I'm preparing for my biology final and was reviewing the Kreb's Cycle, and there is one thing that I'm a little unclear about. After alpha-ketoglutarate is oxidized and decarboxylated, the resulting succinyl group is bound to coenzyme A. Then in the next step, when the succinyl-CoA is cleaved, energy released drives the phosphorylation of GTP. What I didn't understand was that if breaking the bond released enough energy to yield a GTP and then ATP, why was there no input of energy needed to form the bond between the succinyl group and CoA in the first place?

 

This isn't a question on an assignment, I hope it doesn't seem like I'm asking for someone to do my homework for me, just something that puzzled me that I was hoping for some clarification on.

 

Thanks in advance for your time!

Ahh but you see there was energy put into that bond. The release of CO2 from alpha-ketoglutarate contains more energy than is required to form NADH. Metabolism is resourceful and funnels that energy into the addition of CoA so that it may then later be cleaved to transfer it to GTP. If there wasn't the addition of CoA after the decarboxylation there would be alot of energy wasted.

 

Hope that helps.

 

 

*edit*

This ties into the most interesting part of the cycle (at least for me)

The alpha-ketoglutarate dehydrogenase reaction is almost identical to the pyruvate dehydrogenase, so you could ask the same question about that. But the loss of the CoA from acetyl-CoA doesn't go into making GTP, it is used to pull the preceeding unfavorable malate dehydrogenase step that forms oxaloacetate forward.

 

What you have to realize is that the whole thing is called a cycle for a reason. Just like how if you push on one part of a bikes wheel the rest turns so it works the same with the krebs cycle. The energetically favorable steps balance out the unfavorable steps and pull them forward. In the cell the free energy of the whole cycle is practically at 0!! just slighly negative allowing for it to proceed. Any additional energy put into one part drives the rest around. It's very efficient actually.

Bluenoise is correct. Decarboxylation makes many reaction step irreversible and more energetically favorable. For example, in fattly acid synthesis, decarboxylation (among other factors) helps drive the synthesis of fatty acids from malonyl CoA.