A. The Mechanisms of Enzyme Catalysis
We describe the action of biological catalysis in two ways. One way takes into account structural features of the enzyme (active site shape, overall conformation, the affinities of the enzyme for its substrates). The other way involves the energetic of enzyme action. We’ll see that enzymes lower the activation energy of a chemical reaction. Activation energy is an inherent energy barrier to the reaction. Of course, structural and energy considerations of enzyme catalysis are related.
1. Structural Considerations of Catalysis
From a chemistry course, you may recall that the rate of an uncatalyzed reaction is dependent on the concentration of the reactants in solution. This is the Law of Mass Action, recognized in the 19th century. Look at this simple reaction:
\[A + B ⇔ C + D\]
The Law of Mass Action makes two key assumptions:
a) At any given time following the start of the reaction, the rate of product formation is proportional to the concentrations of the reactants and products ([A], [B], [C] and [D] in this case).
b) Chemical reactions in the laboratory eventually reach equilibrium, at which point the net rate of formation of reaction products is zero (i.e., the forward and reverse reactions occur at the same rate).
At the start of the reaction written above, since there are no products yet, the reaction rate should be directly proportional only to the concentration of the reactants. Therefore, the Law of Mass Action predicts that a chemical reaction will occur faster at higher concentrations of A & B. This is because there are more reactant molecules in solution and a greater likelihood that they will collide in an orientation that allows the bond rearrangements for the reaction to occur.
Of course, reactant concentrations decline as products accumulate over time. Then the rate of formation of C & D should slows down, now affected by product as well as reactant concentrations; remember, all chemical reactions are inherently reversible! You may recognize the chemical rate equations from a chemistry course; these enable quantitation of reaction rates for our sample reaction. Here is the rate of formation or the products, C and D:
Rate of formation of products
\[(C & D) = k1[A][B] – k-1[C][D]\]
This equation recognizes that the reaction is reversible. Thus, the net reaction rate is equal to the rate of the forward reaction \(k1[A][B]\) minus the rate of the back reaction \(k-1[C][D]\). The equation is valid (applicable) at any time during the reaction. \(k1\) and \(k-1\) are rate constants for the forward and reverse reactions, respectively.
So how do catalysts work? Catalysts increase chemical reaction rates by bringing reactants together more rapidly than they would encounter each other based just on random molecular motion in solution. This is possible because catalysts have an affinity for their substrates.
In the case of inorganic catalysts, relatively weak, generic forces account for the affinity of reactants and inorganic catalysts. Thus, a metallic catalyst (e.g., silver, platinum) attracts molecules with the appropriate (e.g., charge) configuration. If the attraction (affinity) is sufficient, the metal will hold reactants in place long enough to catalyze the bond rearrangements of a chemical reaction.
Unlike inorganic catalysts, enzymes have evolved highly specific shapes with physical-chemical properties. As a result, enzymes typically attract only the substrates necessary for a particular biochemical reaction. The active site of an enzyme has the exquisitely selective affinity for its substrate(s). This affinity is many times greater than those of inorganic catalysts for generic reactants. The result is that enzymes are more efficient, faster catalysts.
Early ideas of how substrate-enzyme interaction could be so specific suggested a Lock and Key mechanism, illustrated below.
According to this model, the affinity of enzyme for substrate brings them together, after which the substrate uniquely fits into the active site like a key into a lock. Once in the active site, the substrate(s) would undergo the bond rearrangements specific for the catalyzed reaction to generate products and regenerate an unchanged enzyme. But X-ray crystallography of enzyme-substrate interaction revealed that the active site of the enzyme changes shape during catalysis. This allosteric change suggested the revised, Induced Fit mechanism of enzyme action modeled below.
In this model, enzyme-substrate affinity causes the substrate to bind to the enzyme surface. Once bound, the enzyme undergoes an allosteric change, drawing the substrate(s) more tightly into the active site and catalyzing the reaction. Of course, after the reaction products come off, the enzyme returns to its original shape.
- Gerald Bergtrom
Danielle Alarid, Orange County biotechnology education
Navanjot Batth, M.S., Santiago Canyon College
Kristine Clarke, M.S., Dana Hills High School
Spiridon Dimitritados, Ph.D. Fullerton College
Denise Foley, Ph.D., Santiago Canyon College
Charles Jang, M.S., Santa Ana College
Anson Lui, M.S, Santiago Canyon College
Emalee MacKenzie, M.S. Irvine Valley College
Kimberly Rosales, Ph.D. Fullerton College
Kathleen Takahashi, Ph.D. Santa Ana College
Jo Wu, Ph.D., Fullerton College