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Coenzyme A question - (Dec/26/2008 )

Note: This is not a homework question. This is part of a study effort for a big upcoming test.

Question: What is the function/purpose of coenzyme A in cellular respiration (specifically, in the transition between glycolysis and Krebs cycle - I mean, I know that coenzyme A attaches to the acetate before the Krebs cycle and then falls off - with an extra hydrogen atom - and goes out of Krebs cycle, seemingly (and I put seemingly because I know that it has to do something) doing nothing. My question is: What does the coenzyme A do?????)

Progress I made on this question: When I first looked at Krebs cycle, I got really confused because in the post-glycolysis yet pre-Krebs cycle reaction, my textbook said in the last reaction before Krebs cycle, coenzyme A binds to the acetate. However, in the first step of Krebs cycle, CoA falls off and goes away. And my textbook did not say what it did. So I consulted my other books, hoping that they would provide me with some answers. They didn't. I went online, still believing that I could find the answer. I was still clueless - many of them said that coenzyme A plays and important role in cellular respiration but failed to expound on that idea. So I turned to here.

-thewax-

It is a coenzyme, and thus is recycled.

-HomeBrew-

Thanks!!!!!!!! smile.gif

But
Question: What is the function/purpose of coenzyme A in cellular respiration (specifically, in the transition between glycolysis and Krebs cycle - I mean, I know that coenzyme A attaches to the acetate before the Krebs cycle and then falls off - with an extra hydrogen atom - and goes out of Krebs cycle, seemingly (and I put seemingly because I know that it has to do something) doing nothing. My question is: What does the coenzyme A do?????)

It's recycled, but why does coenzyme A have to bind to acetate to form acetyl CoA if it's only going to fall off in literally the next step?????????????????

-thewax-

umm... I believe there is some misconception here. CoenzymeA doesn't bind to acetate. There are two pools, one cytosolic and one mitochondrial. And in kreb cycle we are dealing with the mitochondrial CoA pool.

Pyruvate is actively transported into the matrix of the mitochondria, where it will meet pyruvate dehydrogenase, a multi-protein complex. The pyruvate dehydrogenase complex will oxidise pyruvate.

Pyruvate + NAD+ + coA ----pyruvate dehydrogenase----> NADH + H+ + CO2 + acetyl-coA

The formation of Acetyl-coA locks pyruvate into the matrix and prevents it from leaking out again. Acetyl-CoA then feeds the acetate molecule to the TCA cycle.

-perneseblue-

Thanks!!!!!!!!!!!!!!!!!! smile.gif

so when coenzyme a is attached to acetate, it prevents the acetate from leaving the matrix, right? How (does it have some special property or something)? How does it "feed" it into the TCA cycle? How does it assure that it will stay in the TCA cycle and not just leave the matrix?

-thewax-

not quite. This process is to prevent the diffusion of pyruvate out of the matrix. It is a common trick used in biology, that if you want to pump a substrate quickly from compartment A to compartment B, you convert the substrate into something else, once it has move to compartment B. This maintains the gradient and keeps the flow quick and less wasteful.

Handling of acetate is done by cytosolic coA and has little to do with TCA cycle directly.

If you look at wikipedia, you will notice that coA is a rather big molecule that carries a three of phosphates, which gives it quite a negative charge. Thus coA doesn't pass through cell phosphobilayer of the membranes easily.

acetly-coA is feed into the TCA cycle via the citrate synthase enzyme. And a cell is packed with proteins. It really is like a gel rather water.

Look up http://biocyc.org/ for more details and further reading.

-perneseblue-

Thanks!!!!!!!!!!!!! smile.gif

But just one more question: How does it assure that it will stay in the TCA cycle and not just leave the matrix?????????????? I mean, the coenzyme A drops right off the acetate, so can't the acetate just leave the "compartment B" then??????????????

-thewax-

The acetyl group of acetyl-coA does not drop off in animals, we don't have the enzyme to decouple it. In the kreb cycle the acetyl group is transferred from acetyl-coA to Oxaloacetate. Never is there free acetate floating about.

In situation where an animal cell is full of acetyl-coA, NADH and lacking in coA and NAD+, we have a bypass that coverts pyruvate to lactate. THis situation is seen an muscle cells. In plant cells and yeast, the excess pyruvate is converted to ethanol.

In bacteria like e coli, which has a fix acid , yes, there is a decoupler. When there is an excess of acetyl-coA, it will remove the acetyl group, producing acetate and regenerating coA. In this situation, the acetate is excreted from the cell.

-perneseblue-