Biochemistry/Metabolism
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What is life? What makes life work? Life is not static; it is defined by activity. A cell has to perform certain tasks to stay alive. This activity has to be powered by fuel; the fuel is converted into energy, to keep the cell running, building blocks for new biomolecules, and waste products. This process is called metabolism.
[edit] ATP as an intermediate energy carrier
We have already seen how ATP is a universal "energy currency" in the cell, and how its high-energy phosphate can store and release this energy efficiently. But ATP is only a short-term energy storage; a hard-working human can use up to one pound of ATP per minute. How can that be?
The answer lies in the medium- and long-term energy storage molecules, notably phosphoenol pyruvate (PEP), acetyl phosphate, and creatine phosphate. Each of these molecules carries a phosphate group with an ever higher transfer potential than that of ATP. ATP is continuously regenerated by transfering phosphate groups from these molecules to ADP. For example, the reaction of creatine phosphate is
a reaction which is catalyzed by the creatine kinase, and highly in favor of the "ATP side" of the equation.
[edit] Electron carriers
Most of the energy of eucaryotic lifeforms derives from oxidating fuel molecules, that is, the transfer of electrons from fuel molecules to oxygen. The fuel molecules are oxidized, while the oxygen is reduced. A rather radical form of oxidation is fire, which oxidizes its fuel directly with oxygen. As spontanous combustion one ones cells is rather unpleasant, the electrons are carried through several steps from the fuel molecules to the final electron acceptor, oxygen. The flow of electrons causes a proton gradient in the mitochondrial membranes, which in turn is used by the enzyme ATPase (or proton pump) to generate ATP. This process, utilizing anelectron transport chain, is called oxidative phosphorylation.
The electron carrier molecules of choice are NADH and FADH2. They are pyridine nucleotides (also called flavions).
| Reduced form | Oxidized form | |
|---|---|---|
| NADH | NADH | NAD+ |
| FADH2 | FADH2 | FAD |
Both molecules are also for biosyntheses that require reducing power. Especially a variant of NADH, NADPH, is used for this purpose.
With all these high-energy bonds and electron-carrying molecules around, one would expect to see wild, uncontrolled chemical reactions occurring in the cell. However, the opposite is the case: Without a catalyst, ATP is quite slow to hydrolize and give its phosphate group to water. Also, all three electron-carrying molecules, NADH, NADPH, and FADH2, are slow to react with O2 without a catalyst.
