according to M. EIGEN and R. WINKLER, 1975
Every living cell performs continuously thousands of different chemical reactions. Taken nutrients are transformed into a multitude of cell specific components. In this way, sugars, amino acids and their precursors, organic acids, nucleotides, lipids and other substances are produced. The totality of these reactions is summarized as the cell metabolism. In every chemical reaction, a bond is either formed or broken. This process does generally consume energy, consequently, the cell's metabolism and its energy balance to have be regarded as interdependent parameters. Every covalent bond of a molecule contains energy that is set free upon breaking down and that can subsequently be used for other purposes. It could, for example, be used for the formation of a new bond or it could be transformed into another form of energy like movement, warmth, light or electrical energy. Most chemical bonds and nearly all that occur within a cell are stable under physiological conditions. Hence, activation energy is needed for the break-down of a molecule. In chemical laboratories, high temperatures, high pressures or certain inorganic or organic catalysts are normally used.
The first two alternatives are out of question for a cell, it can only use catalysts. Biological catalysts are without exception enzymes (proteins). Their activity is often dependent on the presence of other molecules. Enzymes are highly specific and hardly any futile by-products are generated. Therefore, no superfluous molecules are normally found in a cell. The use of enzymes enables the cell to run thousands of thermodynamically possible reactions in parallel.
The chemist R. WILLSTÄTTER wrote as soon as 1912::
"The vast number of chemical reactions taking part in a living cell are controlled by organic catalysts according to direction and speed. Life is the orchestrated combination of processes catalyzed by enzymes."
How does an enzyme work? Generally spoken, it lowers the activation energy of a reaction. Though it catalyzes a reaction, the task of generating a new chemical bond is never performed by an enzyme alone.
Reaction Kinetics:
A. uncatalysed,
B. enzymatically catalyzed
An additional energy source is needed. Cells are so-called open systems, i.e. they are dependent on the continuous uptake of energy and nutriments. During evolution have only those life-forms survived that were able to use the available raw materials efficiently. Cells have two fundamental ways to generate energy:
The free energy that is produced by the step-wise breaking down of such compounds (fermentation, respiration) is stored in chemical bonds and is subsequently available for anabolic processes (or it is transformed into another state of energy).
To understand the cell's metabolism and its energy balance better will we at first review some basic laws of thermodynamics. We will discuss the nature of oxidation and reduction and finally the mechanisms of enzymatic catalysis in detail.
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