruvate Carrier (MPC) that transports this molecule into the mitochondrial matrix. Within the matrix, there is a highly important complex for the cellular respiration pathway, the Pyruvate Dehydrogenase Complex (PDC). Remember what a dehydrogenase enzyme does? It oxidizes. So, the 3-carbon pyruvate molecule is oxidized by the PDC complex, joining with a coenzyme A (CoA) molecule and an acetate molecule, forming Acetyl-CoA (a 2-carbon molecule), NADH, and a CO2 molecule. This is where the lost carbon goes. Acetyl-CoA is one of the molecules needed to drive the Krebs cycle (remember, we had two pyruvates from the glucose breakdown, so the cycle goes through two turns!).

It's important to note that this is not the only way to initiate the cycle; other metabolites can enter this pathway directly, originating from other biochemical processes (a true biochemical puzzle!). But, let's focus on the cycle and continue with the steps of cellular respiration. Here we go!

The first reaction in this cycle involves an enzyme called Citrate Synthase, a crucial enzyme for the entire cycle as it initiates an irreversible reaction. In this case, one molecule of oxaloacetate plus Acetyl-CoA form one molecule of citrate (synthesized by the enzyme, adding 4 carbons for a total of 6). In the second step of the cycle, the enzyme Aconitase transforms citrate into isocitrate (an isomer of citrate, a stereospecific reaction we can call isomerization).

In the third step of the process, isocitrate is converted into alpha-ketoglutarate by the enzyme Isocitrate Dehydrogenase (which oxidizes), producing NADH and releasing CO2 in the process. Now we have a 5-carbon molecule, while the original pyruvate had only 3. This is essential for reactions in which the structure of molecules is transformed, and the energy released forms the basis of the reaction.

In the fourth step, alpha-ketoglutarate is oxidized by the enzyme Alpha-Ketoglutarate Dehydrogenase, resulting in the loss of one carbon (now 4) and the addition of another Coenzyme A (CoA) molecule to its structure, forming Succinyl-CoA and producing another NADH molecule. Remember the lost carbon? It is released as CO2 in this reaction (this is why you constantly exhale this molecule). Right after that, in the fifth step, another synthesis reaction takes place, forming a molecule similar to ATP, GTP (think of it as ATP), by the enzyme Succinyl-CoA Synthetase, where a phosphate is added to a molecule of GDP (or ADP), removing Coenzyme A from the molecule, resulting in the formation of Succinyl. In this process, it loses another carbon (now 4).

From here, the Succinyl molecule undergoes another dehydrogenation in the sixth step, by Succinate Dehydrogenase, forming fumarate and a molecule similar to NADH, FADH2 (think of it as NADH, and it will all make sense in the end). Fumarate, in turn, is transformed into malate in the seventh step by Fumarate Hydratase (adding water!). Up to this point, it has 4 carbons.

Now, we reach another critical point in the cycle – the control of oxaloacetate's RE-formation. In the final step, malate is converted into oxaloacetate by Malate Dehydrogenase (which oxidizes), forming a 4-carbon molecule and one more NADH molecule, restarting the cycle!!!! Then, another pyruvate, converted into Acetyl-CoA, is "added" to oxaloacetate, returning to step 1. Simple, isn't it?

What's the end result? After the second turn through the Krebs cycle (remember, two pyruvate molecules, right?), we get 6 NADH molecules, 2 FADH2 molecules, and 2 ATP molecules (consider GTP as ATP). Of the 38 ATP molecules that cellular respiration produces, we already have 2 from glycolysis and 2 from the Krebs cycle. That leaves 34 more to find! Where are the rest, you ask? We will discover that in the final stage of cellular respiration, oxidative phosphorylation. For now, we can say that each NADH molecule will be converted into 3 ATP molecules in the mitochondria! Exciting!!!!

Join me inside the mitochondria and embark on this fascinating journey through energy production pathways! 💡🔬🔋 #Biochemistry #CellularRespiration #EnergyProduction #Mitochondria