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Name: John C.
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: August 2003


Question:
Last year in biology my teacher ground into us the process of cell metabolism, the ATP/ADP cycle, etc. Unfortunately, he could not provide an explanation of how the individual cells actually utilized the energy gained from this cycle. The extent of his knowledge was that "cells use the energy they gain from glucose to carry out active transport and other life functions." None of the books or web sites I have searched have given much more than this. I would greatly appreciate knowing the actual process a cell goes through when the energy from ATP is directed to a task. This has long been my major biological question.



Replies:
This is really a very good question. I like that you did not just accept the stock answer, which is true, but not all that informative. One of the major ways that ATP is actually used in the cell is in enzymatic reactions. Enzymes, you probably recall from class, are proteins that act as catalysts. They "do" something, like grabbing two small molecules and joining them (via covalent bonds) to form a larger molecule. After the reaction, the enzyme lets go of the reacting molecules and is free to start all over. It is unchanged by the reaction. A prime example of an enzyme at work is during DNA synthesis. A chain of DNA is built up by an enzyme (called "DNA polymerase") which binds to the building blocks of DNA and joins them in a growing strand of new DNA.

Well, if an enzyme is going to "push" two molecules together until they bond covalently, the energy to do that has to come from somewhere. That "somewhere" is frequently provided by a molecule of ATP. The enzyme, DNA polymerase, not only has binding sites for the molecules it is going to join together, but another binding site for a molecule of ATP. Once the enzyme has bound all the players in the reaction, then the molecule of ATP is partially broken down; one covalent bond in the ATP is broken. Breaking that covalent bond releases the energy that was put into it during its formation. Think cutting a rubber band under tension, as a loose analogy. The energy released when this particular bond in the ATP is broken does not just go up in smoke, so to speak, but is "caught" and harvested by the enzyme. The energy causes the enzyme to change shape in a small way, so that the OTHER things it is holding onto are forced together and covalently bonded together. SO, one covalent bond is broken in ATP, and a new covalent bond is formed in the other molecules the enzyme is binding. All that is left is for the enzyme to release everything and start all over.

Variations on this theme will cover a great deal of cell metabolism. Some enzymes join things together, others move things from one area to another, etc. The basic idea is the same.

PS. Enzymes can do all this stuff FAST, as in dozens or hundreds of times per second, depending on the enzyme!).

Paul Mahoney, PhD


Dear John:

ATP, as well as the NADP & NADPH energy carriers, are necessary as cofactors to drive numerous enzymatic reactions necessary for life. ATP synthesized within the mitochondria is exchanged by an ADP-ATP antiporter for ADP accumulating in the cytoplasm, where most of these reactions occur. Although this process maintains a fairly high intracellular [ATP]/[ADP] ratio at any given time, the actual cytoplasmic [ATP] is not especially high (~5mM). Consequently, many critical or energy-intensive reactions rely on other energetic cofactors, typically generated in a separate ATP-dependent reaction, which can accumulate at a higher concentration to store energy for that specific reaction. Muscle contraction is a classic example of an energy-intensive reaction requiring large amounts of energy on fairly short notice. Consequently, muscle tissue utilizes creatine phosphate, which can accumulate to 4 times the typical [ATP], as an "energy reserve". In general, the ongoing synthesis of new ATP in the mitochondria & its exchange with "spent" ADP in the cytoplasm provides a freely available ATP pool in the cytoplasm for the many energy-dependent enzymatic reactions there. The following section on The Mitochondrion in the on-line text of the "Molecular Biology of the Cell @ NCBI" provides a more detailed description of this general process, if you're interested:

http://www.ncbi.nlm.nih.gov/books/bv.fcgi?tool=bookshelf&call=bv.View.ShowSection&rid=cell. section.3470

Thanks for the good question and I hope that this explanation is helpful,

Jeff Buzby, Ph.D.
CHOC Research Institute
NEWTON AAS
Division of Educational Programs
Argonne National Laboratory



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