Re: Hi, Are enzyme catalyzed reactions, Exothermic? Endothermic?

Hi Fay,

There are thousands of known enzymes, and as a group, enzymes catalyze a very wide variety of reactions; some exothermic (exergonic) and some endothermic (endergonic).

Enzymes are biochemical catalysts, and as catalysts, they speed up particular chemical reactions by lowering the activation energy of the reaction pathway (see http://faculty.abe.ufl.edu/~chyn/age2062/lect/lect_03/3_43.gif, below)

But the one thing enzymes and catalysts in general can’t do is change the thermodynamics of a reaction, e.g., the deltaG (Gibbs Free Energy), deltaH (enthalpy), or deltaS (entropy). deltaG is a thermodynamic parameter that is affected by the nature of the chemical species as well as the conditions (temperature, pressure, concentrations) of the reaction. The value of deltaG tells us whether a given reaction will proceed in the forward or reverse direction. If deltaG is negative, the forward reaction is favored. So, although enzymes catalyze both endothermic (deltaH>0) and exothermic (deltaH<0) reactions, enzymes can only catalyze reactions where deltaG<0 (these area called “spontaneous” reactions).

The name, “spontaneous” can be a bit misleading. All it tells us is that a reaction is thermodynamically favored; it tells us nothing about how fast the reaction might occur. For example, the rusting of iron and the decomposition of a diamond to graphite are both reactions with deltaG<0 under normal conditions, but their rates, especially the latter, can be extremely slow. The role of enzymes is to accelerate the rates of thermodynamically favorable reactions.

-Alex Tobias

[Moderator's Note: While most of Alex' answer is accurate, enzymes alter the rate of a reaction but not the equilibrium of that reaction, so that they can catalyze both forward and reverse reactions. For more information, review a college-level Biochemistry text, such as Biochemistry, by L. Stryer. For example, take a look at Chapter 8, 'Enzymes: Basic Concepts and Kinetics'.]