This time, let’s get into the Krebs Cycle a.k.a. the Citric Acid cycle. This one isn’t too bad and we don’t have a lot to remember. This time, let’s get into the Krebs Cycle a.k.a. the Citric Acid cycle. This one isn’t too bad and we don’t have a lot to remember.

We need the pyruvate dehydrogenase enzyme to convert pyruvate, a 3  carbon molecule, into acetyl-CoA which has 2 carbons. That already is going to produce 1 NADH and 1 CO2. Technically, they don’t count as products of the Citric Acid cycle, though, because pyruvate needs to be turned into acetyl-CoA BEFORE it can begin the cycle.
It kicks off by acetyl-CoA (2 carbons) binding with oxaloacetate (4 carbons) to make citrate (6 carbons). Guess what they’re put together with. Citrate synthase. Man, names can be easy. Well, at least helpful.

Citrate is turned into isocitrate by aconitate. Isocitrate is then turned into alpha-ketoglutarate by isocitrate dehydrogenase. At this stage, we’re kicking out a CO2 and an NADH to be used for energy later. This is also a regulatory stage. ADP will stimulate this process so that it keeps moving forward. That makes sense, though, right? ADP is a low energy state so we’re encouraging energy production. Conversely, ATP and NADH, which are high energy molecules serve to inhibit it.

Alpha-ketoglutarate dehydrogenase turns our substrate into succinyl-CoA. We’re going to kick out another CO2 and also another NADH. This is also a regulatory step. Succinyl-CoA, NADH and ATP are all products of this reaction and they are also all inhibitors of it as well. Well, ATP isn’t a product but its that high energy state thing we talked about.

From succinyl CoA we take off the CoA and make a GTP leaving succinate. Succinate makes FADH2 on its way to fumarate. Fumarate becomes malate. Malate makes an NADH and oxaloacetate. That’s where they cycle part comes in, because now this oxaloacetate is ready to bind with an acetyl-CoA and start the process over again!

All in all TCA is going to make 3 NADH, 1 FADH2, 2 CO2 and 1 GTP for each acetyl-CoA that goes through the process. That will reslt in a total of 10 ATP. Since glucose gets broken down into two pyruvate, we’re actually going to get double all these things, too.

Last, here is a mnemonic for remembering all the products of the Citric Acid cycle.
Citrate Is Kreb’s Starting Substrate For Making Oxaloacetate

Citrate Citrate
Is Isocitrate
Kreb’s alpha-Ketogluterate
Starting Succinyl-CoA
Substrate Succinate
For Fumarate
Making Malate
Oxaloacetate Oxaloacetate