Editing Enzyme (section)

==Thermodynamics==
[[File:Enzyme catalysis energy levels 2.svg|thumb|400px|alt=A two dimensional plot of reaction coordinate (x-axis) vs. energy (y-axis) for catalyzed and uncatalyzed reactions. The energy of the system steadily increases from reactants (x = 0) until a maximum is reached at the transition state (x = 0.5), and steadily decreases to the products (x = 1). However, in an enzyme catalysed reaction, binding generates an enzyme-substrate complex (with slightly reduced energy) then increases up to a transition state with a smaller maximum than the uncatalysed reaction.|The energies of the stages of a [[chemical reaction]]. Uncatalysed (dashed line), substrates need a lot of [[activation energy]] to reach a [[transition state]], which then decays into lower-energy products. When enzyme catalysed (solid line), the enzyme binds the substrates (ES), then stabilizes the transition state (ES<sup>‡</sup>) to reduce the activation energy required to produce products (EP) which are finally released.]]

{{main |Activation energy|Thermodynamic equilibrium|Chemical equilibrium}}

As with all catalysts, enzymes do not alter the position of the chemical equilibrium of the reaction. In the presence of an enzyme, the reaction runs in the same direction as it would without the enzyme, just more quickly.<ref name = "Stryer_2002"/>{{rp|8.2.3}} For example, [[carbonic anhydrase]] catalyzes its reaction in either direction depending on the concentration of its reactants:<ref>{{cite book |vauthors=McArdle WD, Katch F, Katch VL | title = Essentials of Exercise Physiology | date = 2006 | publisher = Lippincott Williams & Wilkins | location = Baltimore, Maryland | isbn = 978-0781749916 | pages = 312–3 | edition = 3rd | chapter = Chapter 9: The Pulmonary System and Exercise | chapter-url = https://books.google.com/books?id=L4aZIDbmV3oC&q=carbonic+anhydrase+lung+tissue+low+high+carbon+dioxide+equilibrium&pg=PA313}}</ref>

{{NumBlk|:| <chem>CO2{} + H2O ->[\text{Carbonic anhydrase}] H2CO3</chem> (in [[Tissue (biology)|tissues]]; high CO<sub>2</sub> concentration)|{{EquationRef|1}}}}

{{NumBlk|:| <chem>CO2{} + H2O <-[\text{Carbonic anhydrase}] H2CO3</chem> (in [[lung]]s; low CO<sub>2</sub> concentration)|{{EquationRef|2}}}}

The rate of a reaction is dependent on the [[activation energy]] needed to form the [[transition state]] which then decays into products. Enzymes increase reaction rates by lowering the energy of the transition state. First, binding forms a low energy enzyme-substrate complex (ES). Second, the enzyme stabilises the transition state such that it requires less energy to achieve compared to the uncatalyzed reaction (ES<sup>‡</sup>). Finally the enzyme-product complex (EP) dissociates to release the products.<ref name = "Stryer_2002"/>{{rp|8.3}}

Enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive" a thermodynamically unfavourable one so that the combined energy of the products is lower than the substrates. For example, the hydrolysis of [[Adenosine triphosphate|ATP]] is often used to drive other chemical reactions.<ref name="Nicholls">{{cite book |vauthors=Ferguson SJ, Nicholls D, Ferguson S | title = Bioenergetics 3 | publisher = Academic | location = San Diego | year = 2002 | isbn = 0-12-518121-3 | edition = 3rd}}</ref>