Most of the chemical reactions that take placewithin a cell involve protein catalysts calledenzymes. Enzymes, like other catalysts, speedup the rates of chemical reactions by loweringthe activation energy of that reaction (i.e.
, theamount of energy needed to start a chemicalreaction). They do so by binding reactants(hereafter referred to as substrates) and holdingthem in a particular orientation that maximizesthe chances that a particular chemical reactionwill occur, converting the substrates intoproducts. Like other catalysts, enzymesthemselves are not permanently altered in thechemical reaction they catalyze—enzymesreturn to their original form at the end of thereaction. Also, like other catalysts, the enzymedoes not provide the free energy necessary todrive otherwise energetically unfavorablereactions, but simply facilitates energeticallyfavorable ones.The ability of enzymes to function ascatalysts depends on the three dimensional shapeof the protein. Recall that all proteins