Enzymes

Enzymes are homogeneous biological catalysts. These ubiquitous compounds are special proteins or nucleic acids that contain an active site, which is responsible for binding the substrates, the reactants, and processing them into products. As is true of any catalyst, the active site returns to its original state after the products are released. Many enzymes consist primarily of proteins, some featuring organic or inorganic co-factors in their active sites. However, certain RNA molecules can also be biological catalysts, forming ribozymes. A very important example of a ribozyme is the ribosome, a large assembly of proteins and catalytically active RNA molecules responsible for the synthesis of proteins in the cell. The structure of the active site is specific to the reaction that it catalyses, with groups in the substrate interacting with groups in the active site by intermolecular interactions, such as hydrogen bonding, electrostatic, or van der Waals interactions. Figure 23.9 shows two models that explain the binding of a substrate to the active site of an enzyme. In the lock-and-key model, the active site and substrate have complementary three-dimensional structures and dock perfectly without the need for major atomic rearrangements. Experimental evidence favours the induced fit model, in which binding of the substrate induces a conformational change in the active site. Only after the change does the substrate fit snugly in the active site. Enzyme-catalysed reactions are prone to inhibition by molecules that interfere with the formation of product. Many drugs for the treatment of disease function by inhibiting enzymes. For example, an important strategy in the treatment of acquired immune deficiency syndrome (AIDS) involves the steady administration of a specially designed protease inhibitor. The drug inhibits an enzyme that is key to the formation of the protein envelope surrounding the genetic material of the human immunodeficiency virus (HIV). Without a properly formed envelope, HIV cannot replicate in the host organism.

Fig. 23.9 Two models that explain the binding of a substrate to the active site of an enzyme. In the lock-and-key model, the active site and substrate have complementary three-dimensional structures and dock perfectly without the need for major atomic rearrangements. In the induced fit model, binding of the substrate induces a conformational change in the active site. The substrate fits well in the active site after the conformational change has taken place.

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مواضيع ذات صلة

The primary quantum yield

Timescales of photophysical processes

The catalytic efficiency of enzymes

The Michaelis–Menten mechanism of enzyme catalysis

Application of the techniques

Monitoring the progress of a reaction

The measurement of diffusion coefficients

Solutions of the diffusion equation

Diffusion with convection

Conductivities and ion–ion interactions

The drift speed

Strong electrolytes