Most of the chemical reactions that take place
within a cell involve protein catalysts called
enzymes. Enzymes, like other catalysts, speed
up the rates of chemical reactions by lowering
the activation energy of that reaction (i.e., the
amount of energy needed to start a chemical
reaction). They do so by binding reactants
(hereafter referred to as substrates) and holding
them in a particular orientation that maximizes
the chances that a particular chemical reaction
will occur, converting the substrates into
products. Like other catalysts, enzymes
themselves are not permanently altered in the
chemical reaction they catalyze—enzymes
return to their original form at the end of the
reaction. Also, like other catalysts, the enzyme
does not provide the free energy necessary to
drive otherwise energetically unfavorable
reactions, but simply facilitates energetically
favorable ones.
The ability of enzymes to function as
catalysts depends on the three dimensional shape
of the protein. Recall that all proteins


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