An enzyme (in Greek en = in and zyme = leaven) is a protein, or protein complex, that catalyzes a chemical reaction and also controls the 3D orientation of the catalyzed substrates.
Like any catalyst, enzymes work by lowering the activation energy of a reaction, thus allowing the reaction to proceed to its steady state or completion much faster than it otherwise would; the enzyme (again, as with any catalyst) remains unaltered by the completed reaction and can therefore continue catalysis.
It is important to note that, as with all catalysts, all reactions catalyzed by enzymes must be 'spontaneous' (containing a net negative Gibbs free energy), i.e. with the enzyme, they run in the same direction as they would without the enzyme, just more quickly; the concept is similar to the likelihood of a ball rolling down a hill versus the likelihood of it rolling up the hill.
Catalysis by an enzyme is analogous to removing a pebble that is stopping the ball from rolling down the hill; the reaction goes to completion more quickly, but the final product is identical. Given a particular starting set of conditions, the end products of a particular reaction (including net energy), once steady state is reached, must always be identical, independent of the specific individual pathway taken from beginning point to end point.
This is required by the Law of Conservation of Energy, which would be violated by the possibility of a cycle of moving down a pathway releasing less net energy and back up a different pathway with higher net energy, or vice versa.
An enzyme can, however, run a normally non-spontaneous reaction 'backwards' by coupling it to a spontaneous one, as long as the net free energy from the total of both reactions is negative.
Enzymes are necessary within biological cells to control molecular shapes and because many chemical reactions would occur too slowly to sustain life; oxidation of organic food compounds to provide energy, for instance.
Enzymes may speed up biochemical reactions by a factor of one thousand times or more. They also provide a means to control the reaction rates by modulating enzymatic activity.
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