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Simple ADP --P bonding

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Simple ADP --P bonding

Postby Edher » Wed Feb 16, 2005 7:18 am

Saludos,

I have a question regarding step six and seven in the Glycolisis process. This is what it consists of,

6. Triose phosphate dehydrogenase catalyzes two sequential reactions while it holds glyceraldehyde-3-phosphate in its active site. First, the sugar is oxidized by the transfer of electrons and H+ to NAD+, forming NADH (a redox reaction). This reaction is very exergonic, and the enzyme uses the released every to attach a phosphate group to the oxidized substrate, making a product of very high potential energy. The source of the phosphates is the pool of inorganic phosphate ions that are always present in the cytosol.

7. Glycolisis produces some ATP by substrate-level phosphorylation. The phosphate group added in the previous step is transferred to ADP in an exergonic reaction.

Now, if there is a pool of inorganic phosphate ions always present in the cytosol, why doesn't ADP simply bond to this inorganic phosphate groups? Why are they dependents of glycolisis? Is it because energy is needed to bond the Phosphate Group to the other two Phosphate Groups (Tail of ADP) and since they're all negative, they repel each other, so they don't bond that easily?

Thank You,
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Postby thank.darwin » Fri Feb 18, 2005 8:40 pm

You're right - Its because bonding the negitive phosphate groups togther takes alot of energy - but not alot to take back apart... I'm not 100% sure that I'm right on that one?
No amount of experimentation can ever prove me right; a single experiment can prove me wrong.
-Albert Einstein
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substrate level -iP'ation

Postby 2810712 » Fri Feb 25, 2005 11:03 am

Hi,
Hey, I also had the same question some days before.
But , the answer I got is quite defferent -
The enzyme involved in sub. lev. phosphorylation holds the substrates in such a way that the reaction can proceed further. In the absense of the enzyme the reaction would take much moooore time to happen because of absense of suitable conditions. Thus, the enzyme is required.

thanks,

hrushikesh
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Postby Jelanen » Fri Mar 04, 2005 2:22 am

Addressing the questions...

Now, if there is a pool of inorganic phosphate ions always present in the cytosol, why doesn't ADP simply bond to this inorganic phosphate groups?

When ATP is converted to ADP+Pi, 31kJ/mol of energy is released. Thats alot. To make the reaction go from ADP+Pi to ATP, that 31kJ/mol would have to be put back and that sort of thing doesn't happen spontaneously (usually).

Why are they dependents of glycolisis? Is it because energy is needed to bond the Phosphate Group to the other two Phosphate Groups (Tail of ADP) and since they're all negative, they repel each other, so they don't bond that easily?

There are two main ways of phosphorylating ADP to ATP. First, its important to understand that ATP isn't the highest energy compound out there that the cell uses. Frequently, in the case of substrate level phosphorylation, a "super-high energy" compound is formed (1,3 bisphosphoglycerate in step 6) and since the dephosphorylation of that compound has a higher negative free energy release than the free energy needed to put the 3rd Pi on to ADP, it can be used to phosphorylate ATP. That is the basic sustrate level phosphorylation. The other type is called oxidative phosphorylation and involves the use of electron carriers to drive proton pumps to create a proton gradient across a membrane. When the protons stream back to the less concentrated side, it creates enough energy to drive a mechanism that phosphorylates ADP to ATP. This is the basis of the CA (citric acid) cycle and the ETS (electron transport system). Its called oxidative phosphorylation because you oxidize compounds to get the energy to set up the proton gradient.

-Jelanen
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